Thread: Expand an array

I have a simple array for math functions, but when adding larger numbers, i will need to add a column to the array. i know you can remove a slot/column using a[i]=NULL, but is there a way to add spaces to an array without re-initializing the array?

Originally Posted by SlyVixsky

I have a simple array for math functions, but when adding larger numbers, i will need to add a column to the array. i know you can remove a slot/column using a[i]=NULL, but is there a way to add spaces to an array without re-initializing the array?

I think the things you know aren't necessarily so. The number of slots in an array is fixed when you declare it, and cannot be adjusted in either direction (up or down).

If you need dynamic memory management, then you'll have to use dynamic memory management (malloc/realloc/free).

Originally Posted by tabstop

I think the things you know aren't necessarily so. The number of slots in an array is fixed when you declare it, and cannot be adjusted in either direction (up or down).

If you need dynamic memory management, then you'll have to use dynamic memory management (malloc/realloc/free).

it was a code sample online, and after some modifying, i did prove my initial assumption was incorrect.

heres the code sample I've drawn up, showing you can easily add numbers to an array, but you have to tell the program that you added numbers. Is there any function to tell toe program you expanded the array, or should i just code in another variable counter?

CODE:

Code:

#include<conio.h>
#include<stdio.h>
int main()
{
int arry[10];                   //create arry and counter
int i;
printf("The origional \n");     //display and fill the origional array
for(i=0; i<10; i++) {
    arry[i]=i;
}
for(i=0; i<10; i++)
    printf("%d  ",arry[i]);


arry[9]=12;                     //add more to the array
arry[10]=15;
arry[11]=18;
printf("\n\nThe new array \n");         //display the new, longer array with exact numbers
for(i=0; i<12; i++)
    printf("%d  ",arry[i]);

printf("\n\nThe new array using size testing \n");   //display new array, but let the program decide its length
for(i=0; i<(sizeof(arry)/sizeof(arry[0])); i++)
    printf("%d  ",arry[i]);

getch();
}

OUTPUT:

Code:

The origional
0  1  2  3  4  5  6  7  8  9

The new array
0  1  2  3  4  5  6  7  8  12  15  18

The new array using size testing
0  1  2  3  4  5  6  7  8  12

Except arry[10]=15 doesn't work. You got "lucky", in that array[10] didn't cause a segfault, or memory access error, or similar. It "should" do so. (By "should" I mean morally, not that it actually will.)

Originally Posted by tabstop

Except arry[10]=15 doesn't work. You got "lucky", in that array[10] didn't cause a segfault, or memory access error, or similar. It "should" do so. (By "should" I mean morally, not that it actually will.)

True, which is why i think this program working is odd, and am looking into it

Originally Posted by grumpy

If you need to access other elements (i.e. indices outside the range 0 to 9) you need to specifically ensure the array is large enough to allow that or, alternatively, dynamically create the array of required size at run time BEFORE accessing it's elements.

thats the issue, i need an array that can be flexible, expanding as desired by the user.

In C, that never occurs automatically. If you want an array to expand, you need to implement the process by which expansion occurs. One way is that, when a user attempts to modify a non-existent element, that you create a new (bigger) array, copy the elements across to the new array, and destroy the original. Another (assuming your arrays are dynamically created with malloc/realloc/calloc) to do that is with realloc() - but you still need to implement the logic by which realloc() is called.

In C++, you can use standard containers to simplify some of that. However, the C++ standard containers need to be used in a particular manner - they still don't resize themselves simply through an attempt to modify or access an element.