C ++中的字符串反转不是用新字符串替换旧字符串

Question

I am making a simple palindrome detector by converting an int to a string, reversing the string and making the string back to an int to compare it (could compare strings too, not the issue) but for some reason, the reverse string keeps the previous values and adds the new ones to the line rather than replacing them... why's that?

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

using namespace std;


int main(){
    string tempreverse;
    string temp;
    stringstream out;
    int tempnumber, tempnumber2; 
    int palindrome = 0;

    for(int i = 100; i < 111; i++){
        for(int j = 100; j < 111; j++){
            tempnumber = i * j;
            out << tempnumber;
            temp = out.str();
            tempreverse = string (temp.rbegin(), temp.rend());
            tempnumber2 = atoi(tempreverse.c_str());
            if (tempnumber == tempnumber2){
                palindrome = tempnumber;
            }
        }
    }

    cout << palindrome << "\n";
    cout << "Press ENTER to continue...";
    cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );
    return 0;
}

Answer 1

You need to clear your stringstream each time. You keep appending stuff to it and making the string bigger.

I would declare it inside the loop so that it goes out of scope each time. I've read about issues clearing stringstreams in the past.

Answer 2

This is because you are declaring your variable out outside of the nested for loops. this means the same out variable is used for every iteration and being appended to each time through the loop.

To fix simply this, you can move the stringsteam out line to inside of the inner for loop.

In general, you should be declaring your variables when you first use them to keep them in the smallest containing scope unless you have a particular reason not to. This also prevents accidental usage of uninitialized values. For instance, you can change the line temp = out.str() to string temp = out.str() and remove the string temp; line from the beginning.