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dave_1234
dave_1234

Reputation: 147

Appending element into an array of strings in C

I have an array of strings with a given size, without using any memory allocation, how do I append something into it?

Say I run the code, its waiting for something you want to enter, you enter "bond", how do I append this into an array ? A[10] ?

Upvotes: 9

Views: 69989

Answers (5)

Taylor Liang
Taylor Liang

Reputation: 1

If you want to append a character or string to it;

strcpy(a, "james")
strcpy(a, "bond")

Upvotes: 0

Naliwe
Naliwe

Reputation: 322

Hi,

It really depends on what you mean by append.

...
int tab[5]; // Your tab, with given size
// Fill the tab, however suits you.
// You then realize at some point you needed more room in the array
tab[6] = 5; // You CAN'T do that, obviously. Memory is not allocated.

The problem here can be two things :

  • Did you misjudge the size you need ? In that case, just make sure this given size you mentioned is correctly 'given', however that might be.
  • Or don't you know how much room you want at the beginning ? In that case, you''ll have to allocate the memory yourself ! There is no other way you can resize a memory chunk on the fly, if I might say. 

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #define STR_MAX_SIZE 255                                // Maximum size for a string. Completely arbitray.

    char *new_string(char *str)
    {
      char *ret;                                            // The future new string;

      ret = (char *) malloc(sizeof(char) * 255);            // Allocate the string
      strcpy(ret, str);                                     // Function from the C string.h standard library
      return (ret);
    }

    int main()
    {
      char  *strings[STR_MAX_SIZE];                         // Your array
      char  in[255];                                        // The current buffer
      int   i = 0, j = 0;                                   // iterators

      while (in[0] != 'q')
        {
          printf("Hi ! Enter smth :\n");
          scanf("%s", in);
          strings[i] = new_string(in);                      // Creation of the new string, with call to malloc
          i++;
        }
      for ( ; j < i ; j++)
        {
          printf("Tab[ %d ] :\t%s\n", j, strings[j]);       // Display
          free(strings[j]);                                 // Memory released. Important, your program
                                                            // should free every bit it malloc's before exiting
        }

      return (0);
    }

This is the easiest solution I could think of. It's probably not the best, but I just wanted to show you the whole process. I could have used the C standard library strdup(char *str) function to create a new string, and could have implemented my own quick list or array.

Upvotes: 2

Vlad from Moscow
Vlad from Moscow

Reputation: 311058

If the array declared like

char A[10];

then you can assign string "bond" to it the following way

#include <string.h>

//...

strcpy( A, "bond" );

If you want to append the array with some other string then you can write

#include <string.h>

//...

strcpy( A, "bond" );
strcat( A, " john" );

Upvotes: 9

Nick Bailey
Nick Bailey

Reputation: 3162

You can't append to an array. When you define the array variable, C asks the is for enough contiguous memory. That's all the memory you ever get. You can modify the elements of the array (A[10]=5) but not the size.

However, you CAN create data structures that allow appending. The two most common are linked lists and dynamic arrays. Note, these are no built into the language. You have to implement them yourself or use a library. The lists and arrays of Python, Ruby and JavaScript are implemented as dynamic arrays.

LearnCThHardWay has a pretty good tutorial on linked lists, though the one on dynamic arrays is a little rough.

Upvotes: 3

fuz
fuz

Reputation: 93082

The size of an array variable cannot change. The only way to append to an array is to use memory allocation. You are looking for the realloc() function.

Upvotes: 0

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