NSIndexSet indexPassingTest
- (NSUInteger)indexPassingTest:(BOOL (^)(NSUInteger idx, BOOL *stop))predicate
Parameters of [NSIndexSet indexPassingTest]
predicate
The Block to apply to elements in the set.
The Block takes two arguments:
idx
The index of the object.
stop
A reference to a Boolean value. The block can set the value to YES to stop further processing of the set. The stop argument is an out-only argument. You should only ever set this Boolean to YES within the Block.
The Block returns a Boolean value that indicates whether obj passed the test.
Return Value of [NSIndexSet indexPassingTest]
The index of the first object that passes the predicate test.
NSIndexSet indexPassingTest example.
- (NSUInteger)indexForImageGivenIndexSet:(NSIndexSet*)indexSet // Set of indexes you want to select from
prizeImageIndex:(NSUInteger)prizeIndex // The prize index
probabilityScale:(CGFloat)probabilityScale // The scale for the prize (8.0 for 8x less than the others)
{
// If invalid, return what was given
if (![indexSet containsIndex:prizeIndex]) {
return prizeIndex;
}
// Calculate our probabilities
// For a set of 4, with a scale of 8 on the prize, our probabilities would be
// 0.04 (prize), 0.32, 0.32, 0.32
double prizeProbability = (1.0 / indexSet.count) * (1.0 / probabilityScale);
double val = (double)arc4random() / RAND_MAX;
if (val < prizeProbability) {
return prizeIndex;
}
else {
// Select from the others in range equally
NSMutableIndexSet* newSet = [[NSMutableIndexSet alloc] initWithIndexSet:indexSet];
[newSet removeIndex:prizeIndex];
NSInteger selectedPosition = arc4random() % newSet.count;
__block NSInteger count = 0;
return [newSet indexPassingTest:^BOOL(NSUInteger idx, BOOL *stop) {
if (count == selectedPosition) {
*stop = YES;
return YES;
}
else {
count++;
return NO;
}
}];
}
}
prizeImageIndex:(NSUInteger)prizeIndex // The prize index
probabilityScale:(CGFloat)probabilityScale // The scale for the prize (8.0 for 8x less than the others)
{
// If invalid, return what was given
if (![indexSet containsIndex:prizeIndex]) {
return prizeIndex;
}
// Calculate our probabilities
// For a set of 4, with a scale of 8 on the prize, our probabilities would be
// 0.04 (prize), 0.32, 0.32, 0.32
double prizeProbability = (1.0 / indexSet.count) * (1.0 / probabilityScale);
double val = (double)arc4random() / RAND_MAX;
if (val < prizeProbability) {
return prizeIndex;
}
else {
// Select from the others in range equally
NSMutableIndexSet* newSet = [[NSMutableIndexSet alloc] initWithIndexSet:indexSet];
[newSet removeIndex:prizeIndex];
NSInteger selectedPosition = arc4random() % newSet.count;
__block NSInteger count = 0;
return [newSet indexPassingTest:^BOOL(NSUInteger idx, BOOL *stop) {
if (count == selectedPosition) {
*stop = YES;
return YES;
}
else {
count++;
return NO;
}
}];
}
}
