.. _conditionals:

Conditionals
============

Learn about `if statements` in Python so that you can decide whether to
do things or not.

Look at each question and see if you can answer it, then check with
the answer we've given. Do try these out in the :ref:`repl`.

**Remember:** *understanding* is the most imporant thing, don't move
on until you really understand the question and answer.

----

You've already seen a conditional in the last section :ref:`loops`, do
you remember where?

* Was it in this line of code? ::

    while v > 0:

Yes, that tests whether the variable `v` is greater than `0` and while
it is it runs the code inside the block that follows that line.

If `v = 4` what does this code do? ::

  if v < 5:
    print("small")
    print(v)
  else:
    print("large")
    print(v)

* It prints `small` followed by `4`. 

And if `v = 5` what does it do?

* It prints `large` followed by `5`.
  
If `v = 1` what does this code do? ::

  if v == 1:
    print("one")
  print("end")

* It prints `one` followed by `end`.

If `v` is any other number does it only print `end`?

* Yes, `end` is always printed because that line is not *inside* the
  body of the `if statement`.

Why do we use `==` when comparing variables and numbers?

* A single `=` means assign a value to a variable, a double `==` means
  test whether two expressions are equal. Python uses two different
  operators to avoid confusion.

So what does this code do? ::

  v = 5
  if v = 1:
    v*10

* It raises a `SyntaxError` because we are not allowed to assign a
  value to a variable in an `if statement`.

OK, so back to the challenge: how do we produce the even squares of
numbers 1 to 20?

* I need to know how to test for an even number.

Good point, you can do that using the remainder or modulus operator:
`%` - this returns the remainder after a divition, e.g. `11 % 2` produces
the remainder after dividing by `2`. 

* So `11` divided by `2` has remainder `1`, so we would write that in
  Python like this: ::
     
    11 % 2 == 1

Yes, so what does the code look like? 

* This code should do it because even numbers have zero remainder when
  dividing by two. ::

    for v in range(1, 21):
      if v*v % 2 == 0:
        v*v