FUNCTIONS AND PROCEDURES IN PASCALby Ed Nelson IIIThe main difference between a function and a procedure is that a function mustreturn a value, whereas a procedure does not. Procedures can, however, returna value. This article discusses the use of functions and procedures in Pascalprograms.A function will return the value in its name. For example, if a function isdeclared as "Function Paint: char;", then the word Paint will contain thecharacter that has been returned by the function. This is limited in itsusefulness; to retain the value returned by a function you must assign itsvalue to a variable. However, functions are ideal for testing loopingconditions since they provide the necessary information with a minimum ofPascal commands.There are two types of procedures -- call by value and call by reference. Thecall by value procedure is declared like Demo3 in the demonstration programlisted below. Although these procedures do not return values, they can affectglobal variables. Call by value procedures can be used to do anything thatdoes not require the return of information. For example, a call by valueprocedure can display information on the screen.The other kind of procedure is the call by reference. The call by referenceprocedure is declared like Demo4 in the demonstration program. This kind ofprocedure returns its values in variables declared as VAR in the procedureheading. Once the procedure has completed execution, the variables containtheir new values, where the information is retained until it is changed. Note: ISO Pascal does not allow certain data structures to be returned by aFunction (this is the 'Error 47' described elsewhere in this newsletter). Away around this is to use a procedure with variables passed by reference.Functions and procedures have similar abilities. As a general rule of thumb,anything a function can do, a procedure can do, and vice versa. The largestdifference between functions and procedures does not lie in their respectiveabilities and limitations, but in how they are used.Functions are generally used to find and return a value. Procedures are usedto do other things, such as displaying data, or opening and closing files. Themain difference is stylistic, not something enforced by the language.In the demonstration program below, functions and procedures are used fordifferent aspects of pattern matching in strings. S2 will be the pattern thatis looked for, and S1 will be the string that is searched. S3 is not useduntil it is passed into Demo4. Note: There was a misprint in some early editions of the manual regarding theIndex function. The correct description of this function is: the first stringpassed (S1) is the string to be searched. The second string passed (S2) is thepattern looked for in the first string.The two functions, Demo1 and Demo2, are used to find a value and return it.Demo1 returns a TRUE if S2 is in S1 and returns a FALSE if S2 is not in S1. Itis worth noting that a boolean function can be used wherever a booleanexpression is used, such as in IF-THEN, UNTIL, and WHILE statements. For anexample, look at the IF-THEN statement in the main program. The secondfunction, Demo2, counts the number of occurrences of S2 in S1. The mainprogram prints out the information received.Procedure Demo3 is a call by value procedure. It does not pass any informationback to the main program. Instead, it displays its information on the screen.Procedure Demo4 is a call by reference procedure. This procedure breaks up S1and return parts of it in S1, S2, and S3. After this procedure is called, S1contains the part of the string before the pattern, S2 contains the pattern,and S3 contains everything after the pattern. If you are going to pass large data types in a program, you may want to use acall by reference procedure rather than a function. The reason is that in acall by reference procedure, a point (that points to the data structure) ispassed rather than the whole data structure (which is what happens with afunction). Thus this technique saves time and stack space.Following is the Demonstration program:PROGRAM Demo (input, output); Const maxstring = 20; (* maximum string length is 20 *) empty = ' '; (* 20 spaces *) Type string = array [1..maxstring] of char; Var S1, S2, S3 : string; i : integer;{The following function is an include file on the Kyan Pascal disk. String A1is searched to see if it contains string A2. If it is, the location of thefirst character the string A2 is returned. If it isn't, a zero is returned.For example, if A1 is 'that theory is not viable at this time' and A2 is'viable', the function INDEX (A1, A2) will return the value 20 which is theposition of the first letter of A2 in A1.} FUNCTION Index (VAR A1, A2: string): integer; VAR I, J, K, L: integer; BEGIN I := Maxstring; WHILE ((A2[I] = ' ') AND (I <> 1)) DO I := I - 1; K := 0; REPEAT J := 1; L := 1; WHILE (J <= I) DO BEGIN IF (A1[J+K] <> A2[J]) THEN L := 0; J := J+1; END; K := K+1; UNTIL ((L = 1) OR ((I+K) > Maxstring)); IF (L = 1) THEN Index := K ELSE Index := 0; END;{The next function is an include file on the Kyan Pascal disk. It returns thelength of the string passed to it.} FUNCTION Length (VAR A1: String): Integer; VAR I: Integer; BEGIN I := Maxstring; WHILE ((A1[I] = ' ') and (I <> 1)) DO I := I-1; LENGTH := I; END;{The next procedure is also an include file on the Kyan Pascal disk. It copiespart of A1 into A2, as specified by I (the position in A1 of the firstcharacter copied) and J (the number of characters copies to A2).} PROCEDURE Substring (VAR A1, A2: String; I,J: Integer); VAR K : Integer; BEGIN FOR K := 1 TO Maxstring DO A2[K] := ' '; I := I - 1; FOR K := 1 TO J DO A2[K] := A1[I+K] END;{This next function return a value TRUE if S2 is in S1, otherwise it returns aFALSE.} FUNCTION Demo1 (S1, S2: String) : Boolean; VAR Result : integer; BEGIN Result := Index (S1, S2); IF Result = 0 THEN {If result = 0, then S2 is not in S1} Demo1 := FALSE ELSE Demo1 := TRUE {If result <> 0, then S2 is in S1} END;{The next function returns an integer equal to the number of times S2 appearsin S1. It will not work if the first character in S2 is a space.} FUNCTION Demo2 (S1, S2: string): Integer; VAR Result : integer; Dem : integer; BEGIN Result := Index (S1, S2); Dem := 0; WHILE Result <> 0 DO BEGIN Dem := Dem +1; S1 [Result] := ' '; Result := Index (S1, S2); END; Demo2 := Dem; END;{This procedure prints out the position and length of S2 in S1} PROCEDURE Demo3 (S1, S2: String); VAR Result : integer; BEGIN Result := Index (S1, S2); Writeln (S1: length (S1)); Writeln ('position: ', result : 2); Writeln ('length: ', length (S2) : 2); END;{If S2 is in S1, this procedure will return everything before the firstoccurrence of S2 in S1 (in S1), everything after the first occurrence of S2 inS1 (in S3), and S2 itself (in S2). If S2 is not in S1, then S1, S2, and S3 areunchanged. For example, if S1 is 'Gumby and Pokey live' and S2 is 'and' (plus17 spaces), then Procedure Demo4 will change S1, S2, and S3 into the following:S1 will be 'Gumby', S2 will be 'and', and S3 will be 'Pokey live'. All threewill be padded with spaces after the last letter.} PROCEDURE Demo4 (Var S1, S2, S3: string); VAR Position, len, p, l : integer; temp : string; BEGIN Position := Index (S1, S2); IF Position <> 0 THEN BEGIN len := length (S2); p := 1; l := Position -1; Substring (S1, temp, p, l); p := position + len; l := maxstring - p +1; Substring (S1, S3, p, l); Substring (S1, S2, position, len); S1 := temp END; END;BEGIN {Main program] S1 := empty; S2 := empty; S3 := empty; S1 := 'Gumby and Pokey live'; S2 := 'and '; IF Demo1 (S1, S2) THEN (* Boolean function instead of Boolean expression*) Writeln ('Demo1: It's there!'); Writeln;
