Problem with effective function and arrays in C - arrays

Question

I'm trying to write an effective function that receives an array the size of n and a and b.
The function should search all the numbers in the array such that b-a < array[i] and collect them to a new sorted array called incoming.
For instance, for the input 11,12,8,15,3,12,3,12 , b=15, a=8 the output would be a 6 size array that will contain the values 8,11,12,12,12,15 (anything that is higher than (b)15-(a)8 ).
This is my own code attempt:
#include<stdio.h>
#include<stdlib.h>
int* f5(int arr[], int n, int a, int b, int* p)
{
int i,min,minIndex;
int* incoming = (int*)malloc(*p*sizeof(int));
for ( i = 0; i < n; i++)
{
if (arr[i]>(b - a))
{
incoming[i] = arr[i];
(*p)++;
}
}
return *incoming;
}
void main()
{
int arr[] = { 12,3,12,3,15,8,12,11 };
int p, i;
int incoming[] = f5(arr, sizeof(arr) / sizeof(arr[0]), 8,15, &p);
printf("The size is: %d and the new marahc is: ", p);
for (i = 0; i < p; i++) {
printf("%d", incoming[i]);
}
free(incoming);
}

Answer 1

So I corrected your f5 function because you were not allocating your new array properly, since you were doing the malloc with (*p) before it was set to the number of elements you needed to store. After that, I ordered the incoming array in ascending order as shown in the example output with qsort. I also added the part where you take input values from the user, since in the code you've posted you were probably just trying that specific test case.
#include<stdio.h>
#include<stdlib.h>
// compare function for qsort
int mycompare (const void* a, const void* b) {
int val1 = *(const int*)a;
int val2 = *(const int*)b;
return (val1 - val2);
}
int* f5(int arr[], int n, int a, int b, int* p) {
int target = b - a;
int i;
for (i=0;i<n;i++) { // storing number of values > (a-b)
if (arr[i]>target) (*p)+=1;
}
int* incoming;
if ((incoming= malloc((*p)*sizeof(int)))==NULL); // should always check malloc errors
{
perror("malloc");
exit(EXIT_FAILURE);
}
int j = 0;
i = 0;
while (i<n && j<(*p)) { // storing values in new array
if (arr[i]>target) {
incoming[j] = arr[i];
j++;
}
i++;
}
return incoming;
}
void main() {
int n, a, b, i;
printf("Insert 'a' value: ");
scanf("%d", &a);
printf("Insert 'b' value: ");
scanf("%d", &b);
printf("Insert array size: ");
scanf("%d", &n);
int arr[n];
printf("Insert array values: \n");
for (i=0;i<n;i++) {
scanf("%d", &arr[i]);
}
int size = 0;
int *incoming = f5(arr, n, a, b, &size);
qsort(incoming, size, sizeof(int), mycompare); // sorting array
printf("The size is: %d and the new marahc is: ", size);
for (i=0; i<size;i++) {
printf("%d ", incoming[i]);
}
free(incoming);
}

Answer 2

In f5 you need to determine the required length to allocate. You could either do an initial pass of the arr[] contents to determine the required length, or just set the length to the upper bound n and resize it once the actual length is known.
First approach using an initial pass:
int* f5(int arr[], int n, int a, int b, int* p)
{
int i;
int j;
int* incoming;
j = 0;
for ( i = 0; i < n; i++)
{
if (arr[i]>(b - a))
{
j++;
}
}
incoming = malloc(j*sizeof(int));
if (incoming == NULL)
{
return NULL;
}
j = 0;
for ( i = 0; i < n; i++)
{
if (arr[i]>(b - a))
{
incoming[j++] = arr[i];
}
}
*p = j;
return incoming;
}
Second approach using upper bound for allocated size:
int* f5(int arr[], int n, int a, int b, int* p)
{
int i;
int j;
int* incoming = malloc(n*sizeof(int));
if (incoming == NULL)
{
return NULL;
}
j = 0;
for ( i = 0; i < n; i++)
{
if (arr[i]>(b - a))
{
incoming[j++] = arr[i];
}
}
int* resized_incoming = realloc(incoming, j*sizeof(int));
if (resized_incoming != NULL)
{
incoming = resized_incoming;
}
*p = j;
return incoming;
}
The above have been written to return NULL if malloc returns NULL.
A third approach would be to start with a small amount for incoming and reallocate to a larger amount when necessary.
In main, the variable incoming needs to be changed from an array type to a pointer:
int* incoming = f5(arr, sizeof(arr) / sizeof(arr[0]), 8,15, &p);
Since f5 can now return NULL, the return value should be checked and appropriate action taken:
if (incoming == NULL)
{
fprintf(stderr, "Failed to allocate memory!\n");
exit(EXIT_FAILURE);
}
You also need to add code to sort the filtered numbers.