Type Checking In Pascal

Question

I'm just wondering how it's possible to do type checking in pascal? I have been searching for hours now but I haven't been able to find anything useful.

Example:

var 
number: Integer;

begin
  write('Enter a number: ');
  read(number);

  if {How am I supposed to check if 'number' is an Integer here?}
  then writeln(number)
  else writeln('Invalid input')
end.

Answer 1

You are actually hitting the I/O type checking. You can work around this by disabling it temporarily and then checking the result:

 {$I-}  //turn off IO checking temporarily
 read(i);
 {$I+}  // and back on

 if ioresult=0 then  // check the result of the last IO operation
   writeln('integer successfully read:',number)
 else
   writeln('invalid input');

Note: the typical answer is often "just read a string and do the conversion yourself", however it is difficult to do that nicely without making assumptions about the terminal type.

For clear and simple programs where you just want somewhat validated input, the above trick (and a loop around it that repeats on error) is enough.

Answer 2

Maybe Val procedure can help you. Here is one for fpc. But change your logic to read into a String and validate it using Val. You can find a sample here.

Answer 3

That 's too easy, see my code below:

program int_check;
uses crt;
var n:real;
begin 
     clrscr;
     write('Enter a number: ');readln(n);
     if n-round(n)=0 then write('Integer!') else write('Not an Integer!');
     readln;
end.

You see, no string, no IOcheck, and fits your form!

Answer 4

Since number is an Integer, the app will fail if the user types a non-numeric value. You'll never reach the if statement.

Answer 5

Use frac(n) directly

program int_check; uses crt; var n:real; begin clrscr; write('Enter a number: ');readln(n); if frac(n)=0 then write('Integer!') else write('Not an Integer!'); readln; end.