C++ 无法解决我的问题。 我有控制台错误

Question

I dont know C++ but I need help, solution. I want to read from file words.txt strings and if separator ',' comes then program should cut off this line and go further to the next one. Its working 50/50 because I have error in console:

terminate called after throwing an instance of 'std::lenght_error'
what(): basic_string::_M_replace_aux

Source code:

#include <iostream>
#include <string>
#include <map>
#include <cmath>
#include <fstream>
#include <stdio.h>

using namespace std;

void print( string::size_type n, string const & s )
{
    if( n == string::npos ) {
        std::cout << "not found\n";
    } else {
        cout << "found: " << s.substr( n ) << '\n';
    }
}


int main()
{
    map < char, int > pythagorean;

    pythagorean[ 'a' ] = 7;
    pythagorean[ 'b' ] = 8;
    pythagorean[ 'c' ] = 1;
    pythagorean[ 'd' ] = 2;
    pythagorean[ 'e' ] = 3;
    pythagorean[ 'f' ] = 4;
    pythagorean[ 'g' ] = 5;
    pythagorean[ 'h' ] = 6;
    pythagorean[ 'i' ] = 7;
    pythagorean[ 'j' ] = 8;
    pythagorean[ 'k' ] = 9;
    pythagorean[ 'l' ] = 1;
    pythagorean[ 'm' ] = 2;
    pythagorean[ 'n' ] = 3;
    pythagorean[ 'o' ] = 4;
    pythagorean[ 'p' ] = 5;
    pythagorean[ 'q' ] = 6;
    pythagorean[ 'r' ] = 7;
    pythagorean[ 's' ] = 8;
    pythagorean[ 't' ] = 9;
    pythagorean[ 'u' ] = 1;
    pythagorean[ 'v' ] = 2;
    pythagorean[ 'w' ] = 3;
    pythagorean[ 'x' ] = 4;
    pythagorean[ 'y' ] = 5;
    pythagorean[ 'z' ] = 6;


    string::size_type n;
    ifstream input( "words.txt" );
    ofstream output( "Results.txt" );
    string str;
    int counter = 0, cnt = 0;


    while( getline( input, str ) )
    {
        int sum = 0;

        n = str.find( ',' );
        str.resize( n );

        for( char charr: str )
        {
            sum += pythagorean[ charr ];
        }

        if( sum == 11 )
        {
            cout << str << "(" << sum << ")" << '\n' << "Pythagorean: " << '\t';
            output << "Pythagorean: " << '\t' << str << " = " << sum << endl;
            counter++;
        }
    }

    cout << "\n\n You have found: " << counter << " words" << '\n';
    output << "\n\n You have found: " << counter << " words" << '\n';

    return 0;
}

Can someone give me a solution? Im really bad in C++ and I need it to check a lot of datas in.txt file. Please:)

Answer 1

In

n = str.find( ',' );
str.resize( n );

if there is no comma in str , find will return npos and npos is an incredibly large number that a std::string cannot be resized to fit. This is precisely why npos is used to signal no match found: npos cannot possibly be a valid index in the string.

To avoid the problem test n against npos

n = str.find( ',' );
if (n != std::string::npos)
{ // found a comma
    str.resize( n );
}
else
{ // did not find a comma
    // Handle the error as best fits the needs of the program.
    // Perhaps this is not an error and the program can continue parsing with 
    // the whole string.
    // But if there must be a comma and one cannot be found, there is a protocol
    // error in the file and the user should be notified before continuing or 
    // halting the program.
}