all? Method

First, the code:

@numbers = [1, 0, 3, 2, 5, 4, 7, 6, 9, 8]

# are all numbers even?
@numbers.all?(&:even?)    #=> false

# are all numbers positive? 
@numbers.all?{|number| number >= 0}    #=> true

# if list is empty, is everything true?
p [].all?(&:even?)    #=> true

This last example is something I found interesting while playing with this method. If your list is empty, what does the all? method return? By default, it returns true because no items failed the comparison test. Technically, no items passed either, which is a bit confusing, but I guess the authors picked the default that would work as you expect most of the time.

# do all pets have legs?
@inventory.all?{|pet| pet.legs > 0}    #=> false

# are all pets in stock?
@inventory.all?(&:in_stock?)    #=> false

# all pets have a name? 
@inventory.all?{|pet| !pet.name.nil?}    #=> true

Let's walk through these examples. Do all pets have legs? No, fish and rocks don't pass the legs > 0 test. Are all pets in stock? No, we're out of pet rocks. Do all pets have a name? Yes, because no pet names are nil.

# do all pokey things have at least 3 letters?
@pokey_things.seek(0)  # starting at the beginning of the file
@pokey_things.all?{|line| line.size >= 3}    #=> true

# do all pokey things have the letter 'e'?
@pokey_things.seek(0) # starting at the beginning of the file
@pokey_things.all?{|line| line.include?('e')}    #=> false

These examples are pretty straightforward - every pokey thing on our list has at least three letters, but "cactus" doesn't have an "e".