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I'm writing a program which prompts the user to say how many elements an array should have, then fill that array with numbers:

    int n;
    int i;
    float numbers[100];

    printf("Enter number of elements:");
    scanf("%d", &n);

    for(i = 0; i < n - 1; i++)
        scanf("%f\n", &numbers[i]);

I'm trying to write a function which takes that array of numbers, and sorts it in ascending order. I read somewhere online that there's actually a built in function in C that does this - qsort(). Here's my code:

void sort(int n, float array[])
{
    int i;
    int cmpfunc (const void * a, const void * b)
    {
        return(*(int*)a - *(int*)b );
    }

    qsort(array, n, sizeof(float), cmpfunc);

    for(i = 1; i <= n; i++)
        printf("%.2f\n", array[i]);
}

Basically the problem is I don't think I'm implementing the function correctly. For example, I'll run the program, set the array size to 5, and input the following numbers:

1,9,2,8,5

The output I get is:

1,2,8,9,0

There is a similar pattern in all my outputs.

It sorts it somewhat decently, except it always ends with a 0, and for some unknown reason, randomly skips one number of the array and doesn't sort it (in the example above 5 is skipped). In the for loop, I had i set to 0, but then my output would also skip a number when sorting, and it would still have a 0, except this time it would be at the beginning of the output.

Finally, it should be noted that I have absolutely no idea how the qsort() function works, or why it needs that other compare function to work, I just sort of copy pasted it in there.

If anyone knows the answer to my problem please help me out I'd really appreciate it, thank you.

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8
  • 2
    Functions cannot be nested in C. And for(i = 1; i <= n; i++) looks like an off-by-one. Commented Dec 29, 2020 at 0:07
  • 1
    You are sorting an array of float, so the pointers passed to the comparison function point to float values, not int values. Commented Dec 29, 2020 at 0:13
  • 2
    Casting pointers pointing at float to int* doesn't look like a good idea. Commented Dec 29, 2020 at 0:13
  • 1
    @wildplasser Functions can be nested in GCC extension. Commented Dec 29, 2020 at 0:13
  • 1
    for(i = 0; i < n - 1; i++) should be for(i = 0; i < n ; i++), and for(i = 1; i <= n; i++) should be for(i = 0; i < n; i++). Commented Dec 29, 2020 at 0:16

2 Answers 2

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2

You are not filling the last element of your array here

for(i = 0; i < n - 1; i++)
    scanf("%f\n", &numbers[i]);

And in the sorting function you are: not printing the first element; printing the last element, which can be anything; printing an element out of the array's bounds

for(i = 1; i <= n; i++)
    printf("%.2f\n", array[i]);

Both the loops should be

for(i = 0; i < n; i++)

And it seems that using qsort with a float array isn't straightforward, you should adjust its comparison function as said here: Problem trying to use the C qsort function

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7 Comments

This answer misses the fact that float* values are cast as int* and dereferenced inside the cmpfunc, which invokes undefined behavior
If I switch the for loop used for filling the array, I end up getting prompted one additional time for an extra element. (i.e. if I specify I want 5 elements, I get prompted for 6 elements). Any idea what this is?
@Wacy if you enter 5 the first time, then you are asked to enter 5 elements
Using qsort with a float array should be OK. The comparision function is wrong. The linked question is also saying so.
Yes I updated the answer to reflect the content of the link
|
2

OP's compare fails when

a, b not within about rage of int.

a, b unequal, but (int) a == (int) b

The subtraction overflows.

A better compare

int cmpfunc (const void * a, const void * b) {
  float fa = *(const float *)a;
  float fb = *(const float *)b;
  return (fa > fb) - (fa < fb);
}

Use for(i = 0; i < n; i++) as well answered by @Davide.

Fails then n > 100.

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