Question

How do I create an exclusion for a array map in Ruby.
Here’s what I want to achieve,

a = [1,2,3,4]
b = [5,6,7,8]
a.map.each do |x|
  b.map.each do |y|
    if !(x == 1 && y == 7)
      puts "#{x} and #{y}"
    elsif !(x == 4 && y == 8)
      puts "#{x} and #{y}"
    end
  end
end

1 and 5
1 and 6
1 and 7 # still here
1 and 8
2 and 5
2 and 6
2 and 7
2 and 8
3 and 5
3 and 6
3 and 7
3 and 8
4 and 5
4 and 6
4 and 7
4 and 8 # still here

However, it doesn’t work, how do I add an exception to these values being processed by map? Also if it’s possible to use inject/reject/filter function with the same goal.

Answer 1

To explain why 1 and 7 is still printing, step through the logic:

1. if !(x == 1 && y == 7)
   • x == 1 is true and y == 7 is true, therefore !(true && true) is false, this is skipped.
2. elsif !(x == 4 && y == 8)
   • the if was skipped, so the elsif is evaluated. x == 4 is false (since x is still 1) and y == 8 is false (since y is still 7). Therefore, !(false && false) is true, and the puts is reached.

Because x can never be both 1 and 4 at the same time and y can never be 7 and 8 at the same time, either your if statement or your elsif statement will always succeed, and since both branches print, the values will be always printed, no matter what x and y are.

As other answers said, you need to combine your clauses.