operator precedence - Why does the block of this if statement seem to be executed before the conditional in ruby? -

this question has answer here:

• undefined local variable based on syntax in ruby 1 answer
• variable scope , order of parsing vs. operations: assignment in “if” 3 answers

i'm working on a codegolf challenge, , i'm trying use few characters possible.

this code:

#example data x=[1,2,3] a=5.times.map{5.times.map{' '}} #problematic code: a[b][c]=x.pop if a[b=rand(5)][c=rand(5)] == ' ' 

returns (quite strange) error:

test.rb:5:in `<main>': undefined local variable or method `b' main:object (nameerror) did mean?  b 

which boils down "can't find b, did mean b?". (to answer yes).

this happens 2d array inline if statement.

this code runs fine:

if a[b=rand(5)][c=rand(5)] == ' '   a[b][c]=x.pop end 

it seems should same, behaves differently. why this?

i thought might have precedence, had @ the ruby precedence table , seem expect, i'm not sure.