error: implicit conversion changes signedness: 'int' to 'unsigned long'

Question

I am using clang++ "file.cpp" -std=c++14 -Weverything -Wno-c++98-compat to compile my program. Now I get this following error code:

warning: implicit conversion changes signedness: 'int' to 'unsigned long' [-Wsign-conversion]

The error line is this:

v_rand = rand() % (english.size());

The whole code of error:

void trainer_es (string lang)
{

    ifstream in("dictionary_es.txt");
    if(!in)
    {
        cout<<"ERROR"<<endl;
        }

    string line;
    int right=0;
    int wrong=0;
    vector<string> english, spanish;
    bool is_english = true; 
    while(in.eof()==0)  
        { 
            getline(in,line);
            if(is_english   
            {
                english.push_back(line);
            }
                else 
            {
            spanish.push_back(line);

            }
        is_english = !is_english;   
        }

    in.close();

    unsigned long v_rand;
    srand(unsigned (time(nullptr)));

    if (lang == "english")
    {
        for (unsigned long int i=0; i<deutsch.size()-1; i++)
        {

            v_rand = rand() % (english.size());
            cout<<"Translate the word: "<<english.at(v_rand)<<endl;
            cin>>line;

            if(line == spanish.at(v_rand))
            {
                cout<<"right!"<<endl;
                right++;
            }

            else 
            {
                cout<<"wrong"<<endl;
                cout<<"the correct word is: "<<spanish.at(v_rand)<<endl;
                wrong++;
            }

        } 
    }

Thanks for your help

What part of the error message do you not understand? – Scott Hunter Jan 31 '20 at 20:10 — Scott Hunter, Jan 31 '20 at 20:10

scohe001 · Answer 1 · 2020-01-31T20:52:32.180

rand() returns an int, which is signed. english.size() returns a size_t, which is unsigned. So, to compute the expression rand() % english.size(), the return value of rand() will need to be cast to an unsigned type.

This causes an implicit conversion from int to unsigned long, which is throwing this warning.

I'd strongly suggest looking into std::uniform_int_distribution, which you can specify a return type for. Here's a good example of how to use it.

error: implicit conversion changes signedness: 'int' to 'unsigned long'

1 Answers1