Number of Occurrences of a Character in NSString

Question

I have an NSString or NSMutableString and would like to get the number of occurrences of a particular character.

I need to do this for quite a few characters -- uppercase English characters in this case -- so it would be nice for it to be quick.

Solution 1

replaceOccurrencesOfString:withString:options:range: will return the number of characters replaced in a NSMutableString.

[string replaceOccurrencesOfString:@"A" 
                        withString:@"B" 
                           options:NSLiteralSearch 
                             range:NSMakeRange(0, [receiver length])];

Solution 2

You can do this in one line. For example, this counts the number of spaces:

NSUInteger numberOfOccurrences = [[yourString componentsSeparatedByString:@" "] count] - 1;

Solution 3

Try this category on NSString:

@implementation NSString (OccurrenceCount)

- (NSUInteger)occurrenceCountOfCharacter:(UniChar)character
{
    CFStringRef selfAsCFStr = (__bridge CFStringRef)self;

    CFStringInlineBuffer inlineBuffer;
    CFIndex length = CFStringGetLength(selfAsCFStr);
    CFStringInitInlineBuffer(selfAsCFStr, &inlineBuffer, CFRangeMake(0, length));

    NSUInteger counter = 0;

    for (CFIndex i = 0; i < length; i++) {
        UniChar c = CFStringGetCharacterFromInlineBuffer(&inlineBuffer, i);
        if (c == character) counter += 1;
    }

    return counter;
}

@end

This one is approximately 5 times faster than the componentsSeparatedByString: approach.

Solution 4

Whenever you are looking for things in a NSString, try using NSScanner first.

NSString *yourString = @"ABCCDEDRFFED"; // For example
NSScanner *scanner = [NSScanner scannerWithString:yourString];

NSCharacterSet *charactersToCount = [NSCharacterSet characterSetWithCharactersInString:@"C"]; // For example
NSString *charactersFromString;

if (!([scanner scanCharactersFromSet:charactersToCount 
                          intoString:&charactersFromString])) {
    // No characters found
    NSLog(@"No characters found");
}

// should return 2 for this
NSInteger characterCount = [charactersFromString length];

Solution 5

Nowadays the first thing that come to my mind for something like that: NSCountedSet

NSString *string = @"AAATTC";

NSMutableArray *array = [NSMutableArray array];

[string enumerateSubstringsInRange:NSMakeRange(0, [string length]) options:NSStringEnumerationByComposedCharacterSequences usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
    [array addObject:substring];
}] ;
NSCountedSet * set = [[NSCountedSet alloc] initWithArray:array];

for (NSString *nucleobase in @[@"C", @"G", @"A", @"T"]){
    NSUInteger count = [set countForObject:nucleobase];
    NSLog(@"%@: %lu", nucleobase, (unsigned long)count);
}

logs:

C: 1
G: 0
A: 3
T: 2

View more solutions

Comments

[Elliot]

If I'm understanding the Documentation correctly, this would give the range of any of the characters in the set. But I need the count of each character.

[Nubzor]

my idea is to keep dictionary of char -> count pairs and then get the char at given index and increment it's count in dictionary ... or you could just iterate over the string and check if each character is in your set, if it is then increment it's count

[lawrence]

i have not been able to get this to work at all. I'm trying to count the number of spaces.

[borrrden]

@lawrence By default, spaces and whitespace are ignored by NSScanner.

[Bjinse]

This category works fine, but I have a question: can you use both UniChar (from CFString.h) and unichar (from NSString) as being the same?

[fabian789]

Creates a temporary array though.. For speed, Jacque's answer should be preferred.

[Gavin Hope]

You can call setCharactersToBeSkipped:(NSCharacterSet *)skipSet with nil as the skipSet - and the NSScanner won't skip any characters.

[meaning-matters]

Does this count a space at the beginning and end of yourString?

[Ethan Holshouser]

The speed difference will only matter if you're doing it thousands of times.

[Cœur]

@Bjinse both of them are typedef to unsigned short, so yes.

[Cœur]

It's unclear if you're referring to Abizern post or user251442 post.

[Cœur]

This won't return 2, because scanCharactersFromSet:intoString: will stop as soon as it doesn't get a match.

[Cœur]

@EthanHolshouser actually depends on the length of the string. And from my tests, a variation of CynicismRising answer is the fastest (see my answer for details).