Passing a pointer to an array and allocating memory

Question

I'm trying to pass a pointer from main to a function I wrote, which suppose to allocate memory with the size integer and receive input from the user.

Now I know I'm doing something wrong because the pointer doesn't change back in the main function. could someone tell me what exactly I did wrong?

int rows, *newCol=NULL // the pointer inside main();//rows is the size of the array.

the function:

void buildArr(int *newCol, int rows);

void buildArr(int *newCol, int rows)
{
    int i;
    newCol = (int*)malloc(sizeof(int)*rows);
    for (i = 0; i<rows; i++)
    {
        printf("enter the value of the newCol:\n");
        printf("value #%d\n", i + 1);
        scanf("%d", &newCol[i]);
    }


}

user2736738 · Accepted Answer · 2018-01-01T17:39:08.080

3

The pointer shouldn't change in the callee. C is pass by value.

Pass the address of the pointer variable

Call it like

buildArr(&newCol,rows);
...
...
void buildArr(int **newCol, int rows)
{
    int i;
    *newCol = malloc(sizeof(int)*rows);
    ...
        scanf("%d", &(*newCol)[i]);
    }
    // nothing is returned.
}

Or return the address of the allocated chunk

newCol = buildArr(newCol, rows);
...
...
int* buildArr(int *newCol, int rows)
{
    int i;
    newCol = malloc(sizeof(int)*rows);
    ...
        scanf("%d", &newCol[i]);
    ...
    return newCol;
}

edited Jan 01 '18 at 17:39

answered Jan 01 '18 at 17:36

user2736738

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I should send &newCol instead? – Alex Kreinis Jan 01 '18 at 17:37
yes but I wish to do it like that, for learning purposes, what did I do wrong? – Alex Kreinis Jan 01 '18 at 17:38
@AlexKreinis.: Wil explain.wait – user2736738 Jan 01 '18 at 17:41
@coderredoc I think you already did in your 1st sentence. – JFMR Jan 01 '18 at 17:43
I'm sorry I did as you suggested at the void option and I still get garbage in my newCol, maybe I missed something? – Alex Kreinis Jan 01 '18 at 17:43
@水飲み鳥.: Yes...really :) – user2736738 Jan 01 '18 at 17:43
oh ok , you edited a few times, now I see what you did there and it worked, thank you so much.! – Alex Kreinis Jan 01 '18 at 17:44
@AlexKreinis.: Check few edits to know few extra things. That wil help you later. – user2736738 Jan 01 '18 at 17:47
I have another question please @ coderredoc if I want to send my newCol to another function after I inputed it, and in the function it appears as *newCol, do I need to send it as (&newCol) aswell? – Alex Kreinis Jan 01 '18 at 17:48
1

@AlexKreinis.: If you again don't want to change the number of variables (resizing or shrinking..) you can simply pass the `int*`..that's it. – user2736738 Jan 01 '18 at 17:51
so if I want to add the pointer I send it by adress, and if I want to use its value I just send it as (*newCol)? – Alex Kreinis Jan 01 '18 at 17:52
1

@AlexKreinis.: Like if you want to change the content of the array like setting the `0`th element to `2018` then just pass the pointer. `f(p)` where `void f(int *ptr){ ptr[0]=2018;}` – user2736738 Jan 01 '18 at 17:53

Passing a pointer to an array and allocating memory

1 Answers1