Here is the code:

#include<iostream>
using namespace std;

int lengthOfLastWord(string s) {
    int i,j,n=0;
    for(i=0;s[i]!=0;i  ){
        n  ;
    }
    string s1;
    for(i=n,j=0;s[i]!=' ';i--,j  ){
        s1[j]=s[i];
    }
    cout<<s1;
};

int main(){
    string s;
    getline(cin,s);
    lengthOfLastWord(s);
}

What is the problem with the string s1? If s1 is in the for loop, s1 prints successfully.

CodePudding user response：

First and foremost, your lengthOfLastWord() function doesn't return anything so have its return type be void:

void lengthOfLastWord(string const& s) {
    //                       ^^^^^^ Preferably use const reference here to avoid making unnecessary copies at each invocation to 'lengthOfLastWord()'
    /* ... */
}

Now, the real problem is that you never initialized s1 before accessing it with s1[j] which leads to Undefined Behavior.

So, to fix your problem, just replace this line:

string s1;

with this:

string s1(n, '\0');

Demo

Alternatively, you can use std::string::push_back():

// ...
string s1;
for(i = n; s[i] != ' '; i--)
    s1.push_back(s[i]);
cout << s1;

Demo using std::string::push_back