Think Perl 6

Chapter 6. Iteration

This chapter is about iteration, which is the ability to run a block of statements repeatedly. We saw a kind of iteration, using recursion, in “Recursion”. We saw another kind, using a for loop, in “for Loops”. In this chapter we’ll see yet another kind, using a while statement. But first I want to say a little more about variable assignment.

Assignment Versus Equality

Before going further, I want to address a common source of confusion. Because Perl uses the equals sign (=) for assignment, it is tempting to interpret a statement like $a = $b as a mathematical proposition of equality, that is, the claim that $a and $b are equal. But this interpretation is wrong.

First, equality is a symmetric relationship and assignment is not. For example, in mathematics, if then . But in Perl, the statement $a = 7 is legal and 7 = $a is not.

Also, in mathematics, a proposition of equality is either true or false for all time. If now, then a will always equal b. In Perl, an assignment statement can make two variables equal, but they don’t have to stay that way:

> my $a = 5;
5
> my $b = $a;   # $a and $b are now equal
5
> $a = 3;       # $a and $b are no longer equal
3
> say $b;
5

The third ...