beginner question about stopping a for-loop.

This is a discussion on beginner question about stopping a for-loop. within the C Programming forums, part of the General Programming Boards category; I am currenty writing a program that needs to read in an unknown number of integers from a file and ...

  1. #1
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    beginner question about stopping a for-loop.

    I am currenty writing a program that needs to read in an unknown number of integers from a file and sort them using selection sort (with a max 100,000 integers). Here is my code so far:

    Code:
    #include <stdio.h>
    
    void selectionsort ( int x[], int size ) ;
    
    int main ( int argc, char *argv[] ) {
    
      int x[100000], i ;
    
      FILE *fin = fopen ( argv [1], "r" ) ;
    
      for ( i = 0 ; i < 100000 ; i++ ) {
    
        fscanf ( fin, "%d", &x[i] ) ;
    
      }
    
      selectionsort ( x, i ) ;
    
      printf ( "\nThe first five sorted numbers are: %d %d %d %d %d\n", x[0], x[1], x[2], x[3], x[4] ) ;
    
      printf ( "The last five sorted numbers are: %d %d %d %d %d\n\n", x[i-5], x[i-4], x[i-3], x[i-2], x[i-1] ) ;
    
      fclose ( fin ) ;
    
      return 0 ;
    }
    
    void selectionsort ( int x[], int size ) {
    
      int i, j, min_index, temp ;
    
      for ( i = 0 ; i < size - 1 ; i++ ) {
    
        min_index = i ;
    
        for ( j = i + 1 ; j < size ; j++ ) {
    
    	if ( x[j] < x[min_index] ) min_index = j ;
    
        }
    
        temp = x[i];
    
        x[i] = x[min_index] ;
    
        x[min_index] = temp ;
    
      }
    
    }
    Now my question is, how would I go about stopping the for-loop in main when fscanf starts scanning just blanks. I tried adding " if ( x[ i ] == ' ' ) break ; " after the fscanf but it doesn't work. What would I need to add so it stops scanning the file when it runs out of numbers?

    Thank you very much

  2. #2
    and the hat of wrongness Salem's Avatar
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    Study the return result(s) of fscanf

    Then break; out of the loop.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    Never mind, I figured it out. I just initialized x[i] to be \0, then when fscanf scanned a \0 it broke out of the loop.
    Last edited by Techboy10; 12-11-2008 at 03:31 PM.

  4. #4
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    Quote Originally Posted by Techboy10 View Post
    Never mind, I figured it out. I just initialized x[i] to be \0, then when fscanf scanned a \0 it broke out of the loop.
    That means that the value 0 is not allowed in the numbers - is that how it will always be?

    Yes, the scanf() family of functions do not modify the data when an error happens while reading the data, but like Salem suggests, check if the return value from fscanf() is what you expect, as it returns the number of items successfully read, so you expect 1 in this case - if not, its a failure to read the data. That could be "rubbish" (such as "abc" for a number) or the end of the file (in which case you get EOF back - but it's better to check for "not equal to the number of items I expect")

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