Question

I’m trying to write a method that generates all substrings of a given NSString with a fixed length, but I’m getting stuck. Here’s the idea of what I want:

-(int)countGoodSubstringsOfString:(NSString*)str ofLength:(int)len {
    int cnt = 0;
    for (NSString *substr substring of str of length len) {
        if ([self isGoodSubstring:substr ofString:str])
            cnt++;
    }
    return cnt;
}

By substring, I mean all possible combinations of len characters from str. If str = @"ABCDE" and len = 4, then I need to get 5 substrings: @"ABCD", @"ABCE", @"ABDE", @"ACDE", @"BCDE".

If len were fixed, then I could just write nested for loops to get all the substrings like this

NSString *substr = @"";
for (int i=0; i<str.length-len+1; ++i) {
    substr = [substr stringByAppendingString:[str substringWithRange:NSMakeRange(i,1)]];
    for (int j=i+1; j<str.length-len+2; ++j) {
        substr = [substr stringByAppendingString:[str substringWithRange:NSMakeRange(j,1)]];
        ...
            for(int k=j+1; k<str.length; ++k) {
                substr = [substr stringByAppendingString:[str substringWithRange:NSMakeRange(k,1)]];
                if ([self isGoodSubstring:substr ofString:str])
                    cnt++;

But since len is part of the input, I can’t know it in advance. How can I get around this problem?

Answer 1

EDIT: I finally managed to find the solution. It uses same ideas as yours, but it’s recursive, so you don’t have to know the length.

-(int)countGoodSubstringsOfString:(NSString*)str ofLength:(int)len recStr:(NSString *)recStr from:(int)from{
  /// C(str.length, str.length-len)
  if (!recStr) {
    recStr = @"";
  }
  if ([recStr length]==len) {
    if ([self isGoodSubstring:substr ofString:str])
      return 1;
    return 0;
  }
  int cnt = 0;
  for (int i=from; i<=str.length-len+[recStr length]; i++) {
    NSString *substr = [recStr stringByAppendingString:[str substringWithRange:NSMakeRange(i, 1)]];
    cnt += [self countGoodSubstringsOfString:str ofLength:len recStr:substr from:i+1];
  }
  return cnt;
}

Initial call should look something like this:

int cnt = [self countGoodSubstringsOfString:@"ABCDEF" ofLength:4 recStr:nil from:0];