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How to pass dynamic arrays?

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shmay

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How to pass dynamic arrays?

Apr 1st, 2007

For my C class, I have to write a program where the user enters how many fahrenheits he will be entering, but the fahrenheits have to be in an array. So a dynamic array seems the obvious choice. In main, we can only have initializations and function calls.

For example, if i have:

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 C Syntax (Toggle Plain Text) 
int main (void)
{
     int *dynArray;
int main (void)
{
     int *dynArray;

How would I pass the dynamic array to another function? Do I have to use malloc in only the first function that I call it to? Help!

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Re: How to pass dynamic arrays?

Apr 1st, 2007

pass the pointer and the length of the array.

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Re: How to pass dynamic arrays?

Apr 1st, 2007

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int arr[10];
int *pArr = malloc ( 10 * sizeof *pArr );
myFunc( arr );
myFunc( pArr );
int arr[10];
int *pArr = malloc ( 10 * sizeof *pArr );
myFunc( arr );
myFunc( pArr );

The function itself doesn't care whether you have a real array or a pointer to allocated memory, the function is declared and written in exactly the same way.

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Re: How to pass dynamic arrays?

Apr 3rd, 2007

I understand that, but the user has to enter the size of the array, and in a different function than main.

Also, what would the receiving functions look like in your example? Thanks!

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Re: How to pass dynamic arrays?

Apr 3rd, 2007

>I understand that, but the user has to enter the size of the array, and in a different function than main.
Then use a pointer to modify the memory in the variables in main(). Or if you can, return the value that the user entered in the function.

>Also, what would the receiving functions look like in your example?
They'd look exactly the same as if they had been written for static arrays. One thing you have to remember though, is to always pass the size of the dynamic array to the function (even though it's not a requirement), because otherwise you run the risk of overrunning the array boundaries.

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Re: How to pass dynamic arrays?

Apr 3rd, 2007

Ah, I see. Thanks. I'm still left with one problem I can't solve though: how to have the user decide the size of the array through another function.

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 C Syntax (Toggle Plain Text) 
{
int *pArr, Size;
 
Size = /* Function call here*/
myFunc( pArr, Size );
}
 
/* Put function that returns size here*/
 
void myFunc(int dArr[], int Size)
{
      dArr = malloc (sizeof (int) * Size);
}
{
int *pArr, Size;

Size = /* Function call here*/
myFunc( pArr, Size );
}

/* Put function that returns size here*/

void myFunc(int dArr[], int Size)
{
      dArr = malloc (sizeof (int) * Size);
}

That's my shitty attempt at it. I have tried many other variations. Which is the correct way?

Last edited by shmay; Apr 3rd, 2007 at 9:29 pm.

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Re: How to pass dynamic arrays?

Apr 3rd, 2007

>how to have the user decide the size of the array through another function.
What's so hard about that? Just get the user to input a number, and return that value back. (although you might want to validate the number first, so as to avoid negative array sizes...)

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Re: How to pass dynamic arrays?

Apr 4th, 2007

You don't even necessarily have to have the user choose the size of the array...
you could let the program change the size of the array on the fly...ie. if the user enters another value that is beyond the size of the current array, the program could double the size variable, create a new array with that size, copy the old values from the old array, add the new value entered by the user, then reassign the new array to the variable of the old array...it sounds complicated, but it's not too bad...