What is precedence in this expression?

Question

When I run this program:

print(rand*100)

I get values from [0,1) range.

But for this:

print(100*rand)

I get values from [0,100) range.

What is precedence here? and why first expression does not return values from [0,100) range?

Answer 1

rand has two syntax:

• rand
• rand EXPR

If what follows rand can be the start of an expression (EXPR), Perl assumes you are using the latter form.

* can start an EXPR, so rand*... is parsed as rand EXPR. This means that rand*100 is equivalent to rand(*100).

$ perl -MO=Deparse,-p -wle'print(rand*100)'
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
print(rand(*100));
-e syntax OK

$ perl -wle'print(rand*100)'
Argument "*main::100" isn't numeric in rand at -e line 1.
0.57355563536203

Answer 2

You can always use B::Deparse to see how Perl is parsing an expression.

$ perl -MO=Deparse -e'print(100*rand)'
print 100 * (rand);
-e syntax OK
$ perl -MO=Deparse -e'print(rand*100)'
print rand *100;
-e syntax OK