get 'random' unique strings question

rickrets · 07-22-2010, 05:01 AM

Hey guys, I can't figure this out...

I have like 100 questions online, and grab 50 of them in this way:

Code:

array = [[NSMutableArray alloc] initWithCapacity:50];

for (int i = 0; i < 50; i ++) {

NSDictionary *stream = (NSDictionary *)[result objectAtIndex:arc4random()%result.count];

questions *q = [[questions alloc] initWithName:[stream valueForKey:@"bablabla.........];

[array addObject:q];

}

How can I ensure that it gets 50 different questions?

Can I somehow check if the object that's being added already exists?

rickrets · 07-22-2010, 06:37 AM

So I see stuff about fisher-yates, but does that also works for me?

I think I need to randomize the informatio server side

That Don Guy · 07-22-2010, 08:50 AM

Here is how I would do it:

Code:

int questionNumbers[100];
int i;
int j;
int r;

for (i = 0; i < 100; i++)
{
    questionNumbers[i] = i;
}
for (i = 99; i > 0; i--)
{
    r = arc4random() % (i + 1);
    if (r < i)
    {
        j = questionNumbers[i];
        questionNumbers[i] = questionNumbers[r];
        questionNumbers[r] = j;
    }
}

questionNumbers[] now contains the integers from 0 to 99 in a random order. Take the first 50 entries one at a time to determine which questions to use.

-- Don

rickrets · 07-22-2010, 09:24 AM

That's awesome Don!

That is working like a charm

Changed it a (tiny) bit:

Code:

int questionNumbers[result.count];
int i;
int j;
int r;

for (i = 0; i < result.count; i++)
{
    questionNumbers[i] = i;
}
for (i = 99; i > 0; i--)
{
    r = arc4random() % (i + 1);
    if (r < i)
    {
        j = questionNumbers[i];
        questionNumbers[i] = questionNumbers[r];
        questionNumbers[r] = j;
    }
}

But how can I change this:

Code:

for (i = 99; i > 0; i--)
{
    r = arc4random() % (i + 1);
    if (r < i)
    {
        j = questionNumbers[i];
        questionNumbers[i] = questionNumbers[r];
        questionNumbers[r] = j;
    }
}

To make it work like this:

Code:

for (i = result.count; i > 0; i--)
{
    r = arc4random() % (i + 1);
    if (r < i)
    {
        j = questionNumbers[i];
        questionNumbers[i] = questionNumbers[r];
        questionNumbers[r] = j;
    }
}

With this I'm getting an error..

rickrets · 07-22-2010, 09:26 AM

Code:

index 112498768 beyond bounds [0 .. 101]'

While the result.count is 102 when reading it with NSLog.

Nevermind, count starts from 1 I guess, so it IS out of bounds... did this:

Code:

for (i = (result.count - 1); i > 0; i--)
{
    r = arc4random() % (i + 1);
    if (r < i)
    {
        j = questionNumbers[i];
        questionNumbers[i] = questionNumbers[r];
        questionNumbers[r] = j;
    }
}

That Don Guy · 07-22-2010, 09:38 AM

You don't want:

Code:

for (i = result.count; i > 0; i--)

You want:

Code:

for (i = result.count - 1; i > 0; i--)

questionNumbers[] only has result.count entries, so the largest index for questionNumbers[] is (result.count - 1), but if i = result.count, the loop is trying to access questionNumbers[result.count], which does not exist.

-- Don

rickrets · 07-22-2010, 09:40 AM

Hehe, just fixed that yeah, thanks for the help on the subject! Cheers

headkaze · 07-22-2010, 10:19 AM

Quote:

Originally Posted by That Don Guy

You want:

Code:

for (i = result.count - 1; i > 0; i--)

Actually you want

Code:

for (i = result.count - 1; i >= 0; i--)

That Don Guy · 07-22-2010, 10:36 AM

Quote:

Originally Posted by headkaze

Actually you want

Code:

for (i = result.count - 1; i >= 0; i--)

No, as when i = 0, arc4random() % (i + 1) will always return 0, so no switching will take place (since it would always switch questionNumbers[0] with questionNumbers[0]).

-- Don