copy first letters from string to another string-CodePudding

Well, I tried to find an answer to my question.

Basically, I have two string variables, and I want to copy the first letters until to space, to another string variable, but without success. I can't use strcpy().

#include <iostream>
#include <string>

using namespace std;
    
int main() {

    string string1;
    string string2;

    cout << "String1:" << endl;
    getline(cin, string1);

    for (int i = 0; (i < string1.length()) && (string1[i] != ' '); i  ){
        string2[i] = string1[i];
    }

    cout << string2;

    return 0;

}

CodePudding user response：

The variable string2 is defined to be empty, therefore string2[i] is undefined.

Use string2.push_back(string1[i]);

CodePudding user response：

There is no need to reinvent the wheel. Just use standard C library:

auto pos = string1.find(' ');
string2 = (std::string::npos == pos) ? string1 : string1.substr(0, pos);

PS: Remy is right, you can use std::string::npos as second argument in substr. So it will be even simplier:

string2 = string1.substr(0, string1.find(' '));

CodePudding user response：

string2 is empty, so you can't copy from string1[i] into string2[i]. You can append to the string like this though:

string2  = string1[i];

but with insuccess. I can't use the strcpy.

No need. std::string has a copy assignment operator and a substr member function so to copy the part of the string before the first space:

std::string string2;

auto pos = string1.find(' ');

if(pos != std::string::npos) {        // space found
    string2 = string1.substr(0, pos); // copy the substring [0, pos)
} else {
    string2 = string1;                // copy te whole string
}

If pos returns npos you can use that to copy the whole string, so you could strip the if/else above:

std::string string2;

auto pos = string1.find(' ');

string2 = string1.substr(0, pos);

So your loop is not needed.

You could construct string2 directly from the find too:

std::string string2(string1, 0, string1.find(' '));