How to fix infinite printing of random number?

Question

I wrote a program that scans an unknown amount of integers into an array but when I run it, it print the last value it has gotten an infinite amount of times.

For example for the input: 1 2 3 4 5

The output would be 55555555555555555555555...

Why does this happen and how can I fix that?

My goal here is to create a array, for an instance {1, 2, 3, 4, 5} and then print what it scanned into the array, ONLY ONCE...

int *pSet = (int*) malloc(sizeof(int)); int i; int c;
printf("Please enter a stream of numbers to make a set out of them: ");
printf("\n");

scanf("%d", &c);
pSet[0] = c;
printf("%d ", c);
for(i = 1; c != EOF; i++) {
    pSet = (int*) realloc(pSet, sizeof(int)*(i+1));
    if(pSet == NULL) {
        return FAIL;        
    }
    scanf("%d", &c);
    pSet[i] = c;
    printf("%d ", c);
}

free(pSet);

Answer 1

Why does this happen (?) (print ... an infinite amount of times.)

Look at the loop terminating conditions c != EOF.

int c;
scanf("%d", &c);
for(i = 1; c != EOF; i++) {  // Not good code
  scanf("%d", &c);
}

EOF is some negative value, often -1. scanf("%d", &c) attempts to read user input and convert to an int. scanf() returns a 1,0,EOF depending on if it 1) succeeded, 2) failed to find numeric text or 3) end-of-file or input error occurred. Unfortunately code does not use that return value. Instead code used the number read, c and checked if that number read was the same as EOF.

---

how can I fix that?

Only loop when the return value of scanf() is as expected (1).

for(i = 1; scanf("%d", &c) == 1; i++) {
  ...
}

---

Putting this together with some other ideas

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

int main(void) {
  printf("Please enter a stream of numbers to make a set out of them:\n");
  int *pSet = NULL;  // Start with no allocation
  size_t i = 0;

  int c;
  for (i = 0; scanf("%d", &c) == 1; i++) {
    //        +---------------------------  No cast needed.
    //        v               v----------v  Use sizeof de-referenced pointer
    void *p =   realloc(pSet, sizeof *pSet * (i + 1));
    if (p == NULL) {
      free(pSet);
      return EXIT_FAILURE;
    }
    pSet = p;
    pSet[i] = c;
  }

  for (size_t j = 0; j < i; j++) {
    printf("%d ", pSet[j]);
  }

  free(pSet);
  return 0;
}

Answer 2

You should stop your loop when scanf fails. According to the manual:

On success, [scanf] return[s] the number of input items successfully matched and assigned; this can be fewer than provided for, or even zero, in the event of an early matching failure. The value EOF is returned if the end of input is reached before either the first successful conversion or a matching failure occurs. EOF is also returned if a read error occurs. [...]

So you can turn your for loop into a while one.

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

#define FAIL 0
int main() {
  int *pSet = (int*) malloc(sizeof(int));
  int c;
  int i=0;
  printf("Please enter a stream of numbers to make a set out of them: ");
  while(scanf("%d", &c) == 1) {
    pSet[i] = c;
    pSetNew = (int*) realloc(pSet, sizeof(int)*(i+1));
    if(pSetNew == NULL) {
      free(pSet);
      return FAIL;        
    } else {
      pSet = pSetNew;
    }
    printf("%d ", c);
    i++;
  }

  free(pSet);
}

But if you want a more robust piece of code, I suggest you to retrieve the answer as a string (NULL-terminated array of char), and then parse it with dedicated functions like strtol which let you check if the whole string is a valid entry, and not only the first characters.

Note: HengLi fixed a potential memory leak in the code sample above

Answer 3

There are a number of problems.

1) Terminate the loop when scanf fails instead of using EOF. Do that by checking that the return value is 1 (i.e. the number of input items successfully matched)

2) Don't allocate memory until it's needed

3) Never do realloc directly into the target pointer - always use a temp variable.

Fixing this your code could be:

#include <stdio.h>

int main(void) {
    int *pSet = NULL;
    printf("Please enter a stream of numbers to make a set out of them: ");
    printf("\n");

    int i = 0;
    int c;
    while (1) {
         if (scanf("%d", &c) != 1)
         {
             printf("Terminating input loop\n");
             break;
         }

         int* tmp = realloc(pSet, sizeof(int)*(i+1));
         if(tmp == NULL) {
            printf("oh dear...\n");
            break;
         }
         pSet = tmp;
         pSet[i++] = c;
         printf("%d ", c);
    }

    for (int j=0; j < i; ++j) printf("%d\n", pSet[j]);
    free(pSet);
    return 0;
}

Input:

1 2 3 4 5 6 7 stop

Output:

Please enter a stream of numbers to make a set out of them: 
1 2 3 4 5 6 7 
Terminating input loop
1
2
3
4
5
6
7