Another look at why it's important to understand what the method you are using is returning and how it works. There are some significant advantages and disadvantages to methods that might generate the same, correct, output for you.

gistfile1.rb

require 'benchmark'
array = (1..900_000).to_a
Benchmark.bmbm do |x|
  #reject returns the array without the rejected number, so it has to re-index it without the rejected number
  x.report("reject 890,000") { array.reject{|num| num == 890_000}}
  x.report("reject 2") { array.dup.reject{|num| num == 2}}
  #select goes through whole array no matter what bc it returns all matched elements
  x.report("select 890,000") { array.select{|num| num == 890_000}}
  x.report("select 2") { array.select{|num| num == 2}}
  #detect returns the first match for what's in the block, so the later the match, the longer it will take
  x.report("detect 890,000") { array.detect{|num| num == 890_000}}
  x.report("detect 2") { array.detect{|num| num == 2}}
end
=begin

# TERMINAL OUTPUT (reordered by speed)
                     user     system      total        real
detect 2         0.000000   0.000000   0.000000 (  0.000007)
select 2         0.060000   0.000000   0.060000 (  0.055447)
select 890,000   0.060000   0.000000   0.060000 (  0.055771)                  
reject 2         0.060000   0.000000   0.060000 (  0.074060)
reject 890,000   0.070000   0.010000   0.080000 (  0.074298)
detect 890,000   0.080000   0.000000   0.080000 (  0.078985)

=end

array = (1..900_000).to_a
Benchmark.bmbm do |x|
  x.report("reject 890,000") { array.reject{|num| num == 890_000}}
  x.report("select 890,000") { array.select{|num| num == 890_000}}
  x.report("detect 890,000") { array.detect{|num| num == 890_000}}
  
  x.report("reject 2") { array.dup.reject{|num| num == 2}}
  x.report("select 2") { array.select{|num| num == 2}}
  x.report("detect 2") { array.detect{|num| num == 2}}
end

=begin
# TERMINAL OUTPUT (reordered by speed and num val)
                     user     system      total        real
select 890,000   0.060000   0.000000   0.060000 (  0.055352)                
reject 890,000   0.070000   0.000000   0.070000 (  0.072121)
detect 890,000   0.080000   0.000000   0.080000 (  0.079082)

detect 2         0.000000   0.000000   0.000000 (  0.000007)
select 2         0.050000   0.000000   0.050000 (  0.054411)
reject 2         0.070000   0.000000   0.070000 (  0.071456)
=end