Array/Pointer and scanf

Question

I have a problem with my code. I am creating a dynamic array and ask for some values. But when I print them on screen, the array seems uninitialized.

int main(void)
{
    int i, j;
    double *p;
    printf("How much numbers?\n");
    scanf("%d", &i);
    p = malloc (sizeof(double)*i);
    for(j = 0; j < i; j++)
    {
        printf("Set nr. %d :\n", j);
        scanf("%f", p+j);
    }
    for(j = 0; j < i; j++)
    {
        printf("Nr. %d = %f\n", j, *(p+j));
    }
}

I think the problem should be in this line, but i cannot figure out why?

scanf("%f", p+j);

I also tried this versions of code:

scanf("%f", (p+j));
scanf("%f", &p[j]);

Thank you!

Possible duplicate of [Problems with scanf and doubles](https://stackoverflow.com/questions/19890748/problems-with-scanf-and-doubles) — phuclv, Sep 09 '18 at 04:42

score 3 · Accepted Answer · edited May 23 '17 at 12:05

3

You have wrong format specifier for reading a double:

    scanf("%f", p+j);

Use %lf for reading a double:

    scanf("%lf", p+j);

Your possible confusion arises from the fact that printf() uses %f for printing both float and double while scanf() needs different format specifier. Read this post:

Why does scanf() need "%lf" for doubles, when printf() is okay with just "%f"?

edited May 23 '17 at 12:05

Community

1
1

answered May 27 '14 at 20:41

P.P

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David C. Rankin · Answer 2 · 2014-05-27T21:20:17.963

0

Your problem is double *p. You need an array of pointers to type double:

double **p;

You sill need to do something like this:

int main(void)
{
    int i, j;
    double **p;
    printf("How much numbers?\n");
    scanf("%d", &i);
    p = (double **)malloc (sizeof(double*)*i);
    for(j = 0; j < i; j++)
    {
        printf("Set nr. %d :\n", j);
        p[j] = (double *)malloc(sizeof(double));
        scanf("%lf", p[j] );
        printf ("p[%d]: %f\n", j, *p[j]);
    }
    for(j = 0; j < i; j++)
    {
        printf("Nr. %d = %f\n", j, *p[j]);
    }
    return 0;
}

edited May 27 '14 at 21:20

answered May 27 '14 at 20:39

David C. Rankin

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His original code allocates memory correctly, I don't see why you want to expand a 1-d array to 2-d. And you have added bogus casts to malloc. – M.M May 27 '14 at 21:45

score 0 · Answer 3 · answered May 27 '14 at 20:42

0

Use this:

scanf("%f", p+(j*sizeof(double));

The pointer p stores the address for the first element of the array. The second element will be at the address:

[First Address] PLUS one element. The elements you want are double so there you go!

answered May 27 '14 at 20:42

TheCrafter

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no... adding `1` to a pointer makes it point to the next element, regardless of element size. `p+j` (aka. `&p[j]`) is correct. – M.M May 27 '14 at 21:49

Array/Pointer and scanf

3 Answers3