Type Checking In Pascal

pascal fpc
Radix picture Radix · Apr 7, 2012 · Viewed 10.7k times · Source

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

Marco van de Voort picture Marco van de Voort · Apr 7, 2012

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.

Related questions

Reading a file in FreePascal

I have a text file with specified structure, namely (for each line): char, space, char, space, double value, endline. For instance q w 1.23 e r 4.56 t y 7.89 What is the proper way to "extract" those values in Free Pascal?

Read More
How to convert a integer to float in Delphi?

How to convert a integer to float in Delphi? E.g int_h:= int_var/1.5* int_var;

Read More
How to get a random number in pascal?

I want to get a random number in pascal from between a range. Basically something like this: r = random(100,200); The above code would then have a random number between 100 and 200. Any ideas? The built in pascal function only lets you …

Read More