Regarding passing pointers as arguments - c

Question

I am trying to code a basic zero threshold function. such that if the value of the array element is greater than zero must remain same else it must be zero. But my problem is with passing the array values from main to the function using pointers. Here's the snippet. im is the input array and im2 is the array to store result. t is the threshold which is 0 and m is the order. On passing the input array from main to thresh function. I just checked the values of im in thresh function but all were showing 0, as commented in code below, instead of original values. where am i going wrong??
int thresh(double *im[], double *im2[], int t, int m)
{
int i, j;
printf("im:%f", im[0]); //here i am getting output as zero instead of 1
for (i = 0; i < m; i++)
{
if (im[i] > t)
im2[i] = im[i];
else
im2[i] = 0;
}
return 0;
}
int main()
{
float im[4] = { 1,-2,3,-4 };
float im2[4];
int th = 0;
thresh((float*)im, (float*)im2, th, 2);
getch();
return 0;
}

Answer 1

Turn on compiler warnings and read them. They are there to help. This is what I got on compilation:
$ gcc main.c
main.c: In function ‘thresh’:
main.c:10:23: warning: comparison between pointer and integer
if (im[i] > t)
^
main.c: In function ‘main’:
main.c:24:12: warning: passing argument 1 of ‘thresh’ from incompatible pointer type [-Wincompatible-pointer-types]
thresh((float*)im, (float*)im2, th, 2);
^
main.c:4:5: note: expected ‘double **’ but argument is of type ‘float *’
int thresh(double *im[], double *im2[], int t, int m)
^~~~~~
main.c:24:24: warning: passing argument 2 of ‘thresh’ from incompatible pointer type [-Wincompatible-pointer-types]
thresh((float*)im, (float*)im2, th, 2);
^
main.c:4:5: note: expected ‘double **’ but argument is of type ‘float *’
int thresh(double *im[], double *im2[], int t, int m)
^~~~~~
So there is some stuff to fix.
First of, the prototype for thresh should be int thresh(double *im, double *im2, int t, int m) or even better int thresh(const double *im, double *im2, int t, int m)
Second, why are you mixing float and double? Stick to one, and stick to double unless you have a really good reason.