How to "delete" every element in array by value in C

Question

I have been trying to solve this problem for about 5 days..can't find any solution please send help. I am supposed to implement a function to "delete" every element in an array by value. Let's say my array is "Hello" and I want to delete every "l". So far I can only delete l once. By the way keep in mind I am not allowed to use pointers for this function...(we haven't learned that yet in my school) Here's my code:

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

void strdel(char array[], char c);

int main(void)
{
    char source[40];
    printf("\nStrdel test: ");
    strcpy(source, "Hello");
    printf("\nsource = %s", source);
    strdel(source, 'l');
    printf("\nStrdel: new source = %s", source);
    return 0;
}

void strdel(char array[], char c)
{
    int string_lenght;
    int i;
    for (string_lenght = 0; array[string_lenght] != '\0'; string_lenght++) {} 

    for (i = 0; i < string_lenght; i++) {
        if (array[i] == c) {
            for (i = i; array[i] != '\0'; ++i)
                array[i] = array[i + 1];
        }
    }
}

Tip - use a 'read' index and a 'write' index. Start them the same and increment each as necessary... — Carl Norum
– Carl Norum, Commented Nov 26, 2018 at 19:06
i'm not allowed to use other functions to implement strdel that's why i don't use strlen... I can only write them in main to test if my function is correct — Compil3
– Compil3, Commented Nov 26, 2018 at 19:10

chux · Accepted Answer · 2018-11-26 21:12:09Z

5

Simple use 2 indexes, one for reading and one for writing. @Carl Norum

void strdel(char array[], char c) {
  int read_index = 0;
  int write_index = 0;
  while (array[read_index] != '\0') {
    if (array[read_index] != c) {
      array[write_index] = array[read_index];
      write_index++;  // Only advance write_index when a character is copied
    }
    read_index++;     // Always advance read_index
  }
  array[write_index] = '\0';
}

The has O(n) performance, much faster than using nested for() loops which is O(n*n).

Details:

OP: By the way keep in mind I am not allowed to use pointers for this function.

Note that array in void strdel(char array[], char c) is a pointer, even though it might look like an array.

int for array indexing is OK for learner and much code, yet better to use size_t. int may lack the range needed. Type size_t is an unsigned type that is neither too narrow nor too wide for array indexing needs. This becomes important for very long strings.

edited Nov 26, 2018 at 21:12

answered Nov 26, 2018 at 19:27

chux

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6 Comments

Brydon Gibson Over a year ago

The while(array[read_index]) may be confusing for a new programmer - I'd suggest explaining that when it's null, that statement will fail (or even using a for loop in the example and explaining how it can be compressed to a while - that way the comparison is still visible)

chux Over a year ago

@BrydonGibson Amended while (array[read_index]) to while (array[read_index] != '\0') which is functionally equivalent.

Compil3 Over a year ago

No idea what while(array[read_index]) does. How does that work ? the rest is somewhat clear

chux Over a year ago

@Saty while (test_condition) tests if test_condition != 0.

Compil3 Over a year ago

I've tested your code and for my array: "Hello" and "l" I only get ll instead of Heo ? Why is that ? I'm still trying to understand why you use only 1 while loop

|

4386427 · Accepted Answer · 2018-11-26 20:48:42Z

3

Your problem is related to using the variable i in both loops. So once the inner loop is executed, outer loop will terminate right after.

Use another variable for the inner loop.

void strdel(char array[], char c)
{
    int string_lenght;
    int i, j;
    for (string_lenght = 0; array[string_lenght] != '\0'; string_lenght++) {} 

    for (i = 0; i < string_lenght; i++) {
        if (array[i] == c) {
            for (j = i; array[j] != '\0'; ++j)  // Use variable j instead of i
                array[j] = array[j + 1];

           --i;              // Decrement i to "stay" at the same index
           --string_lenght;  // As one character were just removed
        }
    }
}

The above shows how to make OPs approach work. For a better solution see the answer from @chux : https://stackoverflow.com/a/53487767/4386427

edited Nov 26, 2018 at 20:48

answered Nov 26, 2018 at 19:15

4386427

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2 Comments

Compil3 Over a year ago

Thanks so much ! I used read_index and write_index for i and j so that it's a bit clearer.... Could you please explain why you need to use --i after the second for loop ? That was the key step that i was always missing...

4386427 Over a year ago

@Saty The --i is needed to stay at the same index in the next loop. In other words - it cancels the ++i in the for statement

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