Wrong output when printing elements of a vector of vectors in C++

Question

In my main I have this:

vector<vector<u8>> matrix(size.first, vector<u8>(size.second));
for(auto i = 0; i < matrix.size(); ++i){
    for(auto j = 0; j < matrix[0].size(); ++j){
        matrix[i][j] = 5;
    }
}
print_matrix(matrix);

The function print_matrix is:

void print_matrix(vector<vector<u8>>& m){
    cout << "_______________________\n";
    for(size_t i (0) ; i < m.size(); ++i){
        auto& row = m[i];
        for(size_t j (0); j < row.size(); ++j){
            cout << row[j] << "-";
        }
        cout << endl;
    }
}

I was expecting to get a matrix full of 5 printed on the stdout, but instead I get:

_______________________
-----
-----
-----
-----
-----

Why there are no numbers printed in the output?

Don't forget about the [range-based `for` loop](https://en.cppreference.com/w/cpp/language/range-for). — tadman, Apr 14 '20 at 20:05
it works with the range based for loop suggested in the answer, but why isn't my code working? — ninazzo, Apr 14 '20 at 20:08
ARe you sure you're not getting the character 0x05 printed to your output and your terminal may not have a glyph to print it? — Stephen M. Webb, Apr 14 '20 at 20:15
Change the 5 to a 7, and depending on your output device, herald the sound of the beeps. — PaulMcKenzie, Apr 14 '20 at 20:16
you already have an answer, but still you should clarify in the question what type is `u8`, without that the question (and answer) is not really useful — 463035818_is_not_an_ai, Apr 14 '20 at 20:23

NutCracker · Accepted Answer · 2020-04-14T20:18:07.760

1

Try using range-based for loops like:

void print_matrix(std::vector<std::vector<u8>> const& m){
    std::cout << "_______________________\n";
    for (auto const& i : m) {
        for (auto const j : i){
            std::cout << static_cast<unsigned>(j) << "-";
            // or
            // std::cout << +j << "-";
        }
        std::cout << std::endl;
    }
}

Remember that uint8_t is considered to be a raw character and, thus, it need to be converted to unsigned int in order to get it outputted as a numeric value.

edited Apr 14 '20 at 20:18

answered Apr 14 '20 at 20:04

NutCracker

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I'll try, but what's the problem in my code? – ninazzo Apr 14 '20 at 20:05
1

@ninazzo take a look at my further explanation about outputting `uint8_t` – NutCracker Apr 14 '20 at 20:09
1

`uint8_t` is considered a raw character, and `5` is the ENQ character. – tadman Apr 14 '20 at 20:10
Why `static_cast`? Why not just `for ( unsigned j : i )`? – Ben Voigt Apr 14 '20 at 20:25
@BenVoigt you are right... that can be used too... – NutCracker Apr 14 '20 at 20:28

Wrong output when printing elements of a vector of vectors in C++

1 Answers1