How to add/remove a string in a dynamic array in c language

Question

I have a defined array sample:

char *arguments[] = {"test-1","test-2","test-3"};

I am trying to add an argument input given by the command line. I tried the strcpy function and also to pass it through the array element e.g. arguments[num+1] = argv[1] but again not successfully.

I know that this is a very simple question but I am not an experienced programmer and all my experience comes from higher level programming languages (PHP, Perl).

The closest sample of work that I found online is C program to insert an element in an array and C program to delete an element from an array. But are not exactly what I am looking for and they are working with intigers not characters that I need.

My goal is to find a way to add and remove strings from a dynamic array that can grow and shrink based on the process of the script.

Thanks everyone for their time and effort to assist me.

Sample of a working code is given under:

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

/* Set as minimum parameters 2 */
#define MIN_REQUIRED 2
#define MAX_CHARACTERS 46

/* Usage Instructions */
int help() {
  printf("Usage: test.c [-s <arg0>]\n");
  printf("\t-s: a string program name <arg0>\n");
  printf("\t-s: a string sample name <arg1>\n");
  return (1);
}

int main(int argc, char *argv[]) {

  if ( argc < MIN_REQUIRED ) {
    printf ("Please follow the instructions: not less than %i argument inputs\n",MIN_REQUIRED);
    return help();
  }
  else if ( argc > MIN_REQUIRED ) {
    printf ("Please follow the instructions: not more than %i argument inputs\n",MIN_REQUIRED);
    return help();
  }
  else {
    int size, realsize;
    char *input = NULL;

    char *arguments[] = {"test-1","test-2","test-3"};

    int num = sizeof(arguments) / sizeof(arguments[0]);

    printf("This is the number of elements before: %i\n",num);

    int i;
    for (i=0; i<num; i++) {
      printf("This is the arguments before: [%i]: %s\n",i,arguments[i]);
    }

    printf("This is the input argument: %s\n",argv[1]);
    printf("This is the array element: %i\n",num+1);

    input = (char *)malloc(MAX_CHARACTERS);
    if (input == NULL) {
      printf("malloc_in failled\n");
      exit(0);
    }

    memset ( input , '\0' , MAX_CHARACTERS);

    int length_before = strlen(input);
    printf("This is the length before: %i\n",length_before);
    strcpy(input , argv[1]);
    int length_after = strlen(input);
    printf("This is the length after: %i\n",length_after);

    //arguments[num+1] = input;

    strcpy(arguments[num+1],input);

    int num_2 = sizeof(arguments) / sizeof(arguments[0]);

    printf("This is the number of elements after: %i\n",num);

    for (i=0; i<num_2; i++) {
      printf("This is the arguments after [%i]: %s\n",i,arguments[i]);
    }

  } // End of else condition

  return 0;
} // Enf of int main ()

Answer 1

1. "My goal is to find a way to add and remove strings from a dynamic array":

   char *arguments[] = {...} is statically allocated, so it cannot serve as a "dynamic array".

2. strcpy(arguments[num+1],input):

   You cannot access arguments[num+1] when this array has only num entries.

---

Suggested fix - allocate and initialize arguments dynamically, according to the value of argc:

char* strings[] = {"test-1","test-2","test-3"};
int i, num = sizeof(strings) / sizeof(*strings);
char** arguments = malloc((num+argc-1)*sizeof(char*));
if (arguments == NULL)
    ; // Exit with a failure

for (i=0; i<num; i++)
{
    arguments[i] = malloc(strlen(strings[i])+1);
    if (arguments[i] == NULL)
        ; // Deallocate what's already been allocated, and exit with a failure
    strcpy(arguments[i],strings[i]);
}

for (i=0; i<argc-1; i++)
{
    arguments[num+i] = malloc(strlen(argv[i+1])+1);
    if (arguments[num+i] == NULL)
        ; // Deallocate what's already been allocated, and exit with a failure
    strcpy(arguments[num+i],argv[i+1]);
}

...

// Deallocate everything before ending the program

Answer 2

There are no dynamic arrays in C, arguments has a static size, capable of holding 3 elements,

strcpy(arguments[num+1],input);

is simply undefined behaviour, the expression arguments[num+1] accesses an array out of bounds (two elements after the last); no magical reallocation or something will happen.

In general, you have three options:

1. You have an upper bound of how many items you want to be able to store in the array and declare the array to have that size. Either keep track of the numbers actually stored in it or add some sentinel value (which needs additional space!) indicating the end.
2. You have an upper bound, and if the number of items you want to store happen to exceed this limit, you abort (returning an error indicator, telling the user that the input data is too big, …).
3. Look for malloc, realloc and free.