Search a string for all occurrences of a substring in C++

Question

Write a function countMatches that searches the substring in the given string and returns how many times the substring appears in the string.

I've been stuck on this awhile now (6+ hours) and would really appreciate any help I can get. I would really like to understand this better.

int countMatches(string str, string comp)
{
    int small = comp.length();
    int large = str.length();
    int count = 0;

    // If string is empty
    if (small == 0 || large == 0) {
        return -1;
    }

    // Increment i over string length
    for (int i = 0; i < small; i++) {
        // Output substring stored in string
        for (int j = 0; j < large; j++) {
            if (comp.substr(i, small) == str.substr(j, large)) {
                count++;
            }
        }
    }

    cout << count << endl;
    return count;
}

When I call this function from main, with countMatches("Hello", "Hello"); I get the output of 5. Which is completely wrong as it should return 1. I just want to know what I'm doing wrong here so I don't repeat the mistake and actually understand what I am doing.

Did you try stepping through your code with a debugger, while investigating the values, of variables, at each execution step? — Algirdas Preidžius
– Algirdas Preidžius, Commented Sep 30, 2019 at 14:21

RJ Trujillo · Accepted Answer · 2019-09-30 14:27:13Z

1

I figured it out. I did not need a nested for loop because I was only comparing the secondary string to that of the string. It also removed the need to take the substring of the first string. SOOO... For those interested, it should have looked like this:

int countMatches(string str, string comp)
{
    int small = comp.length();
    int large = str.length();
    int count = 0;

    // If string is empty
    if (small == 0 || large == 0) {
        return -1;
    }

    // Increment i over string length
    for (int i = 0; i < large; i++) {
        // Output substring stored in string
        if (comp == str.substr(i, small)) {
            count++;
        }
    }

    cout << count << endl;
    return count;
}

answered Sep 30, 2019 at 14:27

RJ Trujillo

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2 Comments

Superlokkus Over a year ago

Damn I was in the middle of an answer

Pete Becker Over a year ago

Yes, that works. But each of those calls to substr creates a temporary string, which gets destroyed after comparing. That's a lot of memory thrashing. See my answer for another approach.

Pete Becker · Accepted Answer · 2019-09-30 15:42:20Z

0

The usual approach is to search in place:

std::string::size_type pos = 0;
int count = 0;
for (;;) {
    pos = large.find(small, pos);
    if (pos == std::string::npos)
        break;
    ++count;
    ++pos;
}

That can be tweaked if you're not concerned about overlapping matches (i.e., looking for all occurrences of "ll" in the string "llll", the answer could be 3, which the above algorithm will give, or it could be 2, if you don't allow the next match to overlap the first. To do that, just change ++pos to pos += small.size() to resume the search after the entire preceding match.

answered Sep 30, 2019 at 15:42

Pete Becker

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Comments

Davide Spataro · Accepted Answer · 2019-09-30 14:31:01Z

0

The problem with your function is that you are checking that:

Hello is substring of Hello
ello is substring of ello
llo is substring of llo
...

of course this matches 5 times in this case.

What you really need is:

For each position i of str
check if the substring of str starting at i and of length = comp.size() is exactly comp.

The following code should do exactly that:

size_t countMatches(const string& str, const string& comp)
{
    size_t count = 0;
    for (int j = 0; j < str.size()-comp.size()+1; j++)
         if (comp == str.substr(j, comp.size()))
              count++;
    return count;
}

answered Sep 30, 2019 at 14:31

Davide Spataro

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