Swapping Strings!

[shaneos]

I'm writing a code to put names in alphabetical order. The code i've written wont compile as its teling me that 'left operand needs l value'. The lines it refers to is where i'm trying to swap around strins using pointers. I could use some help with this, thanks!

#include<stdio.h>
#include<string.h>
main(void){

#define CLASS_SIZE 5
#define NAME_LEN   30

	char *string[CLASS_SIZE][NAME_LEN];
	int i=0;
	int j=0;
    char *temp;

	for(i=0;i<CLASS_SIZE;i++){
		printf("Enter a surname, first  name and age\n");
		gets(string[i]);
	}

	for(i=0;i<CLASS_SIZE;i++){
		puts(string[i]);
	}
     
	for(j=0;j<CLASS_SIZE;j++){

    for(i=0;i<CLASS_SIZE;i++){
		if(strncmp(string[i],string[i+1],30)){
	   
	    temp=string[i];
	    string[i]=string[i+1];
	    string[i+1]=temp;
		}
	}
	}
	return 0;
}

[Smeagel13]

This wouldn't come about during compilation, but when it runs there will be a problem.

for(i=0; i<CLASS_SIZE; i++) {  // array size is 5 (CLASS_SIZE), variable i will equal 4 at some point ...  

  // when i is 4, string[4+1] is outside desired memory bounds, so compare result will be undesired.
  if(strncmp(string[i], string[i+1], 30)) {

    temp=string[i];
    string[i]=string[i+1];
    string[i+1]=temp;
	
  }

}

[shaneos]

Cheers, didnt think of that, i just replaced the 'i+1' with '(i+1)%5', i think that should solve the problem i think? Any ideas on the string swapping? It might be the combination of pointers and arrays?

[Smeagel13]

I would have gone with:

for(i = 0; i < (CLASS_SIZE - 1); i++) {

As for the strings, string[i]=string[i+1]; doesn't work because it's a multi dimensional array and your trying to assign outside a valid address.

Think of it as :

string[5]
  [30]
  [30]
  [30]
  [30]
  [30]

string[0][0] = 0x0A // Valid
string[0] = 0x0A // Invalid, there is no l-value to modify.

Change the declaration of string[][] to string[] and recompile.

At the minute you have an array of arrays holding char pointers, 30 * 5 = storage for 150 pointers, I don't think that was your intention.

In C, you can't copy strings (char arrays) using the assignment operator. What you are doing is re-assigning the places in memory.

You will need to do a bit of copying using loops and strcpy.

[Tumlee]

You're going to have to swap each string manually, character by character, if you're going to want this to work.

[Smeagel13]

We aren't swapping actual strings here, we're swapping the pointers.

Example:

char * ap_string[2];
char * temp_ptr = NULL;

// Set The Strings Initially

// Memory Location: ap_string[0] = 0xAA -> 'Bob'
// Memory Location: ap_string[1] = 0XBB -> 'Dan' 

temp_ptr = ap_string[0];

// temp_ptr = 0xAA

ap_string[0] = ap_string[1];
ap_string[1] = temp_ptr;

// Memory Location: ap_string[0] = 0xBB -> 'Dan'
// Memory Location: ap_string[1] = 0XAA -> 'Bob'