c-exercise-map-filter.org

Function pointers in C

Problems

Map

Implement a function called map that accepts 3 arguments: an list of integers, the length of the list, and a pointer to a function. The function should accept a single argument as an integer and return an integer.

The function should return a new array of integers, with each one applied to the argument passed into the function. Here’s a sample input and output of the function.

int f(int x) { return x * x; }

int main() {
  int nums[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
  int *mapped = map(nums, 10, f);
  // { 1, 4, 9, 16, 25, 36, 49, 64, 81, 100 }

  return 0;
}

int *map(int *nums, int size, int (*func)(int)) {
  int *newnums = malloc(sizeof(int) * size);

  for (int i = 0; i < size; i++) {
    newnums[i] = func(nums[i]);
  }

  return newnums;
}

Filter

Implement a function called filter that accepts 3 arguments: a list of integers, a reference to the length of the list, and a pointer to a predicate function. The function should accept a single argument as an integer and return an integer.

The function should return a new array of integers, but only the ones for which the predicate function returns true. Here’s a sample input and output of the function.

int isEven(int x) { return x % 2 == 0; }

int main() {
  int nums[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
  int len = 10;
  int *evens = filter(nums, &len, even);
  // { 2, 4, 6, 8, 10 }

  return 0;
}

int *filter(int *nums, int *size, int (*pred)(int)) {
  int *newnums = malloc(sizeof(int) * *size);

  int j = 0;
  for (int i = 0; i < *size; i++) {
    if (pred(nums[i])) {
      newnums[j++] = nums[i];
    }
  }

  newnums = realloc(newnums, sizeof(int) * j);
  *size = j;
  return newnums;
}