C align columns w/ printf() w/ exactly 2 spaces after each column

One way to do it is to save the values as they arrive in an array, and keep track of the largest value in each column, then format that value to find out how long it is, and from that deduce the values to specify.

Here's some code that does that:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    int cols = 3;
    int prec = 13;

    if (argc > 3)
    {
        fprintf(stderr, "Usage: %s [precision [columns]]\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    if (argc > 1)
        prec = atoi(argv[1]);
    if (argc > 2)
        cols = atoi(argv[2]);
    double max[cols];
    for (int i = 0; i < cols; i++)
        max[i] = 0.0;

    enum { MAX_DATA = 100 };
    double data[MAX_DATA];
    int count =  0;
    double x;
    while (scanf("%lf", &x) == 1 && count < MAX_DATA)
    {
        int colnum = count % cols;
        if (x > max[colnum])
            max[colnum] = x;
        data[count++] = x;
    }

    int width[cols];
    const char *pad[cols];
    for (int i = 0; i < cols; i++)
    {
        pad[i] = (i == 0) ? "" : "  ";
        width[i] = snprintf(0, 0, "%.*f", prec, max[i]);
        if (width[i] < 4 + prec)
            width[i] = 4 + prec;
    }

    for (int i = 0; i < count; i++)
    {
        int colnum = i % cols;
        printf("%s%*.*f", pad[colnum], width[colnum], prec, data[i]);
        if (colnum == cols - 1)
            putchar('\n');
    }
    if (count % cols != 0)
        putchar('\n');

    return 0;
}

Note that the code is careful to reject too many arguments, and not to access arguments that don't exist. I also removed the prompt; there's no way I'm typing the numbers by hand, and when the input comes from a file, the prompt is just a nuisance.

Also, the use of snprintf() with zero length (and optionally a null pointer) is documented by the standard as a way to get the amount of space needed to format the string.

Given some sample data like this (randomly generated, but then rigged so that there's a 10k value in each column, and also a 'single digit before the decimal point' value in each column — in the default 3-column layout):

2730.8075416749843 9153.7050562644145 8264.2778874481955
5393.9722906483921 9659.6077493184748 59.6077493184748
4973.9718947965630 3.7623787002290 5975.6392547304667 682.2153319663826
6236.5964619743863 7786.2954141327737 3.7623787002290 6735.6044984396849
1069.6226524395413 8709.7209141371932 3854.7386329491574 3.7623787002290
4960.9318291197014 40.3314639772034 9017.1314461534275
1717.9459363110184 8682.9285936347133 10000 6671.2353105858210
4119.1373095038844 70.3291668437700 4528.3226201367906
1926.8741591097082 2101.4643722293158 760.9213269470772 10000
7366.6932284462664 1287.1299466478447 3418.7415326626078
3144.9791945834349 2385.3575762876035 3779.9164071168789
9743.9571880258318 10000 7432.8398636749780 3011.9532204395937
5883.0779787486517

The default output looks like this:

 2730.8075416749843   9153.7050562644145   8264.2778874481955
 5393.9722906483921   9659.6077493184748     59.6077493184748
 4973.9718947965630      3.7623787002290   5975.6392547304667
  682.2153319663826   6236.5964619743863   7786.2954141327737
    3.7623787002290   6735.6044984396849   1069.6226524395413
 8709.7209141371932   3854.7386329491574      3.7623787002290
 4960.9318291197014     40.3314639772034   9017.1314461534275
 1717.9459363110184   8682.9285936347133  10000.0000000000000
 6671.2353105858210   4119.1373095038844     70.3291668437700
 4528.3226201367906   1926.8741591097082   2101.4643722293158
  760.9213269470772  10000.0000000000000   7366.6932284462664
 1287.1299466478447   3418.7415326626078   3144.9791945834349
 2385.3575762876035   3779.9164071168789   9743.9571880258318
10000.0000000000000   7432.8398636749780   3011.9532204395937
 5883.0779787486517

Or with 6 decimal places and 6 columns, it looks like this:

$ ./fmt53 6 6 < data
2730.807542   9153.705056  8264.277887   5393.972291  9659.607749     59.607749
4973.971895      3.762379  5975.639255    682.215332  6236.596462   7786.295414
   3.762379   6735.604498  1069.622652   8709.720914  3854.738633      3.762379
4960.931829     40.331464  9017.131446   1717.945936  8682.928594  10000.000000
6671.235311   4119.137310    70.329167   4528.322620  1926.874159   2101.464372
 760.921327  10000.000000  7366.693228   1287.129947  3418.741533   3144.979195
2385.357576   3779.916407  9743.957188  10000.000000  7432.839864   3011.953220
5883.077979
$

Or with 5 decimal places and 7 columns:

$ ./fmt53 5 7 < data
2730.80754  9153.70506   8264.27789   5393.97229   9659.60775    59.60775  4973.97189
   3.76238  5975.63925    682.21533   6236.59646   7786.29541     3.76238  6735.60450
1069.62265  8709.72091   3854.73863      3.76238   4960.93183    40.33146  9017.13145
1717.94594  8682.92859  10000.00000   6671.23531   4119.13731    70.32917  4528.32262
1926.87416  2101.46437    760.92133  10000.00000   7366.69323  1287.12995  3418.74153
3144.97919  2385.35758   3779.91641   9743.95719  10000.00000  7432.83986  3011.95322
5883.07798
$