Question

I’ve seen this a couple times :

if foo = bar
  # do something
end

Is this syntactically appropriate? Does it differ from? :

if foo == bar
   # do something
end

Answer 1

if foo = bar
end

it can be read as this:

foo = bar
if foo
  # do stuff
end

Assignment operator in ruby returns a value that was assigned. This makes it very useful in some situations, thanks to the fact that ruby has very simple rules about what’s true and what’s not.

Here’s a typical example where assignment is used in a condition (not a good one, though, because with files you should use each_line and a block, but you get the idea):

f = File.open(...)
while line = f.gets
  # do something with a line from file
end

This, on the other hand, is a comparison operator, not assignment operator. It returns a boolean value if foo and bar were equal.

if foo == bar

[edit] An example that demonstrates that using this pattern may have a real impact on the verbosity of code. Look at this nested conditional expression:

a = f(1)
result = if a
  b = g(a)
  if b
    c = h(b)
    if c
      2 * c
    end
  end
end

Using this pattern we can write instead (note how I surround the assignments):

result = if (a = f(1)) && (b = g(a)) && (c = h(b))
  2 * c 
end