I used the std::string::substr method inside the if block but if block isn't working

Question

There's a string with the word "WUB" in it, and I need to eliminate this word from the string.
So I used the substring method inside the if block so that while traversing the loop, if block can catch the WUB and instead print 1

#include <bits/stdc++.h>

using namespace std;

int main()
{
    string s="WUBhello";
    for(int i=0;i<s.length();i++){
    
        if(s.substr(i,i+2)=="WUB"){
            cout<<"1 ";
            i+=2;
        }
        else{
            cout<<s[i];
        }
    }
    return 0;
 }

I'm expecting it will print only "hello" , but it's printing "WUBhello"

Answer 1

1. The issues in your code:

There are several issues, some bugs and some bad practices (see side notes below).
To begin with, std::string::substr 's second parameter is count - i.e. the number of characters. So in your case it should simply be 3. You also don't check that i < s.length()-3 before using substr.
Then the whole logic of your loop is flawed. Using a debuggers will help you to get more insight. See: What is a debugger and how can it help me diagnose problems?.

---

2. A better approach:

If you want to remove a substring from a string, you can do the following:

1. Use std::string::find to find the substring.
2. Then use std::string::erase to remove it.

Code example:

#include <iostream>
#include <string>

int main()
{
    std::string str = "WUBhello";
    std::string toRemove = "WUB";

    // Find the substring:
    auto pos = str.find(toRemove);
    if (pos != std::string::npos)
    {
        // Remove it:
        str.erase(pos, toRemove.length());
    }

    std::cout << str << std::endl;
    return 0;
}

Output:

hello

If you want to remove multiple occurances of the substring, you can apply similar logic in a loop:

// Find the 1st occurance substring:
auto pos = str.find(toRemove);
while (pos != std::string::npos)
{
    // Remove it:
    str.erase(pos, toRemove.length());
    // Find the next one:
    pos = str.find(toRemove);
}

Some side notes:

1. Why should I not #include <bits/stdc++.h>?
2. Why is "using namespace std;" considered bad practice?

Answer 2

You can use std::stringstream too.

Note: Do check the function signature of any standard library function before using it. std::substr's second argument is length of substring.

#include <string>
#include <sstream>
#include <iostream>

std::string remove_substring(const std::string& s, const std::string& key)
{
    std::stringstream ss;
    for (int i = 0; i < s.length(); )
    {
        if (s.substr(i, key.length()) == key)
        {
            i += key.length();
        }
        else 
        {
            ss << s[i];
            i++;
        }
    }
    return ss.str();
}
int main()
{
    const std::string s = "WUBhello";
    const std::string key = "WUB";
    std::cout << remove_substring(s, key);
 }

Answer 3

The second argument of std::string::substr is exclusive, so it should be i+3. Also, even when the logic is correct, it will print "1 hello".