Thread: This runs..but why not this. Help me to understand, please

valaris : thanks, but your suggestions didn't work for me.

i don't know how to make this damn thing run using a pointer to a pointer, so i'm just gonna stick with returning the pointer at the end of the function (this now makes alot of sense to me).
I've also updated a few coding faux pas (except 2):

some serious

Code:

integers = malloc(*num1 * sizeof(int));  to  integers = malloc((*num1 + 1) * sizeof(int));

Code:

int   *integers = 0; // undefined behavior bad
int   num1 = 0;

some not so serious

Code:

main() to int main(void)

Code:

return 0;

Code:

int *allocate_memory(int *integers, int *num1); // not just the prototypes
int average_of(int *integers, int *num1);
int print_average(int *integers, int *num1);

some just plain stupid (in an attempt to understand)

Code:

 integers = ptr;

But.....

1)fflush(stdin) must stay, because the getchar() doesn't work if its not there...see for yourself, it doesn't wait for a character return.
matsp: fflush(stdout) does not make sense to me. It flushes the standard output, after those scanfs i have to flush the standard input to do a getchar().

2)Between copying a value in to a function (int num1) vs copying a pointer to a value in to a function (int *num1). I don't know whether one or the other is more efficient, or if they are equal. I know i'm not changing the value of num1 (except in the allocate_memory function), but do i really need to change all of the other int *num1's to int num1's... ?

heres my code:

Code:

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


int *allocate_memory(int *integers, int *num1);
int average_of(int *integers, int *num1);
int print_average(int *integers, int *num1);


int *allocate_memory(int *integers, int *num1)
{

            
    printf("How many numbers do you want to average: ");
    scanf("%d", num1);
    
    integers = malloc((*num1 + 1) * sizeof(int));
    
    if(integers == NULL)
      {
         printf("malloc has failed\n");
         exit(1);
      }
    
    return integers;
}


int average_of(int *integers, int *num1)
{
   int    i;

   integers[*num1] = 0;

   for(i = 0; i < *num1; i++)
      {
         printf("#%d Please enter a number: ", i + 1);
         scanf("%d", &integers[i]);
         integers[*num1] += integers[i];
      }
   integers[*num1] /= *num1;
}

int print_average(int *integers, int *num1)
{
   int    i;
   
   printf("The average of ");
   
   for(i = 0; i < *num1; i++)
      printf("%d ", integers[i]);
      
   printf("is : %d\n", integers[*num1]);
}

int main(void)
{
   int   *integers = 0;
   int   num1 = 0;
    
   
  
   integers = allocate_memory(integers, &num1);
   average_of(integers, &num1);
   print_average(integers, &num1);
   
   free(integers);
   
   fflush(stdin);
   getchar();
   
   return 0;
}

Thanks again, to everyone who has posted so far.

Thanks Macgyver. What should i use to flush the standard input?

Originally Posted by tabstop

You can change what is pointed to by a function parameter, but you can never change the thing itself. So in your example, integers was a pointer to some integers -- those integers could be changed, because they were pointed to, but the value of integers itself could not be changed -- integers could not be set to point to a new bunch of integers.

Edit: That's a really confusing name, so maybe the typeface will help.

of course you could always pass a pointer to the pointer as so -

Code:

void foo(int** integers , int num){
   *integers = (int*)malloc(num * sizeof(int));
   
   return;
   }
 
void main(void){ // love you sweety
   
   int* integers;
   int num = 5;
 
   printf("integers* = %d\n" , integers);
 
   foo(&integers , num);
 
   // integers now points to an array of integers
   printf("integers* = %d\n" , integers);
 
   return;
   }

This is weird to me...but its pretty cool. I was not familiar with the ** operator.
So, ** essentially means: a pointer to a pointer? Valaris, your previous post makes more sense to me now. You gave me the answer, thank you.

Abachler: thanks... your code helped me see what i was doing wrong. I wasn't calling allocate_memory(&integers, &num1)... i was calling ( integers, &num1). I thought &integers would be redundant syntax (the address of the address of integers), its not. I'm kinda sorta starting to get it. **integers is a pointer to *integers address, so in the function the address of *integers is alterable...right?

Macgyver and matsp: I only hope, one day i'll understand the language like you too do. From what i can gather the new version of fflush(stdin) is:

Code:

int ch;
while ((ch = getchar()) != '\n' && ch != EOF);

1)This creates an int variable called ch
2)executes an infinite do nothing while loop if the condition ((ch = getchar()) != '\n' && ch != EOF); is met.
3)((ch = getchar()) != '\n' && ch != EOF) means ch equals the return value of getchar . if this is not equal to a newline character and ch is not equal to EOF do nothing and evaluate the condition again.

Is this right? because i don't get it.
It works though.... just replaced fflush(stdin) with
while ((ch = getchar()) != '\n' && ch != EOF); and it does the same thing.
for me though, its more undefined than fflush(stdin) is to the compiler. I would really appreciate an explanation .

here's my new code:

Code:

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


int allocate_memory(int **integers, int *num1);
int average_of(int *integers, int *num1);
int print_average(int *integers, int *num1);


int allocate_memory(int **integers, int *num1)
{

            
    printf("How many numbers do you want to average: ");
    scanf("%d", num1);
    
    *integers = malloc((*num1 + 1) * sizeof(int));
    
    if(integers == NULL)
      {
         printf("malloc has failed\n");
         exit(1);
      }
    
}


int average_of(int *integers, int *num1)
{
   int    i;

   integers[*num1] = 0;

   for(i = 0; i < *num1; i++)
      {
         printf("#%d Please enter a number: ", i + 1);
         scanf("%d", &integers[i]);
         integers[*num1] += integers[i];
      }
   integers[*num1] /= *num1;
}

int print_average(int *integers, int *num1)
{
   int    i;
   
   printf("The average of ");
   
   for(i = 0; i < *num1; i++)
      printf("%d ", integers[i]);
      
   printf("is : %d\n", integers[*num1]);
}

int main(void)
{
   int   *integers = 0;
   int   num1 = 0;
   int   ch; 
   
  
   allocate_memory(&integers, &num1);
   average_of(integers, &num1);
   print_average(integers, &num1);
   
   free(integers);
   
   while ((ch = getchar()) != '\n' && ch != EOF);
   getchar();
   
   return 0;
}

Is this better?
Is this standard C?
Thanks everybody.