macOS C pointer and address question

[larswik]

I am learning about the 'arrays' in my book right now. To better understand it I wrote my own test code but I am having a problem. I know this goes back to pointers and variable addresses in memory. Using a function, I am trying to add the total of the dice rolls and return that value to the 'hold' variable. Here is the code.

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

int dice (void);
int total (int rollNum, int *newValue);

int main (int argc, const char * argv[]) {

	srand(clock()); // seed for random num gen
	
	int rolls [13], i, hold; // create my variables
	
	hold = 0;
	
	for (i=0; i<=12; i++)  // loop till all the rolls have a number
		rolls[i] = dice(); // put a random number into each array var
	
	i = 0; // I rest the counter to 0
	
	for (i=0; i<=12; i++) // This loop adds all the dice rolls and adds the value to hold
		total (rolls[i], &hold);
	
	i = 0; // I rest the counter to 0
	
	for (i=0; i<=12; i++) // loop that displays each dice roll
		printf("Array #[%d] hold a %d value\n\n", i, rolls[i]);
	
	printf("the total added amount is %d", hold); // prints total of dice rolls
	
    return 0;
}

int dice (void) {        // Rolls a random 6 sided die
return (rand() % 6) + 1;
}

int total (int rollNum, int *newValue){

	&hold = rollNum + *newValue;
}

I would be thankful for any help in understanding.

Thanks.

 

[larswik]

Never mind I figured it out. I needed to change my prototype to

 int total (int rollNum, int *hold);

and then the function to this

int total (int rollNum, int *hold){

	*hold = rollNum + *hold;

Thanks though, its nice to solve these alone when I can!

Thanks!

 

[firestarter]

What were your compiler errors?

There are probably other errors in it, but the most glaring problems I can see are in 'total'.

You're declaring 'total' as returning an int, yet you're not... you're treating it as a function that updates 'hold' in place and doesn't return anything.

'total' won't be able to understand what you mean by '&hold' - since the variable name 'hold' only exists in the scope of the 'main' function.

This may be a better definition of 'total':

void total (int rollNum, int *newValue){

*newValue = rollNum + *newValue;
}

There might be other stuff wrong too... but it's past my bedtime so this will have to do.


Edit: OK, you pretty much got it. But you should still change 'total' to return void - because it isn't returning an int.

 

[larswik]

It kept giving me a error at the bottom about that. I switched it to void and it went away. I think I need a better understanding of VOID vs. INT in this case.

To my understanding VOID is a 1 way trip. I am sending something and not expecting a return value. But in this case I am sending something back to the 'hold' variable using pointers. Is that not returning a value? or is it because it is not returning a value to the Function call in main(), but sending the new value to another place outside that Function call, in this case a different variable?

Thanks!

 

[firestarter]

It kept giving me a error at the bottom about that. I switched it to void and it went away. I think I need a better understanding of VOID vs. INT in this case.

To my understanding VOID is a 1 way trip. I am sending something and not expecting a return value. But in this case I am sending something back to the 'hold' variable using pointers. Is that not returning a value? or is it because it is not returning a value to the Function call in main(), but sending the new value to another place outside that Function call, in this case a different variable?

Thanks!

You're not returning anything. You're using a handle/pointer to mess with something that you've been passed.

If you were returning an int, then you'd use the 'return' keyword in the function to make that return. You'd then assign what total returned to a variable in main().

 

[lee1210]

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

int dice (void);
void total (int rollNum, int *newValue);

int main (int argc, const char * argv[]) {

	srand(clock()); // seed for random num gen
	
	int rolls [13], i, hold; // create my variables
	
	hold = 0;
	
	for (i=0; i<=12; i++) *{// loop till all the rolls have a number
		rolls[i] = dice(); // put a random number into each array var
		total (rolls[i], &hold); //add the value to hold
		printf("Array #[%d] hold a %d value\n\n", i, rolls[i]);
	}
	
	printf("the total added amount is %d", hold); // prints total of dice rolls
	
	return 0;
}

int dice (void) { * * * *// Rolls a random 6 sided die
	return (rand() % 6) + 1;
}

void total (int rollNum, int *newValue){
	*newValue += rollNum;
}

the return type of a function says what the type of the single value returned is (that you can, say, assign to a variable). Changing variables when passing pointers is not related to the return type. You can only return one value, you can modify as many arguments passed via pointer as you want.

-Lee

P.S. The range of int is not evenly divisible by 6 so that's not really a fair roll. Not a big deal, just FYI.

 

[larswik]

I understand that concept now.

Thank you.

-Lars