A simple program?

[David Somekh]

Hello Everybody.
I'm a beginner programmer in c.
I'm trying to write a program that will ask the user to input a number and if he does not input a number, it tells him that it's not a number.

When i first thought of the idea it looked very easy. But when i tried to do it, I just couldn't find a way.

Help anybody?

[Noir]

First read the number as a string, then check every character in the string and see if it's a digit. If you get to the end of the string, it's a number.

Spoilers follow, be warned.

#include <stdio.h>
#include <string.h>
#include <ctype.h>


int isnumber( char *number ) {
  while ( *number ) {
    if ( !isdigit( *number++ ) ) {
      return 0;
    }
  }

  return 1;
}


int main( void ) {
  char number[256];

  fgets( number, sizeof number, stdin );
  number[strlen( number ) - 1] = '\0';

  if ( isnumber( number ) ) {
    printf( "It's a number\n" );
  } else {
    printf( "Not a number\n" );
  }

  return 0;
}

[Joelito]

Is it?

[quzah]

If you weren't so lazy, you would have found the FAQ, instead of having someone do it for you. I wish we still had candy.


Quzah.

[KONI]

You can also try to read numbers with a self-written function, only using getchar() and some simple logic:

int c;
signed int number = 0;
int neg = 0;

while ((c = getchar()) != '\n' && c != ' ')
{
    if (c == '-' && number == 0)
        neg = !neg;
    if (c >= '0' && c <= '9')
        if (neg)
            number = (number * 10) - (c - '0');
        else
            number = (number * 10) + (c - '0');
}

The function isn't very robust if you enter stuff like "-356-25" but it should just illustrate how you could do it.