I do not understand the use of std :: setw

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I do not understand the use of std :: setw

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#1 by (5 votes)
#2 by (1 votes)

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I'm doing an exercise in a C ++ book that I use to learn the language, and I can not fully understand the use of std :: setw and its use in conjunction with std :: left (default) and std :: right . In fact, I do not know how, I've found the key to do what I wanted to do, but I do not understand why it works ..

Tell me if I'm wrong, please, but according to what I think:

The for j loop causes the first triangle to unfold *.
The cout < < right < < setw (16 - i); causes the b's to be separated from * in each line to the right of each *, in 16 -i spaces, starting from the immediate character to each *.
The for z loop adds the triangle consisting of letters b.
The cout < < right < < setw (4 + i * 2); is where I already get lost, because I did not get that result that is what I want, and do not tell me why, if it was intuition or what, but I thought put the "* 2" and so if it came out xD

Please, I know it's a lot to ask, but if someone could explain to me why it works and why I messed with that instruction so much, I would greatly appreciate it.

    #include <iostream>

    using std::cout;
    using std::endl;

    #include <iomanip>

    using std::setw;
    using std::right;
    using std::left;

    int main() {
        for(int i = 1; i <= 10; i++) {
            for(int j = i; j >=1; j--)
                cout << '*';

            cout << right << setw(16 - i);

            for(int z = 11 - i; z >= 1; z--)
                cout  << 'b';

            cout << right << setw(4 + i * 2);

            for(int y = 11 - i; y >= 1; y--)
                cout << 'a';

            cout << endl;
        }

        return 0;
    }

I also put the output that gives me, in case it was not portable.

*              bbbbbbbbbb     aaaaaaaaaa
**             bbbbbbbbb       aaaaaaaaa
***            bbbbbbbb         aaaaaaaa
****           bbbbbbb           aaaaaaa
*****          bbbbbb             aaaaaa
******         bbbbb               aaaaa
*******        bbbb                 aaaa
********       bbb                   aaa
*********      bb                     aa
**********     b                       a

c++

asked by Alejandro 02.05.2016 в 19:56

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2 answers

1

You have the i loop that is used to print the rows. For each row you will print an asterisk number equal to the position of the row (row 1 -> 1 asterisk, row 2 -> 2 asterisks, ...).

The next item to print is the b sequence. Since you want this new sequence to start always in the same column you have to add after the asterisk sequence a number of spaces in such a way that número de asteriscos + número de espacios = x or, put another way, número de espacios = x - número de asteriscos , being in your case x=16 . Since the sequence of asterisks is directly proportional to the row, the number of asterisks must be inversely proportional. From here you get the first setw : setw(16 - i); :

Fila i = i asteriscos + (16 - i) espacios

Fila 01: 1 asterisco  + (16-1) espacios = 16
Fila 02: 2 asteriscos + (16-2) espacios = 16
Fila 03: 3 asteriscos + (16-3) espacios = 16
...

Well, now the sequence of b and the sequence of a is printed in such a way that the gap between them forms a pyramid of spaces, the total sequence occupying 24 characters. Both sequences, a and b , evolve inversely proportional to the row number. So we have:

24 = longitud_secuencia_b + espacios + longitud_secuencia_a
24 = (10-i) + espacios + (10-i)
24 = 2 * (10-i) + espacios

And now we try to calculate the number of spaces to insert according to the number of the row:

espacios = 24 - 2*(10 - i)
espacios = 24 - 20 + 2*i
espacios = 4 + 2*i

If you compare this last result with the second setw you will see that the equation is exactly the same: setw(4 + i * 2);

Why is that x2 ? Because the number of elements in the sequence a is reduced by one in each row and the same for the sequence b , consequently, the number of spaces will have to grow by two units between one row and the next if you intend to the total sequence occupies the same number of characters.

answered by 07.02.2017 в 09:58

matrix edition Perform INSERT INTO query with SELECT DISTINCT with data from 2 tables

score 5 · Accepted Answer

If I understood your question cout << right << setw(4 + i * 2); form the pyramid of spaces in the middle because the space you increase to 4 by the sum of i is doubled, think that if you had this instruction cout << right << setw(4 + i); your result would be something as:

*              bbbbbbbbbb    aaaaaaaaaa
**             bbbbbbbbb     aaaaaaaaa
***            bbbbbbbb      aaaaaaaa
****           bbbbbbb       aaaaaaa
*****          bbbbbb        aaaaaa
******         bbbbb         aaaaa
*******        bbbb          aaaa
********       bbb           aaa
*********      bb            aa
**********     b             a

By multiplying i by 2 the space you leave for each line between the b and the a is doubled, when i = 1 will leave 6 spaces, on the next line it will leave 8 and so on progressively.