为什么擦除命令结束我的while循环？

Question

So I have a piece of code which works pretty well. The goal was to filter through my whatsapp - chat history. First thing I wanted was to sort out such that only my messages which start with 'Ben: ...' are sorted out and the rest discarded.

I succeeded with this but only because of a weird feature I can't understand. Because of some older attempts I have an erase command in my code, that for now has no real purpose. Yet without it my while loop, keeps looping infinitely.

Why does the erase command help me finish the loop ?

I tried removing it, and played around with other ideas. Nothing worked.

#include <iostream>
#include <string>
#include <fstream>

using namespace std;

int main(){
    string text;
    string deli = "Ben:";

    string token;
    size_t pos = 0;

    ifstream myfile;
    myfile.open("WAEZ.txt");

    while( getline(myfile, text)){
        while((pos = text.find(deli)) != std::string::npos){
            token = text.substr(pos,std::string::npos);
            cout << token << endl;
            text.erase(0, pos + deli.length());
        }
    }    
}

I'd like to have this code work without the erase command which I am not using.

Answer 1

text.find(deli)) is finding something in your string, and if it is found, then text.find(deli)) != std::string::npos is true. If you call it again with the same text and the same deli , then the result will be the same.

In your loop, text.erase(0, pos + deli.length()); changes the string text . Without this part, nothing in your loop changes the text or the deli . That's why it results in an infinite loop: if text.find(deli)) != std::string::npos is true the first time, it is true forever and nothing will stop it as there also isn't any break or any other kind of logic in your code that can get out of the loop.

Answer 2

To mimic your code without erase, you might use the extra parameter of find to specify start position of the search:

while (getline(myfile, text)) {
    std::size_t origin = 0;
    while((pos = text.find(deli, origin)) != std::string::npos){
        token = text.substr(pos, std::string::npos);
        cout << token << endl;
        origin += deli.length();
    }
}

But as you describe your problem, C++20 starts_with seems fulfill your needs:

while (getline(myfile, text)) {
    if (text.starts_with(deli)){
        std::cout << text << std::endl;
    }
}