for s="0" and k=20 this code is printing YES infinite number of times but the for loop condition (0<=1-20) is not true. it should print NO. Please help me.

#include <bits/stdc  .h>

using namespace std;

int main()
{
    string s;
    cin>>s;
    int k;
    cin>>k;
    for(int i=0;i<=s.size()-k;  i){
        cout<<"YES"<<endl;
    }
    cout<<"NO"<<endl;
    return 0;
}

CodePudding user response：

The short answer is: unsigned integer underflow.

To see it more clearly, let's assign the values to variables of type size_t (which is the type that s.size() returns):

const size_t ssize = s.size();
const size_t ssize_minus_k = ssize-k;
cout << "ssize=" << ssize << " ssize_minus_k=" << ssize_minus_k << endl;
for(int i=0;i<=ssize_minus_k;  i){
    cout<<"YES"<<endl;
}

.... with the above lines, the output of the program looks like this:

$ ./a.out 
0
20
ssize=1 ssize_minus_k=18446744073709551597
YES
YES
[...]

Now to fix the problem, let's change the type of our variables from size_t to ssize_t (note the extra s, meaning "signed"):

const ssize_t ssize = s.size();
const ssize_t ssize_minus_k = ssize-k;
cout << "ssize=" << ssize << " ssize_minus_k=" << ssize_minus_k << endl;
for(int i=0;i<=ssize_minus_k;  i){
    cout<<"YES"<<endl;
}

.... and now we get behavior more like what we were expecting:

$ ./a.out 
0
20
ssize=1 ssize_minus_k=-19
NO