A method call has a value
Suppose `c2f` converts Celsius to Fahrenheit. Calling `obj.c2f(100)` evaluates to `212`: the method call is an expression, and `212` is its return value. The calculation is the method’s only expression, so its value becomes the value of the call. section 2.1.4
A Ruby method normally returns the value of the last expression evaluated while it runs. You can write `return c * 9.0 / 5 + 32` to make that result explicit, but `return` is usually optional. It is needed to leave a method early. An empty method still returns a value: `nil`. section 2.1.4
The value a call returns is useful in several ways:
- A query method can return an instance variable. For example, a `venue` method whose body is `@venue` hands that value back to its caller. section 3.2.1
- A returned value can become part of another expression. Calls such as `ticket.event` and `ticket.venue` supply details for a printed ticket summary. section 2.2.2
- To return several values, use `return a,b,c`, which wraps them in an array, or make the array explicit with `[a,b,c]`. section 2.1.4
With a block, keep two directions of value flow separate. `5.times` yields to its block five times, offering iteration values from `0` through `4`, but the completed method call returns `5` once. Likewise, `each` yields collection elements yet returns its original array when given a block. Lines printed by a block are output, not the method’s return value. section 6.3.5 section 6.3.6
`map` makes the block’s return values central. It yields each array element, collects the value returned by each block execution, and returns a new array of those results. Thus `names.map {|name| name.upcase }` returns an array of uppercase names rather than the original array. section 6.3.7
class Array
def my_map
c = 0
acc = [] #1
until c == size
acc << yield(self[c]) #2
c += 1
end
acc #3
end
end
The `my_map` implementation shows both value flows: `yield(self[c])` receives each block result into `acc`, and the final `acc` expression becomes the method’s return value. section 6.3.7
