Should I use arrays or pointer if i want a user defined sizes

Question

My question is simple if I have to input a data in an array with size user defined in C should I use pointers to store them or arrays

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

int main(){
    int *arr, n, i;
    printf("Enter number of elements: ");
    scanf("%d", &n);
    arr=(int *)malloc(n*sizeof(int));
    printf("Enter number of elements:\n");
    for(i=0; i<n; i++)
        scanf("%d", &arr[i]);
}

Or

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

int main(){
    int n, i;
    printf("Enter number of elements: ");
    scanf("%d", &n);
    int arr[n];
    printf("Enter number of elements:\n");
    for(i=0; i<n; i++)
        scanf("%d", &arr[i]);
}

Personally I find it more comfortable to use pointers, but as C treats array and pointers differently I may face problems in the future.

Those two snippets work differently in how/where the arrays are allocated. With a large n the second one could end up overflowing the stack, breaking your code — UnholySheep
– UnholySheep, Commented Feb 27, 2022 at 10:10
Exactly that's one of the issue I may face later on as in the second one the array has to be declared after declaring n @UnholySheep — Soham Patil
– Soham Patil, Commented Feb 27, 2022 at 10:21
Also variable length arrays have become optional since C11 (they were mandatoy in C99) so the second snippet may not work on all compilers (e.g.: MSVC does not support VLAs) — UnholySheep
– UnholySheep, Commented Feb 27, 2022 at 10:23

ben · Accepted Answer · 2022-02-27 18:52:25Z

The provided snippets don't match in their semantics. The one using 'pointers' allocates memory on the heap using malloc the other one will allocate memory on the stack (using alloca under the hood).

Furthermore C does not treat pointers and arrays that differently:

    int a[10];
    a[2] = 69;

is semanticly identical to:

    int* a = alloca(10 * sizeof(int));
    *(a + 2) = 69;

Besides the sizeof() operator and how they are sometimes treated when passing them as parameters they are pretty much interchangeable.

So there is no real difference you can choose what you prefer.

And because you don't seem to know the difference between stack and heap:

Allocating memory on the stack has the advantages of:

being faster
less memory management

but:

might overflow the stack when you allocate to much (~ 8kb)
will stay allocated and take up system ressources until function goes out of scope

PS: you should always use free() when using malloc() even in examples

pm100 · Accepted Answer · 2022-02-27 19:21:28Z

0

The other huge difference that has been hinted at but not totally explained is the life time of those 2 arrays.

If this was a function to read a file and return an array of ints from it

int* ReadInts(){..}

The malloc one

int*ReadInts(){
     int * result = malloc(....);
     ...
     return result;
}

works fine, but the vla one does not

int*ReadInts(){
     int result[X];
     ...
     return result;
}

your compiler might compile it (maybe with a warning) but this is bad code. result will be released when ReadInts returns, so this is Undefined Behavior

answered Feb 27, 2022 at 19:21

pm100

50.6k25 gold badges95 silver badges158 bronze badges

Collectives™ on Stack Overflow

Should I use arrays or pointer if i want a user defined sizes

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related