macOS NSArray count error

[Richard Birkett]

Hello,
I have this code:

if ([variableFactors count] >= 1) {
		short j = 0;
		bool shouldRun = true;
		while (shouldRun) {
			if ([(RBMVariableFactor *)[variableFactors objectAtIndex:j] exponent] == 0) {
					//if factor is empty then remove
				[variableFactors removeObjectAtIndex:j];
			}else{
				j++;
			}
			shouldRun = (j <= ([variableFactors count] - 1));
		}
	}

When I have variableFactors with 1 object that satisfies the second if statement, it gets removed before the shouldRun line.
At this point I check with breakpoints that j is 0, variableFactors says '0 objects', I set a breakpoint for after shouldRun is reassigned and it is still true!!! The while loops again and the code crashes because I'm accessing an empty array.
Please help...

Richard

 

[robvas]

I'm getting a compile error on the line ([(RBMVariableFactor....

Your last line where you do 'shouldRun = ....' is the problem.

Look at this program:

    NSMutableArray *variableFactors = [NSMutableArray arrayWithObjects:@"Foo", nil];
        
        NSLog(@"Count is %lu", [variableFactors count]);
        [variableFactors removeObjectAtIndex:0];
        
        NSLog(@"Count is %lu after removing object at index 0", [variableFactors count]);

        NSLog(@"Count - 1 is %lu", [variableFactors count] - 1);

It outputs:

MBHelp[1003:903] Count is 1
MBHelp[1003:903] Count is 0 after removing object at index 0
MBHelp[1003:903] Count - 1 is 18446744073709551615

The 'count' method returns an unsigned int - which can't be negative, which is probably what you were expecting to compare j to.

 

[JoshDC]

[NSArray count] returns an unsigned integer. Taking 1 from 0 will cause it to wrap around to its maximum value (NSUIntegerMax), so this:

NSLog(@"%d", 0 <= ([[NSArray array] count] - 1));

Will print 1, as it evaluates to 0 <= NSUIntegerMax.

You can either cast the value to a signed integer:

j <= (NSInteger)([[NSArray array] count] - 1)

Or use a different inequality:

j < [[NSArray array] count]

I'd suggest the second option.

As a side point, you'd usually see people checking for a non-zero count like this:

if ([variableFactors count] > 0)

Or more succinctly as:

if ([variableFactors count])

 

[Richard Birkett]

Thank you both so much, I removed the = and -1, now it works perfectly.

 

[gnasher729]

Hello,
I have this code:

if ([variableFactors count] >= 1) {
		short j = 0;
		bool shouldRun = true;
		while (shouldRun) {
			if ([(RBMVariableFactor *)[variableFactors objectAtIndex:j] exponent] == 0) {
					//if factor is empty then remove
				[variableFactors removeObjectAtIndex:j];
			}else{
				j++;
			}
			shouldRun = (j <= ([variableFactors count] - 1));
		}
	}

When I have variableFactors with 1 object that satisfies the second if statement, it gets removed before the shouldRun line.
At this point I check with breakpoints that j is 0, variableFactors says '0 objects', I set a breakpoint for after shouldRun is reassigned and it is still true!!! The while loops again and the code crashes because I'm accessing an empty array.
Please help...

Richard

Just saying:

for (NSInteger j = [variableFactors count] - 1; j >= 0; --j)
    if ([(RBMVariableFactor *)[variableFactors objectAtIndex:j] exponent] == 0)
        [variableFactors removeObjectAtIndex:j];