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Comparison of Pascal and C

Comparison of two programming languages

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Through Structured programming, Syntax (programming languages) and Semicolon

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Both are descendants of the ALGOL language series. ALGOL introduced programming language support for structured programming, where programs are constructed of single entry and single exit constructs such as if, while, for and case. 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 (for example in case of struct and union) and also Pascal (for example in case of enumerations, const, typedef and Booleans). Some Pascal dialects also incorporated traits from C.

The languages documented here are the Pascal designed by Niklaus Wirth, as standardized as ISO 7185 in 1982, and the C designed by Dennis Ritchie, as standardized as C89 in 1989. The reason is that these versions both represent the mature version of the language, and also because they are comparatively close in time. ANSI C and C99 (the later C standards) features, and features of later implementations of Pascal (Turbo Pascal, Free Pascal etc.) are not included in the comparison, despite the improvements in robustness and functionality that they conferred e.g. Comparison of Pascal and Delphi

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 more ALGOL-like than Pascal regarding (simple) declarations, retaining the type-name variable-name syntax. For example, C can accept declarations at the start of any block, not just the outer block of a function.

Semicolon use

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

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

A superfluous semicolon can be put on the last line before end, thereby formally inserting an empty statement.

Comments

In the first edition of C, there are only /* block comments */. C later added // line comments.

In 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. In both languages, identifiers consist of letters and digits, with the rule that the first character may not be a digit. In C, the underscore 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.

Both C and Pascal use keywords (words reserved for use by the language). 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, subroutine definitions start with keywords procedure (no value returned) or function (a value is returned) and type definitions with type. In C, all subroutines have function definitions (procedures being void functions) and 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 subroutines, begin and end delimit a block of statements preceded by local declarations, while C functions use "{" and "}" to delimit a block of statements optionally preceded by declarations : C (before 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. By its syntax of a subroutine's body, Pascal enforces that declarations occur before statements. Pascal also 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. In contrast, the corresponding Pascal fragments var Y : ^X; and Z := X * Y; are inherently unambiguous; correct parsing does not require a symbol table.

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