Thread: allocating space dynamically for an array of structures

You have done, but what is the assignment for? What is it supposed to do?

Originally Posted by cus

basically set up an array of structures and then use malloc to allocate space dynamically for that structure. I know i have made things difficult but - i am a beginner and trying to teach myself.

Frankly, the first thing that you made difficult for yourself was in not following the instructions. Some teachers force students to use unreadable names, but your teacher asked you to use descriptive names: "for a structure of arrays, that must have the integer elements execution_time and process_number and a float element called average_time". Further more, it is clear that the structure type is to be named exec_time, not time. As such, I would expect:

Code:

typedef struct exec_time
{
    int execution_time;
    int process_number;
    float average_time;
} exec_time;

Now, when you want to allocate n exec_time structures, I would expect something like this:

Code:

exec_time *exec_time_array;
size_t n;

/* Code to get the value of n */

exec_time_array = malloc(n * sizeof(*exec_time_array));

/* Code that uses the dynamically allocated array via exec_time_array */

free(exec_time_array);

(To be robust, you should check that the return value of malloc() is not NULL before using it, as would be the case if an allocation could not be performed.)

>>sizeof(exec_array)
What is the size of a pointer? Not the size of the struct you want to allocate bytes for obviously!

>>(*exec_array).execution_time=10;
>>(*exec_array).process_number=9;
>>(*exec_array).average_time=9.15;
Hint: Try checking if the pointer is NULL BEFORE you assign or read anything from it.

And (*exec_array).average_time is the same as exec_array->average_time. Repeat for the others.

Originally Posted by cus

I obviously tried to over complicate things by asking the user to specify how many integers they would like to reserve, and as a result forgot about the objective.

Well, that makes the dynamic allocation useful, otherwise if n is sufficiently small you would just make the array a fixed size array

Originally Posted by cus

Here is what I have done.

Personally, I still think you should typedef the struct to exec_time, not time. More importantly, you are using malloc() pre-maturely. On the first line of the main() function, n is 0 since n has static storage duration. This means that you are allocating an array of zero size. You then access the first element of the array, but since the array has no elements, you are accessing the array out of bounds.

I would expect something along these lines:

Code:

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

typedef struct exec_time
{
    int execution_time;
    int process_number;
    float average_time;
} exec_time;

int main()
{
    exec_time *exec_array;
    size_t n;

    printf("Enter a non-negative integer: ");
    /* Read in the input to n. */

    exec_array = malloc(n * sizeof(*exec_array));

    if (exec_array == NULL)
    {
        /* Print memory allocation failure error, and possibly exit immediately. */
    }

    /* Use exec_array */
    
    free(exec_array);

    return 0;
}

Does not work if malloc fails.
And when are you going to learn indentation?

Think up a scenario if malloc fails. Then step through your code in your mind with this scenario and watch what happens.

Originally Posted by cus

But what do you mean does not work if malloc intended - have I not written it as you intended?

You probably should end the program immediately after printing the error message. Another option is to ask the user to enter a smaller number, and try again.

Originally Posted by cus

Ok at this moment in time if malloc fails then the program will continue to execute and consequently run the following printf statement which we dont want.

Indeed not. Quite correct.

So therefore, we should make the program exit - I am unsure of how to do this.. would it be:

Code:

EXIT(0)

exit works fine, but in main, you can also use return since the program quits after the main function has finished executing.

Can i do this in the printf statement? Or do i do it afterwards?

What's the point of doing it afterwards if we don't want the print statement to run if malloc fails?

Consequently, when writing the new code, use the same strategy to see if your logic is correct (works for small sections of code).