我将如何分隔我正在寻找的超过 1 个字符的 std::string ？

Question

I'm writing a program that can take in multiple names separated by a space, like "John Paul Andrew" and turns the names into keys for a std::map.

It works fine with two names, but three names or more separate the string into:

Andrew
John
John Paul

Here's how I'm splitting the strings:

for (int i = 0; i < names.size() - 1; i++) {
        std::string::size_type pos = names.find(" ", i);

        std::cout << "Position of space: " << pos << std::endl; // For debugging

        if (pos != std::string::npos) {
            std::string name = names.substr(0, pos);
            scoreboard[name] = 0;
        } else {
            std::string name = names.substr(names.rfind(" ") + 1);
            scoreboard[name] = 0;
        }

}

Is there a replacement for names.substr(0, pos); I can use to fix the issue?

Answer 1

You have i as you indexer, which can help you find the word being processed. So you don't need an old_pos as I had suggested, since i already can serve that purpose.

#include <iostream>
#include <string>
#include <map>

using std::cout;
using std::map;
using std::string;

static auto make_scoreboard(string names) -> map<string, int> {
    auto scoreboard = map<string, int>{};

    for (decltype(names.size()) i = 0; i < names.size() - 1; i++) {
        auto pos = names.find(" ", i);

        cout << "Position of space: " << pos << "\n"; // For debugging

        if (pos != string::npos) {
            auto name = names.substr(i, pos - i);
            scoreboard[name] = 0;
            i = pos;
        } else {
            string name = names.substr(names.rfind(" ") + 1);
            scoreboard[name] = 0;
            i = names.size();
        }
    }

    return scoreboard;
}


int main() {
    auto names = string{"John Paul Andrew"};
    auto scoreboard = make_scoreboard(names);

    for(auto&& s : scoreboard) {
        cout << s.first << "\n";
    }
}

Answer 2

You are looping through each character of names (except the last character,), using the current loop position as the starting position to find() a space character. That is not the logic you should be using.

Let's follow the logic, using your example of "John Paul Andrew" as the names to split.

• On the 1st-5th loop iterations, where i is 0..4 , names.find(" ", i) returns 4 each time. You then call names.substr(0, 4) which returns "John" . So you assign 0 to scoreboard["John"] 5 times .

• On the 6th-10th loop iterations, where i is 5..9 , names.find(" ", i) returns 9 each time. You then call names.substr(0, 9) which returns "John Paul" . So you assign 0 to scoreboard["John Paul"] 5 times .

• On the 11th-15th loop iterations, where i is 10..14 , names.find(" ", i) returns std::string::npos each time. You then call names.rfind(" ") which returns 9, and then you call names.substr(9 + 1) which returns "Andrew" . So you assign 0 to scoreboard["Andrew"] 5 times .

See the problem now?

At the very least, since you are looping through names one character at a time anyway, you should not be using names.find() at all, eg:

std::string::size_type start = 0;

for (std::string::size_type i = 0; i < names.size(); ++i) { // <-- no -1 !
    if (names[i] == ' ') {
        std::cout << "Position of space: " << i << std::endl; // For debugging
        if (i > start) {
            std::string name = names.substr(start, i - start);
            scoreboard[name] = 0;
        }
        start = i + 1;
    }
}

if (start < names.size()) {
    std::string name = names.substr(start);
    scoreboard[name] = 0;
}

Which, if you really want to use find() , can be re-written more like this:

std::string::size_type start = names.find_first_not_of(' ');
std::string name;

while (start != std::string::npos)
{
    std::string::size_type pos = names.find(' ', start);
    if (pos == std::string::npos)
    {
        name = names.substr(start);
        start = pos;
    }
    else
    {
        name = names.substr(start, pos - start);
        start = names.find_first_not_of(' ', pos + 1);
    }

    scoreboard[name] = 0;
}

However, there is a must easier way to split a string on spaces (and other whitespace characters):

std::istringstream iss(names);
std::string name;

while (iss >> name)
{
    scoreboard[name] = 0;
}