Comparison of Pascal and C

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Template:ProgLangCompare The computer programming languages C and Pascal are often compared[1] to each other, sometimes heatedly, probably because the languages have similar times of origin, influences, and purposes, and so represent two philosophical approaches to a similar need. Both were used to design (and compile) their own compilers early in their lifetimes, and both were established and popular during the formative years of the microcomputer age.

Both C and Pascal are quite old languages. An original Pascal definition appeared in 1969 and a first compiler in 1970, the first version of C a few years later, around 1972. Both are arguably descendants of the ALGOL language series.

Algol introduced so-called structured programming, where programs are constructed of single entry and single exit constructs such as if, while, for, case, and so on. Furthermore, only expression syntaxes were (more or less) systematically described before Algol, which defined the entire language syntax in terms of a recursive grammar (BNF). This tended to make the language more general and regular.

Pascal stems directly from Algol-W, while it shared some new ideas with Algol 68. The C language is more indirectly related to Algol, originally through B, BCPL, and CPL, and later through Algol 68 (struct, union etc) and also Pascal (enumerations, const, typedef, booleans). Some Pascal dialects also incorporated characteristics from C.

Some later versions of the languages have become quite similar, especially types and data structuring possibilities, but also control structures and general layout are (very) roughly equivalent. The most important remaining differences concerns declaration syntax, expression evaluation, nested procedures, sets, and modular programming (separate compilation). However, this document covers only the original definitions of the language and not the endless series of new standards, dialects and other additions to both languages.

What is documented here is the Pascal of Niklaus Wirth, as standardized as ISO 7185 in 1982. The C documented is the language of Brian W. Kernighan and Dennis M. Ritchie, as standardized in 1989. The reason is that these versions both represent the mature versions of the language, and also because they are the closest in time. Certainly people may decry the lack of C99 (the later C standard) features, but that would leave us comparing an old Pascal to an ever changing new C standard. The same comment applies to the various new dialects of Pascal. This article does not cover comparisons with these.

Contents

Syntax

Syntactically, Pascal is much more Algol-like than C. English keywords are retained where C uses punctuation symbols — Pascal has and, or, and mod where C uses &&, ||, and % for example. However, C is actually more Algol-like than Pascal regarding (simple) declarations, retaining the type-name variable-name syntax which Pascal abandoned to allow for (easily read) complex type expressions and a better perceived clarity in educational situations.

Semicolon use

Another, more subtle, difference is the role of the semicolon. In Pascal semicolons separate individual statements within a compound statement whereas they terminate the statement in C. They are also syntactically part of the statement itself in C (transforming an expression into a statement). This difference manifests itself primarily in two situations:

  • there can never be a semicolon directly before else in Pascal whereas it is mandatory in C (unless a block statement is used)
  • the last statement before an end is not required to be followed by a semicolon

Some programmers nevertheless put a semicolon on the last line before end, thereby formally inserting an empty statement. This is discouraged by a few educators, worried that it may confuse students' perception of the formal role of the semicolon in Pascal.

Comments

In traditional C, there are only /* block comments */.

In traditional Pascal, there are { block comments } and (* block comments *).

Identifiers and keywords

C and Pascal differ in their interpretation of upper and lower case. C is case sensitive while Pascal is not, thus MyLabel and mylabel are distinct names in C but identical in Pascal (which may cause problems if object files from the two languages are linked together). In both languages, identifiers consist of letters and digits, with the rule that the first character may not be a digit. Many later Pascal implementations allow underscores in identifiers (as defined in ISO Extended Pascal). In C, the underscore traditionally counts as a letter, so even _abc is a valid name. Names with a leading underscore are often used to differentiate special system identifiers in C, and certain Pascal systems use this convention as well.

Both C and Pascal use keywords (words reserved for use by the language itself). Examples are if, while, const, for and goto, which are keywords that happen to be common to both languages. In C, the basic built-in type names are also keywords (e.g. int, char) or combinations of keywords (e.g. unsigned char), while in Pascal the built-in type names are predefined normal identifiers.

Definitions, declarations, and blocks

In Pascal, procedure definitions start with keywords procedure or function and type definitions with type. In C, function definitions are determined by syntactical context while type definitions use the keyword typedef. Both languages use a mix of keywords and punctuation for definitions of complex types; for instance, arrays are defined by the keyword array in Pascal and by punctuation in C, while enumerations are defined by the keyword enum in C but by punctuation in Pascal.

In Pascal functions, begin and end delimit a block of statements (proper), while C functions use "{" and "}" to delimit a block of statements optionally preceded by declarations. C (prior to C99) strictly defines that any declarations must occur before the statements within a particular block but allows blocks to appear within blocks, which is a way to go around this. Pascal is strict that declarations must occur before statements, but allows definitions of types and functions - not only variable declarations - to be encapsulated by function definitions to any level of depth.

Implementation

The grammars of both languages are of a similar size. From an implementation perspective the main difference between the two languages is that to parse C it is necessary to have access to a symbol table for types, while in Pascal there is only one such construct, assignment. For instance, the C fragment X * Y; could be a declaration of Y to be an object whose type is pointer to X, or a statement-expression that multiplies X and Y. The corresponding Pascal fragment var Y:^X; is unambiguous without a symbol table.

Simple types

Integers

Pascal requires all variable and function declarations to explicitly specify their type. In traditional C, a type name may be omitted in most contexts and the default type int (which corresponds to integer in Pascal) is then implicitly assumed (however, such defaults are considered bad practice in C and are often flagged by warnings).

C accommodates different sizes and signed and unsigned modes for integers by using modifiers such as long, short, signed, unsigned, etc. The exact meaning of the resulting integer type is machine-dependent, however, what can be guaranteed is that long int is no shorter than int and int is no shorter than short int.

Subranges

In Pascal, a similar end is performed by declaring a subrange of integer (a compiler may then choose to allocate a smaller amount of storage for the declared variable):

type a = 1..100;
     b = -20..20;
     c = 0..100000;

This subrange feature is not supported by C.

A major, if subtle, difference between C and Pascal is how they promote integer operations. In Pascal, all operations on integers or integer subranges have the same effect, as if all of the operands were promoted to a full integer. In C, there are defined rules as to how to promote different types of integers, typically with the resultant type of an operation between two integers having a precision that is greater than or equal to the precisions of the operands. This can make machine code generated from C efficient on many processors. A highly optimizing Pascal compiler can reduce, but not eliminate, this effect under standard Pascal rules. As a side effect, many Pascal compilersTemplate:Which? extend the language to include special integer types and promotion rules.

Some earlyTemplate:Which? pre-Standard implementations of C formerly allowed integer and pointer types to be relatively freely intermixed.

Character types

In C the character type is char which is a kind of integer that is no longer than short int, . Expressions such as 'x'+1 are therefore perfectly legal, as are declarations such as int i='i'; and char c=74;.

This integer nature of char (an eight-bit byte on most machines) is clearly illustrated by declarations such as

unsigned char uc = 255;  /* common limit */
signed char sc = -128;   /* common negative limit */

There has been some controversy as whether the char type should be regarded as signed or unsigned by default. This is up to the implementation.

In Pascal, characters and integers are distinct types. The inbuilt compiler functions ord() and chr() can be used to effectively typecast single characters to the corresponding integer value of the character set in use, and vice versa. e.g. on systems using the ASCII character set ord('1') = 49 and chr(9) is a TAB character.

Boolean types

In Pascal, boolean is an enumerated type. The possible values of boolean are false and true. For conversion to integer, ord is used:

i := ord(b);

There is no standard function for integer to boolean, however, the conversion is in practice simple:

b := i <> 0;

C has binary valued relational operators (<, >, ==, !=, <=, >=) which may be regarded as boolean in the sense that they always give results which are either zero or one. As all tests (&&, ||, ?:, if, while, etc.) are performed by zero-checks, false is represented by zero, while true is represented by any other value.

Bitwise operations

The C programmer may sometimes use bitwise operators to perform boolean operations. Care needs to be taken because the semantics are different when operands make use of more than one bit to represent a value.

Pascal has another more abstract, high level method of dealing with bitwise data, sets. Sets allow the programmer to set, clear, intersect, and unite bitwise data values, rather than using direct bitwise operators. Example;

Pascal:

Status := Status + [StickyFlag];
Status := Status - [StickyFlag];
if (StickyFlag in Status) then ...

C:

Status |= StickyFlag;
Status &= ~StickyFlag;
if (Status & StickyFlag) { ...

A note on implementation

During expression evaluation, and in both languages, a boolean value may be internally stored as a single bit, a single byte, a full machine word, a position in the generated code, or as a condition code in a status register, depending on machine, compiler, and situation; these factors are usually more important than the language compiled.

Floating point types

C has a less strict model of floating point types than Pascal. In C, integers may be implicitly converted to floating point numbers, and vice versa (though possible precision loss may be flagged by warnings). In Pascal, integers may be implicitly converted to real, but conversion of real to integer (where information may be lost) must be done explicitly via the functions trunc() and round(), which truncate or round off the fraction, respectively.

Enumeration types

Both C and Pascal include enumeration types. A Pascal example:

type
  color = (red, green, blue);
var
  a: color;

A C example:

enum color {red, green, blue};
enum color a;

The behavior of the types in the two languages however is very different. In C, red becomes just a synonym for 0, green for 1, blue for 2, and nothing prevents a value outside this range to be assigned to the variable a. Furthermore, operations like a = a + 1; are strictly forbidden in Pascal instead you would use a := succ(a);. In C, enums can be freely converted to and from ints, but in Pascal, the function ord() must be used to convert from enumerated types to integers, and there is no function to convert from integer to enumerated types.

Structured types

Array types

Both C and Pascal allow arrays of other complex types, including other arrays. However, there the similarity between the languages ends. C arrays are simply defined by a base type and the number of elements:

int a[SIZE];

and are always indexed from 0 up to SIZE-1 (i.e. modulo SIZE).

In Pascal, the range of indices is often specified by a subrange (as introduced under simple types above). The ten elements of

var a : array[0..9] of integer;

would be indexed by 0..9 (just as in C in this case). Array indices can be any ordinal type, however, not just ranges:

type
   TColor = (red, green, blue);       (* enumeration *)
   RGB = array[TColor] of 0..255;
 
var picture : array[1..640, 1..480] of RGB
 
var palette : array[byte, 0..2] of byte

Although Pascal can represent arrays with any index range, a range of 1..N is the preferred format. For example, strings in Pascal have this index form.

Arrays and pointers

In C expressions, an identifier representing an array is treated as a constant pointer to the first element of the array, thus, given the declarations int a[10] and int *p; the assignment p = a is valid and causes p and a to point to the same array. As the identifier a represents a constant address, a = p is not valid however.

While arrays in C are fixed, pointers to them are interchangeable. This flexibility allows C to manipulate any length array using the same code. It also leaves the programmer with the responsibility not to write outside the allocated array, as no checks are built in into the language itself.

In Pascal, arrays are a distinct type from pointers. This makes bounds checking for arrays possible from a compiler perspective. Practically all Pascal compilers support range checking as a compile option. The ability to both have arrays that change length at runtime, and be able to check them under language control, is often termed "dynamic arrays". In Pascal the number of elements in each array type is determined at compile-time and cannot be changed during the execution of the program. Hence, it is not possible to define an array whose length depends in any way on program data.

C has the ability to initialize arrays of arbitrary length. The sizeof operator can be used to obtain the size of a statically initialized array in C code. For instance in the following code, the terminating index for the loop automatically adjusts should the list of strings be changed.

static char *wordlist[] = {
  "print",   "out",   "the",  "text",   "message" };
static int listSize = (sizeof(wordlist)/sizeof(wordlist[0]));
int i;
 
for (i=0; i<listSize; i++)
  puts(wordlist[i]);
for (i=listSize-1; i>=0; i--)
  puts(wordlist[i]);

Pascal has neither array initialization nor a means of determining arbitrary array sizes at compile time.

One way of implementing the above example in Pascal, but without the automatic size adjustment, is:

const
  minlist = 1;
  maxlist = 5;
  maxword = 8;
 
type
  listrange = minlist .. maxlist;
  wordrange = 0..maxword;
  word = record
    contents: packed array [wordrange] of char;
    length: wordrange
  end;
  wordlist = array[listrange] of word;
var
  i: integer;
  words: wordlist;
 
procedure CreateList(var w: wordlist);
begin
  w[1].contents := 'print  ';
  w[1].length := 5;
  w[2].contents := 'out    ';
  w[2].length := 3;
  w[3].contents := 'the    ';
  w[3].length := 3;
  w[4].contents := 'text   ';
  w[4].length := 4;
  w[5].contents := 'message';
  w[5].length := 7;
end;
 
begin
  CreateList(words);
  for i := minlist to maxlist do
    with words[i] do
      WriteLn(contents: length);
  for i := maxlist downto minlist do
    with words[i] do
      WriteLn(contents: length)
end.

Strings[dubious ]

Neither C nor Pascal have native support for strings. On both, it is common to use arrays of characters to achieve this functionality. Pascal suffers from a problem in that it has no support for variable length arrays, and so any set of routines to perform string operations is dependent on a particular string size. However, the standard "conformant parameter" extension solves this to a great extent, and many or even most implementations of Pascal have support for strings native to the language.

C automatically terminates string literals with a trailing null-character as an end-of-string "sentinel":

const char *p;
p = "the rain in Spain";     /* null-terminated */

Null-termination must be manually maintained for string variables stored in arrays (this is often partly handled by library routines). C does not have built in string or array assignment, so the string is not actually being transferred to p, but rather p is being made to point to the constant string in memory.

Record types

Both C and Pascal can declare "record" types. In C, they are termed "structures".

struct a {
   int b;
   char c;
};
type a = record 
   b: integer;
   c: char;
end;

In C, the exact bit length of a field can be specified:

struct a {
   unsigned int b:3;
   unsigned int c:1;
};

This feature may be available in Pascal by using the subrange construct (3 bits gives a range from 0 to 7). It depends on whether the compiler used packs subranges into the smallest available storage:

type a = record
   b: 0..7;
   c: 0..1;
end;

Both C and Pascal support records which can include different fields overlapping each other:

union a {
   int a;
   float b;
};
type a = record
   case boolean of
      false: (a: integer);
      true:  (b: real)
end;

Both language processors are free to allocate only as much space for these records as needed to contain the largest type in the union/record.

The biggest difference between C and Pascal is that Pascal supports the explicit use of a "tagfield" for the language processor to determine if the valid component of the variant record is being accessed:

type a = record
   case q: boolean of
      false: (a: integer);
      true:  (b: real)
end;

In this case, the tagfield q must be set to the right state to access the proper parts of the record.

Pointers

In C, pointers can be made to point at most program entities, including objects or functions:

int a;
int *b;
int (*compare)(int c, int d);
int  MyCompare(int c, int d);
 
b = &a;
compare = &MyCompare;

In C, since arrays and pointers have a close equivalence, the following are the same:

a = b[5];
a = *(b+5);
a = *(5+b);
a = 5[b];

Thus, pointers are often used in C as just another method to access arrays.

To create dynamic data, the library functions malloc() and free() are used to obtain and release dynamic blocks of data. Thus, dynamic memory allocation is not built into the language processor. This is especially valuable when C is being used in operating system kernels or embedded targets as these things are very platform (not just architecture) specific and would require changing the C compiler for each platform (or operating system) that it would be used on.

Pascal doesn't have the same kind of pointers as C, but it does have an indirection operator that covers the most common use of C pointers. Each pointer is bound to a single dynamic data item, and can only be moved by assignment:

type a = ^integer;
 
var b, c: a;
 
new(b);
c := b;

In Pascal, pointers can never point to program objects such as local or global variables. This appears to make Pascal more type safe than C, but Pascal can still have invalid pointer references in several ways. For example, a pointer can be referenced when uninitialized, or it would be referenced after it is disposed, etc. Pointer arithmetic (a common source of programming errors in C, especially when combined with endianness issues and platform-independent type sizes) is not permitted in Pascal.

Expressions

Precedence levels

The languages differ significantly when it comes to expression evaluation, C (although not fully comparable) has almost four times as many precedence levels as Pascal.

Pascal has four levels:

  1. Logical negation: not
  2. Multiplicative: * / div mod and
  3. Additive: + - or
  4. Relational: = <> < > <= >= in

while C has 15 levels:

  1. Unary postfix: [] () . -> ++ --
  2. Unary prefix: & * + - ! ~ ++ -- (type) sizeof
  3. Multiplicative: * / %
  4. Additive: + -
  5. Shift: << >>
  6. Relational: < > <= >=
  7. Equality: == !=
  8. Bitwise and: &
  9. Bitwise xor: ^
  10. Bitwise or: |
  11. Logical and: &&
  12. Logical or: ||
  13. Conditional: ? :
  14. Assignment: = += -= *= /= %= <<= >>= &= ^= |=
  15. Comma operator: ,

Typing

Most operators serve several purposes in Pascal, for instance, the minus sign may be used for negation, subtraction, or set difference (depending on both type and syntactical context), the >= operator may be used to compare numbers, strings, or sets, and so on. C uses dedicated operator symbols to a greater extent.

Assignment and equality tests

The two languages use different operators for assignment. Pascal, like ALGOL, uses the mathematical equality operator for equality test and the symbol := for assignment, whereas C, like B uses the mathematical equality operator for assignment. In C (and B) the new == symbol was therefore introduced for equality test.

It is common for experienced programmers and those new to C to accidentally put assignment expressions in conditional statements such as if (a = 10) { ... }, which will always execute because the assignment expression a = 10 has the value 10 ≠ 0 and is therefore considered "true" in C. Also note that a has now got the value 10, which may affect the following code. Recent C compilers try to detect these cases and warn the user, asking for a less ambiguous syntax like if ((a=10) != 0 ) { ... }. This kind of mistake cannot happen in Pascal, as assignments are not expressions; using the wrong operator will cause a compile error (and it's not likely that anyone would mistake the := symbol for equality test).

Implementation issues

When Niklaus Wirth designed Pascal, the desire was to limit the number of levels of precedence (fewer parse routines, after all). So the OR and exclusive OR operators are treated just like an Addop and processed at the level of a math expression. Similarly, the AND is treated like a Mulop and processed with Term. The precedence levels are

          Level   Syntax Element     Operator

          0       factor             literal, variable
          1       signed factor      unary minus, NOT
          2       term               *, /, AND
          3       expression         +, -, OR

Notice that there is only ONE set of syntax rules, applying to both kinds of operators. According to this grammar, then, expressions like

     x + (y AND NOT z) DIV 3

are perfectly legal. And, in fact, they ARE...as far as the parser is concerned. Pascal doesn't allow the mixing of arithmetic and Boolean variables, and things like this are caught at the SEMANTIC level, when it comes time to generate code for them, rather than at the syntax level.

The authors of C took a diametrically opposite approach: they treat the operators as different, and have something much more akin to our seven levels of precedence. In fact, in C there are no fewer than 17 levels! That's because C also has the operators '=', '+=' and its kin, '<<', '>>', '++', '--', etc. Ironically, although in C the arithmetic and Boolean operators are treated separately, the variables are NOT...there are no Boolean or logical variables in C, so a Boolean test can be made on any integer value.
...
A strange consequence of this grammar [...] is that EVERY expression is potentially a Boolean expression. The parser will always be looking for a Boolean expression, but will "settle" for an arithmetic one.

Logical connectives

In Pascal a boolean expression that relies on a particular evaluation ordering (possibly via side-effects in function calls) is, more or less, regarded as an error. The Pascal compiler has the freedom to use whatever ordering it may prefer and must always evaluate the whole expression even if the result can be determined by partial evaluation.

A C compiler may have similar freedom in cases where & (bitwise and) and | (bitwise or) are used in place of && (logical and) and || (logical or), but it must then also be able to deduce that the boolean result is the sole purpose of the expression and therefore that the bitwise value is of no importance, and so can be discarded.

In C, dependence on boolean evaluation order is perfectly legal, and often systematically employed using the && and || operators together with operators such as ++, +=<code>, the comma operator, etc. The <code>&& and || operators thereby function as combinations of logical operators and conditional statements.

Short circuit expression evaluation has been commonly considered an advantage for C because of the "evaluation problem":

var i: integer;
    a: packed array [1..10] of char;
 
  ...
  i := 1;
  while (i <= 10) and (a[i] <> 'x') do i := i+1;
  ...

This seemingly straightforward search is problematic in Pascal because the array access a[i] would be invalid for i equal to 11.

However, in superscalar processors there is a penalty for all jumps because they cause pipeline stalls, and programs created for them are more efficient if jumps are removed where possible. Pascal's ability to evaluate using a fixed formula without jumps can be an advantage with highly optimizing compilers, whereas C has effectively prevented this by requiring short circuit optimization.

Control structures

Statements for building control structures are roughly analogous and relatively similar (at least the first three).

Pascal has:

  • if cond then stmt else stmt
  • while cond do stmt
  • repeat stmt until cond
  • for id := expr to expr do stmt and for id := expr downto expr do stmt
  • case expr of expr : stmt; ... expr : stmt; else: stmt; end

C has:

  • if (cond) stmt else stmt
  • while (cond) stmt
  • do stmt while (cond)
  • for (expr; cond; expr) stmt
  • switch (expr) { case expr : stmt; ... case expr : stmt; default: stmt }

Pascal, in its original form, did not have an equivalent to default, but an equivalent else clause is a common extension. Pascal programmers otherwise had to guard case-statements with an expression such as: if expr not in [A..B] then default-case.

C has the so called early-out statements break and continue, and some Pascals have them as well. There is controversy about whether the inclusion of these statements is in keeping with structured programming methodology. The best that can be said about this is that the use of break and continue may make programming easier, but there is no case where they cannot be replaced by "orthodox" structured programming constructs.

Both C and Pascal have a goto statement. However, since Pascal has nested procedures/functions, jumps can be done from an inner procedure or function to the containing one; this was commonly used to implement error recovery. C has this capability via the ANSI C setjmp and longjmp. This is equivalent, but arguably less safe, since it stores program specific information like jump addresses and stack frames in a programmer accessible structure.

Functions/procedures

In Pascal, routines that return a value are called functions, and routines that don't return a value are called procedures. In C all routines are called functions, however routines that do not return a value are declared to return "void".

In practice Pascal procedures are equivalent to C functions that return "void", and Pascal functions are equivalent to C functions that return a non-void type.

The following two declarations in C:

int f(int x, int y);
void k(int q);

are equivalent to the following declarations in Pascal:

function f(x, y: integer): integer;
procedure k(q: integer);

Actually "void" is not really a valid variable type, it can only be used when declaring the return type of a function or when declaring pointers which can point to a variable of any type. It is not possible to declare normal variables of this type.

In Pascal, there are two different types of parameters, value and pass by reference or VAR parameters.

function f(var k: integer): integer;
x := f(t);

In C, there are only value parameters, but the C ability to point to any variable allows the programmer to construct their own pass by reference scheme. The following segment is similar to the Pascal segment above:

int f(int *k); //function accepts a pointer as parameter
x = f(&t);

In C, it is also possible to create a function with any number of parameters:

int f(int a, ...);
f(1, 2, 3, 4, 5);

The function f() uses a special set of functions that allow it to access each of the parameters in turn. This set of functions was undefined in original C, but was defined in ANSI C. In practice, this feature is easy to call to, but fairly complex and machine-dependent, to create functions that are called using it. Perhaps because of this, it is mainly used to form the language support library for C, specifically I/O.

Additionally Pascal has I/O statements built into the language to handle variable amount of parameters, like for Writeln. Pascal allows procedures and functions to be nested. This is convenient to allow variables that are local to a group of procedures, but not global. C does not have this feature and the localization of variables or functions could only be done for a compilation module wherein the variables or functions would have been declared static.

C allows functions to be indirectly invoked through a function pointer. In the following example, the statement (*cmpar)(s1, s2) is equivalent to strcmp(s1, s2):

#include <string.h>
 
int (*cmpar)(const char *a, const char *b);
const char *s1 = "hello";
const char *s2 = "world";
 
cmpar = &strcmp;
b = (*cmpar)(s1, s2);

Though, in this example, it would be wiser to use preprocessor statements to accomplish similar ends (unless cmpar was set based upon run-time conditions, which cannot be handled by the preprocessor).

Pascal also allows functions and procedures to be passed as parameters to functions or procedures:

procedure ShowHex(i: integer);
...
end;
 
procedure ShowInt(i: integer);
...
end;
 
procedure Demo(procedure Show(i: integer));
var j: integer;
begin
  Show(j)
end;
 
...
  Demo(ShowHex);
  Demo(ShowInt);
...

Preprocessor

Early C had neither constant declarations nor type declarations, and the C language was originally defined as needing a "preprocessor"; a separate program, and pass, that handled constant, include and macro definitions, in order to keep memory usage down. Later, with ANSI C, it obtained constant and type definitions features and the preprocessor also became part of the language itself, leading to the syntax we see today.

Pascal constant and type defines are built in, but there were programmers using a preprocessor also with Pascal (sometimes the same one used with C), certainly not as common as with C. Although often pointed out as a "lack" in Pascal, technically C doesn't have program modularity nor macros built in either. It has a simple low level separate compilation facility, however (traditionally using the same generic linker used for assembly language), Pascal does not.

Type escapes

C supports the ability to "cast" any type to another type. While some imply a conversion of some sort (truncation, promotion etc.), pointer casts allow the user to simply assume the desired type is pointed:

int *a;
float b;
 
a = (int*) &b;

The meaning of such casts is entirely machine dependent. This feature often helps with low level conversion of data. For example, a floating point value can be output to a file as a series of bytes.

It may be possible to do this in Pascal using an undiscriminated variant record:

var a: integer;
    b: real;
    a2c: record
           case boolean of
             false: (a: integer);
             true:  (b: real);
           end;
         end;
begin
  a2c.b := b;
  a := a2c.a;
end;

Although casting is possible on the most of Pascal compilers and interpreters, even in the code above a2c.a and a2c.b aren't required by any Pascal standardizations to share the same address space. Niklaus Wirth, the designer of Pascal, has written about the problematic nature of attempting type escapes using this approach:

"Most implementors of Pascal decided that this checking would be too expensive, enlarging code and deteriorating program efficiency. As a consequence, the variant record became a favourite feature to breach the type system by all programmers in love with tricks, which usually turn into pitfalls and calamities".

Several languages now specifically exclude such type escapes, for example Java, C# and Wirth's own Oberon.

Files

In C files do not exist as a built-in type (they are defined in a system header) and all I/O takes place via library calls. Pascal has file handling built into the language.

The typical statements used to perform I/O in each language are:

printf("The sum is: %d\n", x);
writeln('the sum is: ', x);

The main difference is that C uses a "format string" that is interpreted to find the arguments to the printf function and convert them, whereas Pascal performs that under the control of the language processor. The Pascal method is arguably faster, because no interpretation takes place, but the C method is highly extensible.

Later Pascal implementations and extensions

Some popular Pascal implementations have incorporated virtually all C constructs into Pascal. Examples include type casts, being able to obtain the address of any variable, local or global, and different types of integers with special promotion properties.

However, the incorporation of C's lenient attitude towards types and type conversions can result in a Pascal that loses some of its type security. For example, Java and C# were created in part to address some of the perceived type security issues of C, and have "managed" pointers that cannot be used to create invalid references. In its original form (as described by Niklaus Wirth), Pascal qualifies as a managed pointer language, some 30 years before either Java or C#. However, a Pascal amalgamated with C would lose that protection by definition. In general, the lower dependence on pointers for basic tasks makes it safer than C in practice.

The Extended Pascal standard extends Pascal to support many things C supports, which the original standard Pascal did not, in a type safer manner. For example, schema types support (besides other uses) variable-length arrays while keeping the type-safety of mandatory carrying the array dimension with the array, allowing automatic run-time checks for out-of-range indices also for dynamically sized arrays.

Epilogue

It is difficult to produce a truly impartial comparison of C and Pascal, and even more difficult to avoid offending one or another language aficionado.

However, C and Pascal are similar languages, if you look at the basic program structures, data types and aims of the two languages. Each time a proponent of C claims that program X cannot be done in Pascal, someone else shows that it can be done and vice versa. A major difference between the languages is the handling of type security. Pascal has a better ability to detect type-related errors at compile time, but C allows more flexible handling of mixed data-types when that is what is required.

One of the limitations of original Pascal is the inability to specify dynamic arrays as procedure parameters, which even the creator of Pascal later agreed was not a good idea. Many later Pascal compilers added an extension for that feature, and the ISO 7185 standard addressed it as well.

Although C was originally described as a "systems" or "low level" language, it is used for all types of applications including high level ones. Pascal, a language with an academic and educational background, did not gain long lasting support in industry in its original standardized form. However, the extended Pascal derivatives (e.g. Delphi) are in active use for all types of applications.

See also

Notes

  1. It would be less interesting to compare, say, C with Lisp, or Pascal versus Perl, as these languages are so clearly different and have very different aims.

Further reading

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