Gnu Awk - Part 8

Introduction

This is the eighth episode of the "Learning Awk" series that Mr. Young and I are doing.

Recap of the last episode

• The while loop: tests a condition and performs commands while the test returns true

• The do while loop: performs commands after the do, then tests afterwards, repeating the commands while the test is true.

• The for loop (type 1): initialises a variable, performs a test, and increments the variable all together, performing commands while the test is true.

• The for loop (type 2): sets a variable to successive indices of an array, performing a collection of commands for each index.

These types of loops were demonstrated by examples in the last episode.

Note that the example for 'do while' was an infinite loop (perhaps as a test of the alertness of the audience!):

#!/usr/bin/awk -f
BEGIN {

    i=2;
    do {
        print "The square of ", i, " is ", i*i;
        i = i + 1
    }
    while (i != 2)

exit;
}

The condition in the while is always true:

The square of  2  is  4
The square of  3  is  9
The square of  4  is  16
The square of  5  is  25
The square of  6  is  36
The square of  7  is  49
The square of  8  is  64
The square of  9  is  81
The square of  10  is  100
...
The square of  1269630  is  1611960336900
The square of  1269631  is  1611962876161
The square of  1269632  is  1611965415424
The square of  1269633  is  1611967954689
The square of  1269634  is  1611970493956
...

The variable i is set to 2, the print is executed, then i is set to 3. The test "i != 2" is true and will be ad infinitum.

Some more statements

We will come back to loops later in this episode, but first this seems like a good point to describe another statement: the switch statement.

Long notes

The notes for rest of this episode can be found by clicking this link.

Links

• GNU Awk User's Guide
• Previous shows in this series on HPR:
  • "Gnu Awk - Part 1" - episode 2114
  • "Gnu Awk - Part 2" - episode 2129
  • "Gnu Awk - Part 3" - episode 2143
  • "Gnu Awk - Part 4" - episode 2163
  • "Gnu Awk - Part 5" - episode 2184
  • "Gnu Awk - Part 6" - episode 2238
  • "Gnu Awk - Part 7" - episode 2330
• Resources:
  • ePub version of these notes
  • PDF version of these notes
  • Demonstration of the switch statement:
    • Script: switch_example.awk
    • Output: switch_example.out
  • Demonstration of the break statement:
    • Script: divisor.awk
    • Output: divisor.out
  • Demonstration of the continue statement:
    • Script: continue_example.awk
  • Demonstration of the next statement:
    • Script: next_example.awk
    • Output: next_example.out