需要一些帮助在我的while循环中发现逻辑错误

Question

I am learning C++, and I am doing some exercises in the book I am using. One of them asks to write a program that asks a user how many numbers they want to add up. Then prompt for the numbers the user wants to add or to enter '|' once finished. The numbers are then pushed into a vector. Part of the program asks to check if the size of the vector is equal to the original number of input items and that is where I keep getting an error.

cout << "Please enter the numbers and | once you are done: ";

while(true)
{

    for(int num; cin >> num; )
    {
        if(num == '|')
        {
            break;
        }

        ints.push_back(num);
    }

    if(ints.size() != n)
    {
        cout << "There are more or less numbers in the vector than originally specified\n"
            << "Vector will be cleared; please re-enter the values: ";
        ints.clear();
        continue;
    }
    else
    {
        break;
    }
}

The problem is that if the number of input is off, the message goes into an infinite loop and I am not sure how to fix it.

EDIT: n is the variable that holds in the number of values user wanted to enter.

Thanks!

Answer 1

num is an integer and cin >> num won't extract | symbol. Comparison num == '|' may not work as expected because num could have the numeric value of | ascii symbol even when user did not input any | symbol. You should properly handle end marker reading:

// loop will break when user enters `|` because it is not an integer
// setting failbit of cin
for(int num; cin >> num;)
{
    ints.push_back(num);
}
cin.clear(); // reset failbit making cin able to read again
// check the end marker entered by user
{
    string end_marker;
    cin >> end_marker;
    if("|" != end_marker)
    {
        // use of endl will flush the stream ensuring that
        // printed text won't stuck in the buffer
        cout << "Please use | as end marker" << endl;
        continue;
    }
}

Answer 2

Here is how I implemented it. I am worried about the logic in your while loop. I had been taught to avoid while(true) whenever possible. You know the logic behind how your code should work. With more practice you'll start to recognize the conditions you need to use. I am sure there are better ways to do it. But this is the way I tried it.

But to answer your question, the main reason it is failing is because integers cannot compare themselves with characters.

if(num == '|') 

That does not work since num is an integer and not a character.

Normally I would implement this in a class and since global variables are not highly looked upon I created my own namespace. You'll have to finish the rest of the logic yourself however:

#include <iostream>
#include <vector>
#include <string>

namespace global
{
    std::vector<std::string> strings;
    std::vector<int> ints;
    std::string a = " ";
    int num = 0;
}   

void doWork()
{   

    std::cout << "Please enter the number of integers you would like to add up: ";
    std::cin >> global::num;

    std::cout << "Please enter the numbers and | once you are done: ";
    while (global::a != "|")
    {
        std::cin >> global::a;
        global::strings.push_back(global::a);
    }

    global::strings.pop_back();

    for(auto &e : global::strings)
    {
         global::ints.push_back(std::stoi(e));
    }
}   


int main()
{

    doWork();

    if(global::ints.size() != global::num)
    {
        std::cout << "Size of vector does not match the size specified. Clearing vector" << std::endl;
        global::ints.clear(); 
        global::strings.clear();
        global::num = 0;
        global::a = " ";
        doWork();
    }   

}

I made a vector of char's and converted those into integers so that way you could add them up. The while loop should be checking for | rather than always running true. It then will check the size of the vector in the end, clear it if it does not match, and ask you to do it again. This is the best way that I could think of doing it.

EDIT: as VTT pointed out, char can only do one character at a time. I have converted it into a string in order to handle the conversion.

EDIT 2: reset the values of global::num and global::a to their default at the end of the failure in order to prevent crashing.