Whats Wrong with while....

[macifone]

I get an error when I d this...

Code: (Objective-C)

#import <Foundation/Foundation.h>
int j;

NSLog(@"Type in a number:");
scanf("%i",&j);

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
	
	while (j < 0) {
		j = -j;
	}
	


    [pool drain];
    return 0;
}

Whats wrong with this?

[toby]

Two things I can see going wrong here:

1. "j = -j;" flips j from positive to negative or vice versa. For example:

Code: (Objective-C)

int j = 154;

NSLog(@"%i", j); // prints 154

j = -j; // flips j from being positive to negative
NSLog(@"%i", j); // prints -154

j = -j; // j is currently negative so it flips back to positive
NSLog(@"%i", j); // prints 154 again

So what is happening is the user types in a negative number, that is obviously less than zero, so your loop enters and turns it into a positive number, this is then clearly greater than zero so the condition is no longer met and your loop exits. If the user enters a positive number, your loop is completely ignored. You might as well have written:

Code: (Objective-C)

if (j < 0)
   j= -j;

If you were intending for j to count down then:

Code: (Objective-C)

j = j -1;  //reduce j by 1
j = j -2;  //reduce j by 2
j--; //reduce j by 1 in shorthand
j -= 3; //reduce j by any number in shorthand;
 

2. Second problem you got going on here is the bit of code here:

Code: (Objective-C)

int j;  
  
NSLog(@"Type in a number:");  
scanf("%i",&j);  

The code itself is fine but were its located is not. The problem is that it is not within a function or method that is being called, and therefore its not being run at all. For the purpose of this program I would recommend you just put it in main for the moment like this:

Code: (Objective-C)

#import <Foundation/Foundation.h>  
  
int main (int argc, const char * argv[]) {  
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];  

int j;  
  
NSLog(@"Type in a number:");  
scanf("%i",&j);  
      
    while (j < 0) {  
        j = -j;  
    }  
      
  
  
    [pool drain];  
    return 0;  
} 

The last thing I would suggest is that you might want to print some output at some point so you know if your program has done anything. 

Code: (Objective-C)

 
#import <Foundation/Foundation.h>  
  
int main (int argc, const char * argv[]) {  
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];  

int j;  
  
NSLog(@"Type in a number:");  
scanf("%i",&j);  
      
    while (j > 0) {  
       NSLog(@"Count: %i", j);
       j--;
    }  
      
  
    [pool drain];  
    return 0;  
} 

Hope this helps

[macifone]

Thank you the part where you said to put the statements in the for function helped.

[toby]

no worries