Dancing with Strings


Hi ! We meet again ! Now, I would like to discuss about strings in depth. How are you doing ? Well, I'm having health problem this day and I need a rest. I am still able to write this to you. Cheer ! Topics discussed this time are :

  1. String as array of char
  2. Limited length string
  3. String manipulation commands :
    Length, Copy, Pos, Val, Str, Concat, Insert, Delete, Fillchar
  4. String as a pointer of characters (PChar), introduction
Let's begin !

Actually, string is an array of characters. So, suppose s is a string. s[1] is equal to the first character of s. s[2] is the second character, and so on. Look at this :

  s : string;
  s:='Hello, dear';
  s[1]:='J';    { Replace the first character with J }
  s[5]:='y';    { Replace the fifth character with y }
  writeln(s);   { Jelly, dear }
  writeln('The length of s is ',ord(s[0]));

Zeroth character is the character form of the length of that string. So, ord(s[0]) denotes the actual length of s. You may replace it with length(s) which gives the same effect.

Normally, strings would hold 80 characters in maximum. Suppose you would have a set of strings that is about 10 characters long. Declaring each as normal string would be a waste of space. Pascal provides facility to limit the string length. If you would like to have a string that has maximum limit of 10 characters, you would write :

  s : string[10];

Pascal also provides routines to manipulate string :

  1. Length : return string length.
    Syntax : length(s)
    Example : n:=length(s);
    Suppose s:='Who are you ?'; n would be 13.
  2. Copy : get an excerpt from a string.
    Syntax : copy(s,from,howmuch)
    Example : st:=copy(s,5,3);
    Get an excerpt of 3 characters from s, beginning from the 5th character.
    Suppose s:='Who are you ?'; st will be 'are'.
    Note that if index from is greater than the length of s, st would be empty, example :
    st:=copy(s,15,4); { empty string }
    If howmuch exceed the end of the string s, it returns the remainder of the string, example :
    st:=copy(s,9,10); st would be equal to 'you ?'
  3. Pos : get the position of a substring from a string.
    Syntax : Pos(substr,s)
    Example : n:=pos('are','Who are you ?'); { n:=5; }
    If the substring is not found, it returns 0.
  4. Val : converts strings to numeric.
    Syntax : val(strvar,numvar,errorcode)
    strvar is a string variable to be converted, numvar is any numeric variable either integer or real, and errorcode is an integer variable that holds the error code. If errorcode is 0, conversion success. Otherwise, it points at the position of strvar that cause failure. Example :
      s : string;
      e : integer;
      r : real;
      write('Enter a number : '); readln(s);
      if e<>0 then
         writeln('Error at position : ',e);
         writeln('That was : ',r:4:3);

  5. Str : converts numeric into strings.
    Syntax : str(numvar,strvar)
    Example :
      s : string;
      i : integer;
      write('Input an integer : '); readln(i);
      writeln('That was : ',s);

    Note that if you deal with real, you may format it before you convert it into strings. Suppose r is a real variable, s can be like this :
    s consists of 4 digits before the decimal point of r, and 3 digits after the decimal point. Example :
      s : string;
      r : real;
      write('Input a real : '); readln(r);
      writeln('That was : ',s);

  6. Concat : Concatenates two or more strings
    Syntax : concat(s1,s2,...,sn)
    Example : st:=concat(s1,s2);
    If s1='ABC' and s2='DEF', st would be 'ABCDEF'
    st:=concat('Borland ','Pascal ','ver. ','7.0'); Would be 'Borland Pascal ver. 7.0'
    You may put as many parameters to concat as possible. If the resulting string length is more than 255, it will be truncated to 255.
    Concat is the same if we use plus sign (+). For example :
    st:=concat('ABC','DEF'); is the same as st:='ABC'+'DEF';
  7. Insert : Insert a string inside another string from indexth character
    Syntax : insert(source,target,index)
    Example :
      s1, s2 : string;
      s1:='not ';
      s2:='I do love you';
      writeln(s2);      { I do not love you }

    If the result string length is more than 255, it will be truncated into 255 as well.
  8. Delete : Deletes n characters from string s starting from index i.
    Syntax : delete(s,i,n);
    If index i is greater than length of s, s is unchanged. If n specifies more characters than remain (starting from i), the remainder of the string is deleted. Example :
      s : string;
      s:='I am not responsible for that !';
      writeln(s);   { I am responsible for that }

  9. Fillchar : fill string s with character c until s is n-1 char long.
    Syntax : fillchar(s,n,c);
    Beware : s[0] is overrided, so don't forget to add s[0]:=chr(n-1); to normalize it.
      s : string;

Actually, those procedures or functions can be read from Pascal's help. So, refer to Borland Pascal help if you want working examples. You can even make your own string functions. If you don't understand, e-mail me.

As Borland Pascal 7.0 arrives, we know that C style string is adopted in. In C, we view strings either as an array of characters or a pointer of characters. Pointer will be discussed in second lesson. The main reason is that Pascal can not maintain strings larger than 255. Then a new type of string is employed : PChar, a pointer of character.

Pascal string consists of : one byte for its length then the contents. In PChar, just like C, we don't recognize the length. All we know is when the string stops -- that is when we encounter ASCII 0 (NULL). That null stop style makes the C style of string called NULL TERMINATED STRING.

All string manipulation routines for PChar are adopted from C. If you would like to know each one of it, you should learn C, where it came from. Nevertheless, you could even learn it in Borland Pascal help too.

The details of pointers and this type of string could be found in my second lesson of Pascal.

You could compare two strings just like numbers, too ! Suppose s1 and s2 are strings, you could do like this :

    if s1 < s2 then .... (and so on ...)

That's all for this time.

Where to go ?

Back to main page
Back to Pascal Tutorial Lesson 1 contents
To the quiz
Back to Chapter 7 about arrays
To Chapter 9 about records
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By : Roby Joehanes, © 1996, 2000