Module | TatungBytes

Introduction

Numbers are half of what programs handle. The other half is text: names, messages, the name of a level in a game. Pascal keeps text in strings, and Turbo gives you a handful of tools for measuring them, cutting them up and putting them together.

Strings

A string variable is declared with the word string and, in square brackets, the most characters it may ever hold:

var
  First, Last, Full: string[30];

Remember that the Einstein draws [ and ] as arrows (S3). They are typed with the LEFT and RIGHT arrow keys.

The number is required - Turbo has no string without a length, and S: string; stops with Error 8: '[' expected - and it may be anything from 1 to 255. Choose it to suit the longest text the variable will ever hold.

You give a string a value with :=, as with a number, and the text goes in single quotes:

First := 'Albert';

If the text is longer than the string can hold, the end is cut off, with no warning. Put 'Tatung Einstein' into a string[5] and you have 'Tatun'.

And '' - two quotes with nothing between - is the empty string, with no characters at all.

To put a quote mark inside a string, write it twice: 'Don''t' prints Don't.

Joining And Measuring

+ joins strings end to end:

Full := First + ' ' + Last;

Length(S) is how many characters S holds now - not how many it could hold. With Full holding Albert Einstein, Length(Full) is 15.

Taking Strings Apart

Tool	Does	With `T` = `'Tatung Einstein'`
`Copy(T, 8, 4)`	4 characters starting at the 8th	`'Eins'`
`Copy(T, 8, 50)`	asking for more than there is just stops at the end	`'Einstein'`
`Pos('stein', T)`	where `'stein'` starts in `T`	`11`
`Pos('x', T)`	0 if it is not there	`0`

And two that change the string in the variable itself:

Tool	Does	Result
`Delete(T, 1, 7)`	removes 7 characters starting at the 1st	`T` is `'Einstein'`
`Insert('Mr ', T, 1)`	puts `'Mr '` in at position 1	`T` is `'Mr Einstein'`

Copy and Pos are functions: they give back an answer, and you use it in a sum, a Writeln or on the right of :=. Delete and Insert are procedures: they are statements on their own, and they change the variable you give them. Pascal is strict about the difference, and the difference is S15's subject.

One Character At A Time

S[i] is the i-th character of S, counting from 1. With S holding 'Einstein', S[1] is E. You can change one character the same way:

S := 'Hello';
S[1] := 'J';

and S is 'Jello'. The last character of any string is S[Length(S)].

A single character has a type of its own, Char:

var
  C: Char;
...
C := S[1];

Every character has a number, its code. Ord(C) gives it: Ord('E') is 69. Chr(n) goes the other way: Chr(66) is B. And UpCase(C) gives the capital of a letter: UpCase('q') is Q.

Numbers And Strings

A string that looks like a number is still a string; Turbo will not put one into an integer (Error 44: Type mismatch). Two procedures convert:

Str(N:w, S) puts the number N into the string S as text, laid out just as Writeln would lay it out with :w. Str(42:5, S) makes S hold ' 42'.
Val(S, N, Code) reads the number in S into N. Code says whether it worked: 0 if it did, otherwise the position of the first character that is not part of a number. Val('123', N, Code) gives 123 and 0; Val('12x', N, Code) gives Code 3, pointing at the x.

The Code

program Names;
var
  First, Last, Full: string[30];
begin
  First := 'Albert';
  Last := 'Einstein';
  Full := First + ' ' + Last;
  Writeln(Full);
  Writeln('Letters: ', Length(Full));
  Writeln('Initials: ',
          First[1], Last[1]);
  Writeln('Surname at: ',
          Pos(Last, Full));
end.

Starting from

Turbo freshly started, Y, a new work file called NAMES.

What you should see

Albert Einstein
Letters: 15
Initials: AE
Surname at: 8

Change One Thing

Declare Full on a line of its own as string[10]. What is printed now, and why does Pos give a different answer?
Change First[1], Last[1] to First[2], Last[2]. What are the "initials" now?
Change Writeln(Full); to Writeln(Copy(Full, 1, 3));. What is printed?

Exercises

10.1 Join 'Turbo' and 'Pascal' with a space between, and print the result and how many characters it has.

10.2 Put a word in a string variable, and print its first letter and its last letter.

10.3 A string holds 'Albert Einstein'. Using Pos to find the space, and Copy, print it the other way round: Einstein, Albert.

Worked solutions are in Appendix II.

When It Goes Wrong

Symptom	Cause
`Error 8: '[' expected`	`string` without its length in square brackets.
`Error 49: Invalid String length`	A length over 255.
The end of your text has gone	The string variable is too short for it. Nothing warns you.
`Error 44: Type mismatch` putting a string into a number	Use `Val`.
`Error 55: String constant exceeds line`	A closing quote is missing - or you meant a quote mark inside the text and wrote only one. Write it twice.
`Pos` gives 0	The text you searched for is not there - check its capitals, and that the string was not cut short.

Summary

string[n] holds up to n characters, cut off silently if you give it more. + joins strings; Length measures; Copy and Pos pick out and find; Delete and Insert change a string in place. S[i] is one character, a Char; Ord and Chr go between characters and codes, and UpCase makes capitals. Str and Val go between numbers and strings.

S11, Asking A Question - getting numbers and words from the person at the keyboard while the program runs.

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