Thread: any help would be appreciated.

Hi all-
Could someone please help me with this assignment? I have a swap code that works, but now I need to alter it so the swap function appears in the main function after I call the function. Does this make sense? Maybe I do not understand correctly. This is where I am.

Code:

#include <stdio.h>


int main(void)
{
	void swap(int x, int y);


	int first_num = 10;
	int second_num = 50;


	printf("\nThe value of first_num is %d \nThe value of second num is %d \n",  
                           first_num,  second_num );


	swap(first_num, second_num);
	{
	int local_int;


	local_int = x;
	x = y;
	y = local_int;


	printf("\nIn the function first_num is %d\nIn the function second_num is %d \n", x, 
                      y );
	}
	
	printf("\nThe value of first_num is %d \nThe value of second num is %d \n",  
                           first_num,  second_num );
return 0;
}

My error reads that 'x and y are not declared in this scope.' Any help would be appreciated. Thanks.

C doesn't allow nested function definitions. I suggest the following design:

Code:

#include <stdio.h>

static void swap(int *x, int *y)
{
    int tmp = *x;

    *x = *y;
    *y = tmp;
}

int main(void)
{
    int a = 10, b = 20;

    printf("%d %d\n", a, b);
    swap(&a, &b);
    printf("%d %d\n", a, b);
    return 0;
}

Originally Posted by rosemary

I have a swap code that works

Your problem is threefold.

First, you need to declare the swap function before the main. The declaration is the void swap(int x, int y); line. Its purpose is to tell the compiler that you'll be defining the the function with that name and signature later on.

Second, you need to define the function somewhere else. Right now, you have some form of definition within the main() function, and that is wrong. Move it either before or after main().

Third, that signature for your swap function will not work. In C, variables are passed by value. That means that any changes to the variables (x and y) will only be visible within the function. What you need to do, is to pass pointers to the variables you wish to swap, so that the function can swap the contents pointed by the two pointers instead. The main function then calls it with e.g. swap(&u, &v);

Personally, I prefer to define a generic swap function, with the signature

Code:

void swap(void *const p1, void *const p2, size_t const size);

Given two variables x and y of the same type, you can swap their contents using swap(&x, &y, sizeof x); no matter what the type actually is.

I didn't think C allowed nested functions. That's why I was confused. I already have a code that works with the function after main. I will contact the professor and ask for a clarification. Thank you.

Sorry, I meant I have another code that works. The one I pasted is the one I was trying to make into a nested function, but obviously it is not possible. Thanks for your input.

Personally, I prefer to define a generic swap function, with the signature
void swap(void *const p1, void *const p2, size_t const size);

What did you do for the temp variable?

What does this mean to you-
"Now modify your program so that the swapping performed by the function will appear in the main program after calling the function." ?

I think he's trying to say the same sort of things grumpy is.

Originally Posted by grumpy

That's not true - it's optional. Function declarations are permitted within function bodies (and the definition is only visible within scope of the containing function). It is function definitions (implementations) that are not permitted in functions.

Could you explain more?

This is my beginning code-

Code:

#include <stdio.h>


int main(void)
{
	void swap(int x, int y);


	int first_num = 10;
	int second_num = 50;


	printf("\nThe value of first_num is %d \nThe value of second num is %d \n",  
                           first_num,  second_num );


	swap(first_num, second_num);


	printf("\nThe value of first_num is %d \nThe value of second num is %d \n",  
                           first_num,  second_num );
}


void swap(int x, int y)
{
	int local_int;


	local_int = x;
	x = y;
	y = local_int;


	printf("\nIn the function first_num is %d\nIn the function second_num is %d \n ", x, 
                      y );
}

Originally Posted by whiteflags

What did you do for the temp variable?

You treat the two as char arrays of the specified size, and swap char by char. A straightforward implementation is a dozen lines of less, plus comments.

An optimized implementation using GCC/Intel CC/Pathscale built-ins and unsigned integer transfers (only if aligned on alignment-sensitive architectures) is a bit longer. The built-ins let you do optimizations when e.g. the size is a compile-time constant, or the compiler knows the pointers are aligned.

Originally Posted by grumpy

Function declarations are permitted within function bodies (and the definition is only visible within scope of the containing function).

Technically, yes. Try submitting code that does that to any project, and you'll be asked to rewrite it.

Just because something is technically allowed, does not mean you should encourage it.

I firmly believe giving learners simple rules that lead to good practices, to start with. Then, when they grasp concepts such as scope, you can point them to the actual rules.