Input numbers into dynamic array

Question

I desire that my programm makes the user input numbers into a dynamic array and if the user type -1, it will stop asking for more numbers. The problem here is probably the condition in my while, that's where I have my doubts.

int i=0, size=0;
float *v;
printf("Write a group of real numbers. Write -1 if you want to stop writing numbers\n");
v=(float*)malloc(size*sizeof(float));
while(v!=-1)
{

    printf("Write a number\n");
    scanf("%f", &v[i]);
    i++;
    size++;
    v=realloc(v, (size)*sizeof(float));

}

Weather Vane · Accepted Answer · 2015-12-13 18:44:57Z

size=0; starts with a 0-length array which you write into out-of-bounds with scanf("%f", &v[i]); before incrementing size. The same out-of bounds write happens on every iteration. I rewrote like this. Note there is no initial malloc because realloc will work when given a NULL pointer.

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

int main(void)
{
    int size = 0, i;
    float *v = NULL;
    float val;
    printf("Write a group of real numbers. Write -1 if you want to stop writing numbers\n");
    printf("Write a number\n");
    while(scanf("%f", &val) == 1 && val != -1)
    {
        v = realloc(v, (size+1) * sizeof(float));
        v[size++] = val;
        printf("Write a number\n");
    }

    printf("Results\n");
    for(i=0; i<size; i++)
        printf("%f\n", v[i]);
    free(v);
    return 0;
}

Program session:

Write a group of real numbers. Write -1 if you want to stop writing numbers
Write a number
1
Write a number
2
Write a number
3
Write a number
-1
Results
1.000000
2.000000
3.000000

fuz · Accepted Answer · 2015-12-16 11:44:07Z

0

Instead of comparing v with -1, you probably want to compare v[i - 1] with -1. Also, you should perform error checking for when scanf couldn't parse a number:

do {
    printf("Write a number\n");
    if (scanf(" %f", &v[i]) != 1) {
        /* error handling here */
    }

    v = realloc(v, size * sizeof *v);
    if (v == NULL) {
        /* error handling here */
    }
} while (v[i++] != -1);

edited Dec 16, 2015 at 11:44

answered Dec 13, 2015 at 18:19

fuz

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