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Align

Automatic column alignment for streamed tables

Align is both a C++ library and a command-line utility.

  • The C++ library io::align provides a simple interface to align columns in tabular output performed through a standard C++ stream (std::ostream).
  • The utility align reads tabular data, aligns columns, and writes the aligned table.

Both tools perform alignment while streaming: alignment occurs line-by-line without buffering the entire table.

The tool align uses the library io::align.

When to use align

  • To quickly reformat tabular data on the command line during programming tasks or system administration.
  • To format tabular data in C++ programs when the number of rows is not known in advance.
  • To approximate aligned output for large tables when the amount of memory is limited, eg in embedded environments.

How to use align

  1. run configure
  2. run make
  3. run make install
  4. use align from the command line.

Example uses:

align -t: /etc/passwd

align -t: -T "name:pw:uid:gid:geos:home:shell" -p </etc/passwd

env | align -t=

See the help text (align -h) or Command description below for more details.

Example output

Suppose the file /etc/passwd contains the following:

##
# User Database
#
# [...]
##
nobody:*:-2:-2:Unprivileged User:/var/empty:/usr/bin/false
root:*:0:0:System Administrator:/var/root:/bin/sh
daemon:*:1:1:System Services:/var/root:/usr/bin/false
_uucp:*:4:4:Unix to Unix Copy Protocol:/var/spool/uucp:/usr/sbin/uucico
_lp:*:26:26:Printing Services:/var/spool/cups:/usr/bin/false
_postfix:*:27:27:Postfix Mail Server:/var/spool/postfix:/usr/bin/false
_mcxalr:*:54:54:MCX AppLaunch:/var/empty:/usr/bin/false
_pcastagent:*:55:55:Podcast Producer Agent:/var/pcast/agent:/usr/bin/false
_pcastserver:*:56:56:Podcast Producer Server:/var/pcast/server:/usr/bin/false
_serialnumberd:*:58:58:Serial Number Daemon:/var/empty:/usr/bin/false
_devdocs:*:59:59:Developer Documentation:/var/empty:/usr/bin/false
_sandbox:*:60:60:Seatbelt:/var/empty:/usr/bin/false
_mdnsresponder:*:65:65:mDNSResponder:/var/empty:/usr/bin/false
_ard:*:67:67:Apple Remote Desktop:/var/empty:/usr/bin/false
_www:*:70:70:World Wide Web Server:/Library/WebServer:/usr/bin/false

We can use align to tabulate this data as follows:

align -t: -T "name:pw:uid:gid:geos:home:shell" -up -n 10 /etc/passwd

the above command then produces the following output:

name pw uid gid geos home shell
---- -- --- --- ---- ---- -----
nobody *  -2  -2  Unprivileged User /var/empty /usr/bin/false
root   *  0   0   System Administrator /var/root  /bin/sh
daemon *  1   1   System Services      /var/root  /usr/bin/false
_uucp  *  4   4   Unix to Unix Copy Protocol /var/spool/uucp /usr/sbin/uucico
_lp    *  26  26  Printing Services          /var/spool/cups /usr/bin/false
_postfix *  27  27  Postfix Mail Server        /var/spool/postfix /usr/bin/false

name     pw uid gid geos                       home               shell
-------- -- --- --- -------------------------- ------------------ ----------------
_mcxalr  *  54  54  MCX AppLaunch              /var/empty         /usr/bin/false
_pcastagent *  55  55  Podcast Producer Agent     /var/pcast/agent   /usr/bin/false
_pcastserver *  56  56  Podcast Producer Server    /var/pcast/server  /usr/bin/false
_serialnumberd *  58  58  Serial Number Daemon       /var/empty         /usr/bin/false
_devdocs       *  59  59  Developer Documentation    /var/empty         /usr/bin/false
_sandbox       *  60  60  Seatbelt                   /var/empty         /usr/bin/false

name           pw uid gid geos                       home               shell
-------------- -- --- --- -------------------------- ------------------ ----------------
_mdnsresponder *  65  65  mDNSResponder              /var/empty         /usr/bin/false
_ard           *  67  67  Apple Remote Desktop       /var/empty         /usr/bin/false
_www           *  70  70  World Wide Web Server      /Library/WebServer /usr/bin/false

Note

As this example shows, align incrementally adjusts a column's width until it encounters a row with maximum width. This progression occurs because align does not pre-scan the data before emitting its output. This is a feature, not a bug.

In this example:

  • the parameter -t: indicates that the colon (:) separates columns in the input.
  • the parameter -T ... provides titles to each column.
  • the parameters -p and -u indicate the output is to be paginated, and column titles repeated at the top of each page.
  • the parameter -n 10 indicates the page size.

Command description

Each row in the input is split into columns at each input separator (-t). Columns are then formatted by padding on the right using the fill character (-f). Columnts are separated on the output by the separator character (-s).

If option -i is specified, the following input row prefixes have a spe‐ cial meaning: '=' produce a horizontal rule; ';' indicates new column titles; and '#' skips the row (comment). These special prefixes are configurable (-R / -H / -C).

Option Description
-t C Set the input tab character to C. (default: tab)
-f C Set the output fill character to C. (default: space)
-s C Set the output separator to C. (default: space)
-r C Set the output horizontal rule character to C. (default: -)
-p Paginate the output and repeat titles on each page.
-n N Set the page height in lines. (default: terminal height or 25)
-u Produce a horizontal ruler after each title row.
-T STR Define column titles from STR.
-i Interpret special row prefixes in the input.
-R C Set the special row prefix for rules. (default: =)
-C C Set the special row prefix for comments. (default: #)
-H C Set the special row prefix for titles. (default: ;)
-h Display this help.

How to use the io::align C++ library

  1. copy ioalign.h in your project,
  2. include ioalign.h in your C++ code,
  3. use io::align on your C++ output streams.

For example:

#include "ioalign.h"
#include <fstream>
int main()
{
    std::ofstream os("/dev/stdout");

    os << "Multiplication table modulo 11:" << std::endl;

    // Create the align data structure.
    io::align a;
    // Create the output proxy.
    io::align_proxy o = a.attach(os);

    // Set the column widths.
    for (int i = 1; i <= 10; ++i)
        o << io::head("", 2);
    o << io::endr;

    // Produce a horizontal rule.
    o << io::hline;

    // Fill the table.
    for (int i = 1; i <= 10; ++i)
        for (int j = 1; j <= 10; ++j)
            o << (i * j % 11) << io::next;

    // Produce another horizontal rule.
    o << io::hline;

    return 0;
}

This program produces the following output:

Multiplication table modulo 11:
-- -- -- -- -- -- -- -- -- --
1  2  3  4  5  6  7  8  9  10
2  4  6  8  10 1  3  5  7  9
3  6  9  1  4  7  10 2  5  8
4  8  1  5  9  2  6  10 3  7
5  10 4  9  3  8  2  7  1  6
6  1  7  2  8  3  9  4  10 5
7  3  10 6  2  9  5  1  8  4
8  5  2  10 7  4  1  9  6  3
9  7  5  3  1  10 8  6  4  2
10 9  8  7  6  5  4  3  2  1
-- -- -- -- -- -- -- -- -- --

An API documentation is provided; check the doc subdirectory after building the package with make doxygen-doc.

Performance

Using the enclosed bench.cc on a Core 2 Duo @ 2.4GHz and optimization level -O3, the following was measured:

C++ toolchain Formatter Performance (rows/sec) Slowdown
g++ 4.8 / libstdc++ none 1149950 N/A
g++ 4.8 / libstdc++ io::align 566160 50% (2x slower)
clang++ 3.3 / libc++ none 701035 N/A
clang++ 3.3 / libc++ io::align 411945 41% (1.7x slower)

The memory usage should be related to the length of the longest row by a constant factor, and does not grow with the number of rows.

Licensing

align and io::align are free software. See the accompanying file LICENSE for details.

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Align: a command-line utility and C++ library to format tables.

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