Split c console input into integers

Question

I have a console-input space separated list of integers of n length(n is read in through the console) which needs to be split into the integers and placed in an array of size n. I have attempted this with both fscan and fgets, to either a segmentation fault or the console locking and not letting me enter the list. Any advice:

int main()
{
        int n;
        scanf("%d", &n);
        int buffer = n + n -1;
        char input[buffer];
        //char * input;
        //int buffer = n+n-1;
        //fgets(input, buffer, stdin);
        scanf("%*c", &input);
        printf("c is %c", input[0]);
        char * end;
        long int l;
        end = input;
        int base = 10;
        int ele = 0;
        long int vals[n];
        while(l = strtol(end, &end, base)){
                vals[ele] = l;
                ele += 1;
        }
        return 0;

try `int buffer = n + n + 1; char *input = malloc(buffer); fgets(input, buffer, stdin);` — BLUEPIXY, Sep 28 '17 at 03:09

Dúthomhas · Answer 1 · 2017-09-28T07:05:35.740

There are a number of things going on in your code. Here is a basic example of reading a variable-sized array from standard input:

#include <stdio.h>
#include <stdlib.h>

int main()
{
  int n;
  int *xs;

  /* read n */
  if (!scanf( "%d", &n ) || (n <= 0))
  {
    fprintf( stderr, "%s\n", "invalid n: n <= 0 or n is not an integer" );
    return 1;
  }

  /* allocate xs[n] */
  xs = (int*)malloc( n * sizeof(int) );
  if (!xs)
  {
    fprintf( stderr, "failure to allocate %d integers\n", n );
    return 1;
  }

  /* read xs[] */
  { 
    int i;
    for (i = 0; i < n; i++)
      if (!scanf( "%d", &(xs[i]) ))
      {
        fprintf( stderr, "%s\n", "invalid x: x is not an integer" );
        free( xs );
        return 1;
      }
  }

  /* ... */

  /* print xs[] in reverse order */
  {
    int i = n;
    while (i--)
      printf( "%d ", xs[i] );
    puts( "" );        // Here's your newline
    fflush( stdout );  // POSIX only mandates flush to terminal, and only by default
  }

  free( xs );
}

This should help you get on track.

[edit] Modified as per commentary below:

Casting to (int*) is not necessary in C, but it:
- declares intent
- can be compiled with a C++ compiler without error
Added very simple error handling
("dog" will now refuse to be accepted as input)
Added newline. Note that using fflush() is fully POSIX compliant. POSIX only mandates that terminal default to flush on newline. No newline → need to flush. Piping to file → please flush.

There is no need to cast the return of `malloc`, it is unnecessary. See: [**Do I cast the result of malloc?**](http://stackoverflow.com/q/605845/995714). What would happen if the input was `1 2 3 4 cat 5 dog 6 ... 7`? How would you know? What about checking the **return** of `scanf` and handling any *matching* or *input* failures? Removing `fflush( stdout );` and adding `putchar ('\n')` after the loop would help make your program POSIX compliant. — David C. Rankin, Sep 28 '17 at 04:59

David C. Rankin · Answer 2 · 2017-09-28T05:36:15.360

Whether your compiler supports the Variable Length Array extensions:

#include <stdio.h>

int main (void) {

    int n, tmp, ndx = 0;
    if (scanf ("%d", &n) != 1 || n < 1) {
        fprintf (stderr, "error: invalid input - n.\n");
        return 1;
    }
    int a[n];

    while (ndx < n && scanf ("%d", &tmp) == 1) {
        a[ndx] = tmp;
        printf ("a[%3d] : %d\n", ndx, a[ndx]);
        ndx++;
    }

    return 0;
}

or you choose to allocate your array dynamically:

#include <stdio.h>
#include <stdlib.h>

int main (void) {

    int n, *a, tmp, ndx = 0;
    if (scanf ("%d", &n) != 1 || n < 1) {
        fprintf (stderr, "error: invalid input - n.\n");
        return 1;
    }

    if (!(a = calloc (n, sizeof *a))) { /* allocate/validate */
        fprintf (stderr, "error: virtual memory exhausted.\n");
        return 1;
    }

    while (ndx < n && scanf ("%d", &tmp) == 1) {
        a[ndx] = tmp;
        printf ("a[%3d] : %d\n", ndx, a[ndx]);
        ndx++;
    }

    free (a);   /* don't forget to free allocated memory */

    return 0;
}

Always, always, validate all user input (before assigning the value from the conversion to your array) scanf has a return just for that purpose. You must handle matching or input failures in some fashion.

Example Input

$ cat dat/n_10_ints.txt
10
8572
-2213
6434
16330
3034
12346
4855
16985
11250

Example Use/Output

$ ./bin/readnints <dat/n_10_ints.txt
a[  0] : 8572
a[  1] : -2213
a[  2] : 6434
a[  3] : 16330
a[  4] : 3034
a[  5] : 12346
a[  6] : 4855
a[  7] : 16985
a[  8] : 11250
a[  9] : 1495

Look things over and let me know if you have further questions.

Split c console input into integers

2 Answers2