Return the number of times query occurs as a substring of src

Question

/** Return the number of times query occurs as a substring of src
 * (different occurrences may overlap).
 * Precondition: query is not the empty string "".
 * Examples: For src = "ab", query = "b", return 1.
 *           For src = "Luke Skywalker", query = "ke", return 2.
 *           For src = "abababab", query = "aba", return 3.
 *           For src = "aaaa", query = "aa", return 3.*/
public static int numOccurrences(String src, String query) {
    /* This should be done with one loop. If at all possible, don't have
     * each iteration of the loop process one character of src. Instead,
     * see whether some method of class String can be used to jump from one
     * occurrence of query to the next. */
    int count = 0;
    for (int i = 0; i < src.length(); i++) {
        int end = i + query.length() - 1;
        if (end < src.length()) {
            String sub = src.substring(i, end);
            if (query.contentEquals(sub)) 
                ++count;
        } 
    }return count;
}

I tested the code. If the src is "cherry" and the query is "err", then the output is expected to be 1 but it turns out to be 0. What's wrong with the code? BTW, I cannot use methods outside the String class.

Answer 1

What's wrong is that you're comparing err to:

i | sub
--|------
0 | ch
1 | he
2 | er
3 | rr

Notice that these strings you're comparing to look short, and you don't even get to the end of "cherry" before you stop checking for a match. So there are two things you need to fix in your code: the way you calculate end and the comparison between end and src.length() .

Hint: the second argument (ending index) to substring is exclusive .

Answer 2

Pseudo code:

init 'count' and 'start' to 0
while true do
  find first occurence of 'query' in 'source', start search at 'start'
  if found
    set 'start' to found position + 1
    set count to count + 1
  else
    break out of while loop
end while
return count

Tip: Use String#indexOf(String str, int fromIndex) when finding occurence of query in source

Answer 3

Check the existence of query in src and loop until it return false. With each occurrence, take the substring , update the count and repeat until query is not found in src.

Pseudo code:

int count = 0;
loop (flag is true) 
  int index = find start index of query in src;
  if (query is found) 
    src = update the src with substring(index + query.length());
    count++;
  else 
    flag = false;      
return count;

Answer 4

This does the job:

public static int numOccurrences(String src, String query) {
    int count = 0;
    for(int i = src.indexOf(query); i > -1;i = src.indexOf(query, i + 1)) 
        count++;
    return count;
}

Here, i is the index of query in src , but the increment term makes use of indexOf(String str, int fromIndex) , which javadoc says:

Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.

which is passed the index i plus 1 to start searching for another occurrence after the previous hit.

This also addresses the NFR hinted at in the comment:

Instead, see whether some method of class String can be used to jump from one occurrence of query to the next.