How can I align the numbers that the console outputs at the same position

Question

I'm trying to write a very simple C++ program which outputs a lookup table with the corresponding x and y values of sinus function. The code that I wrote is the following:

#include "stdafx.h"
#include <iostream>
#include <cmath>

using namespace std;

int main()
{

    double hw = 4.0;
    int nsteps = 30;
    const double PI = 3.14159;
    const double maxx = hw * PI;
    const double deltax = maxx / nsteps;
    double x = 0.0;
    for (int i = 0; i < nsteps; i++) {
        const double f = sin(x);
        cerr << x << "\t" << f << endl;
        x = x + deltax;
    }
    return 0;
}

Now the program is working, but my problem is, that the values are not getting aligned properly as showed in the following picture So is there any way, to achieve that the second column of the values will actually be a column and all the values are aligned at the same position? What could I use instead of \t?

Answer 1

Use std::setprecision() to set count number for decimal after point, and std::setw() to set width of output length. Include <iomanip> needed, example:

#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int main()
{

    double hw = 4.0;
    int nsteps = 30;
    const double PI = 3.14159;
    const double maxx = hw * PI;
    const double deltax = maxx / nsteps;
    double x = 0.0;
    cerr << std::setprecision(8);
    for (int i = 0; i < nsteps; i++) {
        const double f = sin(x);
        cerr << std::setw(20) << x << std::setw(20) << f << endl;
        x = x + deltax;
    }
    return 0;
}

Output is:

                   0                   0
          0.41887867          0.40673632
          0.83775733          0.74314435
            1.256636          0.95105619
           1.6755147          0.99452204
           2.0943933          0.86602629
            2.513272          0.58778697
           2.9321507          0.20791411
           3.3510293         -0.20790892
            3.769908         -0.58778268
           4.1887867         -0.86602363
//...

Answer 2

The above answer provides an incorrect solution because the alignment is not set correctly. I would use a function to handle the formatting:

#include "stdafx.h"
#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

void printxy(double x, double y, int width){
    cout << setw(width) << x << "\t";
    if (y < 0) cout << "\b";
    cout << setw(width) << y << "\n";
}

int main(){
    double hw = 4.0;
    int nsteps = 30;
    const double PI = 3.14159;
    const double maxx = hw * PI;
    const double deltax = maxx / nsteps;
    double x = 0.0;

    int decimals = 6;
    int width = 8;    //Adjust as needed for large numbers/many decimals
    cout << std::setprecision(decimals);
    cout << std::setw(width);
    cout.setf(ios::left);

    for (int i = 0; i < nsteps; i++) {
        const double y = sin(x);
        printxy(x, y, width);
        x = x + deltax;
    }
}

The output is now formatted correctly:

0               0
0.418879        0.406736
0.837757        0.743144
1.25664         0.951056
1.67551         0.994522
2.09439         0.866026
2.51327         0.587787
2.93215         0.207914
3.35103        -0.207909
3.76991        -0.587783
4.18879        -0.866024
4.60767        -0.994521
5.02654        -0.951058
5.44542        -0.743148
5.8643         -0.406741
6.28318        -5.30718e-06
6.70206         0.406731
7.12094         0.743141
7.53982         0.951055
7.95869         0.994523
8.37757         0.866029
8.79645         0.587791
9.21533         0.207919
9.63421        -0.207904
10.0531        -0.587778
10.472         -0.866021
10.8908        -0.994521
11.3097        -0.951059
11.7286        -0.743151
12.1475        -0.406746

I would also discourage the use of cerr for these kinds of printing operations. It is intended for printing errors. Use cout instead (it works the same way for all practical purposes).

I should also mention that endl is a ticking bomb: it flushes the output, meaning that the internal buffer of the stream is written out (be it the console, a file or whatever). When applications scale and become more IO intensive, this can become a significant performance problem: the buffer that is intended to increase the IO performance is potentially unused due to frequent endl insertions. The solution is to use the newline character '\n'.