Strange behavior of a FOR loop in objective-C

Question

Found a situation for which I can not find an explanation.

I have an array of arrays in objective-c. this situation the array has 4 empty arrays.

NSArray *dados = [[NSArray alloc] initWithObjects:
             [[NSArray alloc] init],
             [[NSArray alloc] init],
             [[NSArray alloc] init],
             [[NSArray alloc] init],
             nil];

I want a "for" to go through an internal array if it has two or more numbers.

for(int i=0; i<((NSArray *)[dados objectAtIndex:1]).count-1;i++){
        NSLog(@"do anything");
    }

this "for" enter into infinite loop.
However if I use a "NSLog" tells me that the condition is -1, and the "is" not supposed to happen

NSLog(@"%d",((NSArray *)[dados objectAtIndex:1]).count-1);

2014-02-20 15:37:42.563 iKL Time-sheet[31666:a0b] -1

how is it possible that this "for" becomes an infinite loop when the stop condition is 0 <-1?

thought to be a curious case, if someone can explain what is going to be able to better understand the language I thanked

---------------------//----------------
answer:

for(int i=0; i<(int)(((NSArray *)[dados objectAtIndex:1]).count-1);i++){
            NSLog(@"do anything");
        }

with this cast works well, the explanation for this lies in the answer below

Eric Postpischil · Accepted Answer · 2014-02-20 15:48:22Z

9

count is an unsigned integer (NSUInteger). When the count is zero, subtracting one wraps, producing a large number. Zero is less than a large number.

answered Feb 20, 2014 at 15:48

Eric Postpischil

233k15 gold badges199 silver badges380 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Pedro Over a year ago

I realize what you are saying, but being so "NSLog" should print a large number. right?

Eric Postpischil Over a year ago

@NSLog: You told NSLOG to expect an int, since that is what the %d conversion specifier means. You passed it an unsigned integer. It happened to reinterpret the bits of the unsigned integer as if they were the bits of an int, and it printed the value that an int would have if it had those bits. That is not the value that an NSUInteger has when encoded with those bits.

Grady Player · Accepted Answer · 2014-02-20 15:54:14Z

2

your code is not obvious i would re-write with fast iteration

NSArray *dados = [[NSArray alloc] initWithObjects:
         [[NSArray alloc] init],
         [[NSArray alloc] init],
         [[NSArray alloc] init],
         [[NSArray alloc] init],
         nil];

for(NSArray * a in dados)
{
    if([a count] >1)
    {
        NSLog(@"do something");
    }
}

edit:

or

@synchronized(dados){

    for(int i=0; i<[dados count]);i++){
       if([[dados objectAtIndex:i]count]>1)
       NSLog(@"do anything");
    }
}

because you for loop wont make it to the end... you are combining two tests that logically cant be combined... unless your array was sorted by number of elements first.

edited Feb 20, 2014 at 15:54

answered Feb 20, 2014 at 15:48

Grady Player

14.6k3 gold badges50 silver badges79 bronze badges

3 Comments

StaRbUck42 Over a year ago

He may need to know the index he is currently at

Pedro Over a year ago

this code does not work because I know exactly what's inside that array I want to go. my problem is when it is empty for all other cases the code above is well

Grady Player Over a year ago

@pedro... not 100% sure what you mean... but replace the if() with one that makes sense... and if you know you dont need to loop anymore then break

Merlevede · Accepted Answer · 2014-02-20 15:52:17Z

0

Remove the -1 from count, otherwise even if you array has many objects it will always skip the last element.

for(int i=0; i<((NSArray *)[dados objectAtIndex:1]).count-1;i++){
        NSLog(@"do anything");
}

answered Feb 20, 2014 at 15:52

Merlevede

8,1781 gold badge26 silver badges40 bronze badges

1 Comment

Pedro Over a year ago

I want it to skip the last

geowar · Accepted Answer · 2014-02-21 16:55:09Z

0

NSArray *dados = @[@[],@[],@[]];

// I want a "for" to go through an internal array
// if it has two or more numbers (skipping the last one)
for( NSUInteger idx = 0;
     (dados[1].count >= 2) && (idx < dados[1].count - 1);
     idx++) 
{
    NSLog(@"do anything");
}

edited Feb 21, 2014 at 16:55

answered Feb 21, 2014 at 16:49

geowar

4,4631 gold badge31 silver badges24 bronze badges

Collectives™ on Stack Overflow

Strange behavior of a FOR loop in objective-C

4 Answers 4

2 Comments

3 Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

3 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related