How to add/delete element in Flat array in C-CodePudding

I have a structure inside which char array and int value is maintained. I want to treat this char array as a flat array to store the list of strings and the offset will track the starting position where the string is added in the array.

Structure is shown below:

struct A
{
   char element[256];
   int offset;
}

Also, I want to delete the strings after performing some operation if found.

Please let me know if this feasible. If yes then how?

CodePudding user response：

Since all we have to go on is the question if it's feasible - and the answer is yes. Here's a way showing that it is:

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

typedef struct A A;
struct A {
   char element[256];
   int offset;
};

A *A_create() {
    A *a = malloc(sizeof *a);
    a->offset = 0;
    return a;
}

void A_destroy(A *a) {
    free(a);
}

// return false if the string doesn't fit
//        true if it's successfully added
bool A_add_string(A *a, const char *str) {
    size_t len = strlen(str);
    if(a->offset   len >= sizeof a->element) return false;
    memcpy(a->element   a->offset, str, len   1);
    a->offset  = len   1;
    return true;
}

You can now create an A, add \0 terminated strings to it and finally destroy it:

A *a = A_create();
A_add_string(a, "Hello");
A_add_string(a, "world");
A_destroy(a);

CodePudding user response：

Yes, append to a.element[a.offset].

To delete, set a.element[0] to the null byte. C strings end at a null byte.

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

typedef struct
{
   char element[256];
   int offset;
} A;

void A_append(A* a, const char *str) {
    // Concatenate on the end of element.
    strcat(&a->element[a->offset], str);

    // Increment the offset to the new end.
    a->offset  = strlen(str);
}

void A_delete(A* a) {
    a->element[0] = '\0';
    a->offset = 0;
}

int main() {
    A a = { .element = "" };
    a.offset = 0;

    char string1[] = "one";
    A_append(&a, string1);

    char string2[] = "two";
    A_append(&a, string2);

    puts(a.element);

    A_delete(&a);
    puts(a.element);
}

You can also store a pointer to the end of element. It's the same thing, just more direct.

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

typedef struct
{
   char element[256];
   char *end;
} A;

void A_append(A* a, const char *str) {
    // Concatenate nto the end of element.
    strcat(a->end, str);

    // Increment the pointer to the new end.
    a->end  = strlen(str);
}

void A_delete(A* a) {
    a->element[0] = '\0';
    a->end = a->element;
}

int main() {
    A a = { .element = "" };
    a.end = a.element;

    char string1[] = "one";
    A_append(&a, string1);

    char string2[] = "two";
    A_append(&a, string2);

    puts(a.element);

    A_delete(&a);
    puts(a.element);
}

Finally, if you want to store a list of strings, not concatenate them, consider storing them as pointers.