ANSI C: Array element swapper

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February 27th, 2008, 01:11 AM

angel.white

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ANSI C: Array element swapper

Hello, I am trying to write a function which swaps values. This will be used in a sorter I am writing for a school project.

Currently I can sort by swapping the actual values, ie if
a and b are integers, I can swap them with this code:

Code:

void swap( int *a, int *b)
{
int c;
c=*a;
*a=*b;
*b=c;
}

But that only works for nice simple number arrays. Now I need to apply this to an array list, where the elements are structures and not integers.

I realized that if I can simply swap what each element points to, then I can do this very easily.
ie my_struct[1] points to structure_a
and my_struct[2] points to structure_b

then if I can swap it so that it now goes like this:
my_struct[1] points to structure_b
my_struct[2] points to structure_a

I can easily swap elements in the array of structures without lengthy tedious copying of values, and instead simply redirect pointers.

However, I am having difficulty figuring out how to perform this task, and was hoping someone could help me by modifying the piece of code I wrote so that it worked in the manner I am shooting for. Then I will be able to see how it works, and adjust it appropriately to fit the program that I am writing.

Thank you.

February 27th, 2008, 06:43 AM

Annie79

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Is this what you want?

c Code:

Original - c Code
void swap(struct my_struct* a, struct my_struct* b) { struct my_struct* c; c = a; a = b; b = c; }

 
void swap(struct my_struct* a, struct my_struct* b) {
    struct my_struct* c;
    c = a;
    a = b;
    b = c;
}

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Last edited by Annie79 : February 27th, 2008 at 06:52 AM.

February 27th, 2008, 11:43 AM

angel.white

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Quote:

Originally Posted by Annie79

Is this what you want?

c Code:

Original - c Code
void swap(struct my_struct* a, struct my_struct* b) { struct my_struct* c; c = a; a = b; b = c; }

 
void swap(struct my_struct* a, struct my_struct* b) {
    struct my_struct* c;
    c = a;
    a = b;
    b = c;
}

Thank you, but that doesn't seem to work. That seems to correctly redirect the pointers in the swap() function, but not the pointers that was sent to it.

if we pretend there is a line of yarn from
int1 and int2, to the memory that holds their values, then I want the program to clip the ends of the line of yarn, and move it over to the other address. So each variable now points to the thing in memory that the other guy used to point to. This way I don't have to change any data at all.

I'm not sure if this can be done, because I don't really know how arrays work. If they are just the next consecutive spot in memory, then I don't think my plan will work, because if I reorder what element points to what memory, they won't be consecutive anymore, and the array won't be able to get from one element to the next :/

But I'm hoping that isn't the case.

Anyway, thank you for spending time on it, even though it wasn't exactly what I was looking for.

February 27th, 2008, 11:56 AM

jim mcnamara

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Code:

#include <stdlib.h>
#include <string.h>

void swap(void *a, void *b, size_t element_sz)
{
	unsigned char *tmp=malloc(element_sz);
	memcpy(tmp, a, element_sz);
	memcpy(a, b, element_sz);
	memcpy(b, tmp, element_sz);
	free(tmp);
}

int main()
{
	typedef
	struct
	{
	   int a;
	   char b[4];
	} mystruct_t;

	mystruct_t a={1,"aaa"};
	mystruct_t b={2,"bbb"};
	printf("a=%d %s b=%d %s\n",a.a, a.b, b.a, b.b);
	swap(&a, &b, sizeof(mystruct_t));
	printf("a=%d %s b=%d %s\n",a.a, a.b, b.a, b.b);
	return 0;
}

output:

Quote:

/home/jmcnama> cc t.c
/home/jmcnama> a.out
a=1 aaa b=2 bbb
a=2 bbb b=1 aaa

Comments on this post

Annie79 agrees!

angel.white agrees!

February 27th, 2008, 01:43 PM

jnsg

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Altering Annie79's code slightly:

c Code:

Original - c Code
void swap(struct my_struct_t a, struct my_struct_t b) { struct my_struct_t* c; c = a; a = b; b = c; }

void swap(struct my_struct_t** a, struct my_struct_t** b) {
    struct my_struct_t* c;
    c = *a;
    *a = *b;
    *b = c;
}

Then you can pass pointers to the array elements:

c Code:

Original - c Code
/* assuming my_struct is an array of my_struct_t */ swap(&(my_struct[1]), &(my_struct[2]));

/* assuming my_struct is an array of my_struct_t */
swap(&(my_struct[1]), &(my_struct[2]));

If you were doing this in C++, I would say to pass by reference; this is essentially the C version of that.

Comments on this post

angel.white agrees!

Last edited by jnsg : February 28th, 2008 at 01:10 PM.

February 27th, 2008, 10:35 PM

angel.white

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Thank you everybody for all your help, I've been able to successfully get the swap function to work ^_^

Now my whole sorter works, and I can move onto the next stage in my project.

ANSI C: Array element swapper

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