function that remove an integer from an array using pointer in C

Question

I got a question where I need to write a function that reads an integer X and an array A of type int (size N) from the keyboard and eliminates all occurrences of X in A. for example the input is:

5

1 2 3 4 3

3

and it would return:

A : 1 2 3 4 3

New A : 1 2 4

my code so far is

#include <stdio.h>
#include<stdlib.h>
#define DIM 50
int main() {
    int *A;
    int N, X;
    int *P1, *P2;
    do{
     scanf("%d", &N);
    }while(N<0 || N>DIM);

A= (int*)malloc(N*sizeof(int));

for(P1=A; P1<A+N ; P1++)
 scanf("%d ", P1);
printf("\n");

scanf("%d",&X);

printf("A : ");
for(P1=A; P1<A+N ; P1++)
 printf("%d ", *P1);
printf("\n");

but I don't know how to continue if you could please help

Answer 1

What you need is to write a function that will erase elements equal to the specified value and reallocate the result array.

Here is a demonstration program where such a function is shown in action.

#include <stdio.h>
#include <stdlib.h>

size_t erase_remove( int **a, size_t n, int value )
{
    size_t m = 0;

    for (int *p = *a, *q = *a; p != *a + n; ++p)
    {
        if (*p != value)
        {
            if (q != p) *q = *p;
            ++q;
            ++m;
        }
    }

    if (m != n)
    {
        int *tmp = realloc( *a, m * sizeof( int ) );

        if (tmp != NULL)
        {
            *a = tmp;
        }
        else
        {
            m = -1;
        }
    }

    return m;
}

int main( void )
{
    size_t n = 5;
    int *a = malloc( n * sizeof( int ) );

    size_t i = 0;
    a[i++] = 1, a[i++] = 2, a[i++] = 3, a[i++] = 4, a[i++] = 3;

    int value = 3;

    size_t m = erase_remove( &a, n, value );

    if (m != -1) n = m;

    for (const int *p = a; p != a + n; ++p)
    {
        printf( "%d ", *p );
    }
    putchar( '\n' );

    free( a );
}

The program output is

1 2 4

If the memory reallocation for the array within the function was not successful the function returns the value (size_t)-1.

The function preserves the order of elements after removing elements equal to the target value.

If to make the function more general that can deal not only with arrays dynamically allocated then it can look very simply.

size_t erase_remove( int *a, size_t n, int value )
{
    size_t m = 0;

    for (int *p = a, *q = a; p != a + n; ++p)
    {
        if (*p != value)
        {
            if (q != p) *q = *p;
            ++q;
            ++m;
        }
    }

    return m;
}

In this case the caller of the function should reallocate the result dynamically allocated array (if it is required) based on the returned value m from the function.

Answer 2

#define N_MAX 50
#define N_MIN 0

int main(void) {
    int n;

    do{
        scanf("%d", &n);
    }while(N<N_MIN  || N>N_MAX);

    int *array = (int*) malloc(sizeof(int) * n);

    int i; // number of elements in array
    for (i = 0; i < n; i++) {
        scanf("%d", array + i);
    }

    int x;
    scanf("%d", &x);

    //remove x from arr
    for (int j = 0; j <= i; j++) {
        if (*(array + j) == x) {
            *(array + j) = *(array + i); // replace removed value by last value in array
            i--; // decremment number of elements in array
        }
    }

    // print
    for (int j = 0; j <= i; j++) {
        print("%d", *(array + j)); 
    }

    free(array)
}

Answer 3

Try this out!

#include <stdio.h>
#include <stdlib.h>

// Required Prototypes
int *get_nums(char *, size_t *);
int *remove_num(int *, size_t *, int);
void display(char *, int *, size_t);

int main(int argc, char *argv[])
{
    size_t size = 0;
    int *arr = get_nums("Enter numbers (seperated by space): ", &size);
    
    int num;
    printf("Enter number to be removed: ");
    scanf("%d", &num);
    display("Old Array: ", arr, size);
    
    arr = remove_num(arr, &size, num);
    display("New Array: ", arr, size);
    
    free(arr);
    return 0;
}

int *get_nums(char *label, size_t *size)
{
  size_t length = 0;
  int *arr = NULL;
  
  printf("%s", label);
    
    int c, num;
    do {
      scanf("%d", &num);
      arr = realloc(arr, (length + 1) * sizeof(int));
      arr[length++] = num;
    } while ( (c = getchar()) != '\n' && c != EOF);
    
    *size = length;
    return arr;
}

int *remove_num(int *arr, size_t *size, int num)
{
  // Copy elements to the new array
  // Return the new array
  size_t new_size = 0;
  int *new_arr = NULL;
  
  for (size_t i = 0; i < *size; ++i) {
    if (arr[i] != num) {
      new_arr = realloc(new_arr, (new_size + 1) * sizeof(int));
      new_arr[new_size++] = arr[i];
    }
  }
  
  *size = new_size;
  free(arr);
  return new_arr;
}

void display(char *label, int *arr, size_t size)
{
  printf("%s", label);
  
  for (size_t i = 0; i < size; ++i)
    printf("%d ", arr[i]);
    
  printf("\n");
}

The main idea is you create an array of integers. Then you copy those elements to a new array which you do not want to remove. And finally you display the new array. That's all. Yes, it's that simple. ;-)

Enter numbers (seperated by space): 1 2 3 4 3
Enter number to be removed: 3
Old Array: 1 2 3 4 3
New Array: 1 2 4

As @Ahmed Masud said in comments about too many reallocations, here's my modified answer. Please do note that the code below is little bit complex but far more efficient than my previous one.

#include <stdio.h>
#include <stdlib.h>

typedef struct {
  int *a;
  size_t length;
  size_t capacity;
} Array;

// Required Prototypes
Array *init_Array(void);
void destroy(Array *);

Array *get_nums(char *);
void remove_num(Array *, int);
void display(char *, Array *);

int main(int argc, char *argv[])
{
    Array *arr = get_nums("Enter Numbers (seperated by space): ");
    
    int num;
    printf("Enter number to be removed: ");
    scanf("%d", &num);
    display("Old Array: ", arr);
    
    remove_num(arr, num);
    display("New Array: ", arr);
    
    destroy(arr);
    return 0;
}

Array *init_Array(void)
{
  Array *arr = malloc( sizeof(Array) );
  arr->capacity = 1;
  arr->length = 0;
  arr->a = malloc( sizeof(int) );
  return arr;
}

Array *get_nums(char *label)
{
  printf("%s", label);
  
  Array *arr = init_Array();
  int c, num;
  
  do {
    scanf("%d", &num);
    // check and reallocate
    if (arr->length == arr->capacity) {
      arr->a = realloc(
        arr->a, 
        (2 * arr->capacity) * sizeof(int)
      );
      arr->capacity *= 2;
    }
    
    arr->a[arr->length++] = num;
  } while ((c = getchar()) != '\n' && c != EOF);
  
  return arr;
}

void remove_num(Array *arr, int num)
{
  int remv_idx = -1;
  int *a = arr->a;
  size_t count = 0;
  
  for (size_t i = 0; i < arr->length; ++i) {
    if (a[i] == num) count++;
    
    if (a[i] == num && remv_idx == -1)
      remv_idx = i;
      
    if (remv_idx != -1 && remv_idx < i && a[i] != num)
      a[remv_idx++] = a[i];
  }
  
  arr->length -= count;
  arr->capacity = arr->length;
  arr->a = realloc(a, arr->capacity * sizeof(int));
}

void display(char *label, Array *arr)
{
  printf("%s", label);
  
  for (size_t i = 0; i < arr->length; ++i)
    printf("%d ", arr->a[i]);
    
  printf("\n");
}

void destroy(Array *arr)
{
  free(arr->a);
  free(arr);
}

Here I did not consider any new array but removed the elements in place. I'm keeping both of my solution because you might not need the 2nd one if your input space is small. One more thing, since the question did not mention about any reallocations failures so I did not check it in my code.

Answer 4

Here is an approach:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void print_arr(int *arr, size_t size);

size_t remove_by_value(int *arr, size_t len, int value)
{
  int count = 0; // maintain how many times we see value
  int k;
  for(k = 0; k < len ; k++) {
      if ( arr[k] == value ) {
          while(arr[count+k] == value) {
              count++;
          } // skip over conscutive values
          if ( count + k >= len )
              break;
          arr[k] = arr[k+count];
          arr[k+count] = value;
          print_arr(arr, len);
      }
  }
  return len-count;
}

void print_arr(int *arr, size_t size)
{
  for(int k = 0; k < size; k++) {
    printf("%02d ", arr[k]);
  }
  printf("---\n");
}

int main()
{
  int test_values[] = { 0, 1, 3, 2, 3, 5, 4, 7, 8 };
  size_t len = sizeof(test_values)/sizeof(int);
  int *arr = malloc(len*sizeof(int));
  memcpy(arr, test_values, len*sizeof(int));
  print_arr(arr, len);
  len = remove_by_value(arr, len, 3);
  print_arr(arr, len);
  arr = realloc(arr, len);
  print_arr(arr, sizeof(int)*len);
  return 0;
}

It bubbles the value to be extracted to the end of the array and lops it off. The nice thing is that it doesn't use any extra memory to do its work.

The second part that is that it is NOT O(n^2) I have to think a bit about the complexity of it (seems bigger than O(n))

However it's a simple solution that keeps the order of the array, removes unneeded values simply.

i've put in the print_arr function at each step of the loop so that you can see what's happening.

Hopefully the code is clear in its purpose, if you have questions please comment and I will further explanation.

Notes

1. I expressly did not use sizeof(*arr) because i wanted it to be a bit more clear as to what it is. In production code one would use sizeof(*arr) instead of sizeof(int) .... However I would not be able to create a consistent example (e.g. remove_by_value would have to be similarly made generic) so I didn't to keep the example simple to understand.