40_control_statements.md

+++ title = "Control Statements" date = 2020-08-29T16:56:29-07:00 weight = 40 +++

The if Statement and Conditionals

if in Python means: only run the rest of this code once, if the condition evaluates to True.

Anatomy of an if statement:

1. Start with the if keyword, followed by a boolean value, an expression that evaluates to True, or a value with "Truthiness".
2. Add a colon : and a new line
3. Write the code that will run if the statement is True under a level of indentation

{{% notice note %}} Remember, just like with functions, we know that code is associated with an if statement by it's level of indentation. All the lines indented under the if statement will run if it evaluates to True. {{% /notice %}}

>>> if 3 < 5:
...     print("Hello, World!")
...
Hello, World!

{{% notice tip %}} Remember, your if statements only run if the expression in them evaluates to True and just like with functions, you'll need to enter an extra space in the REPL to run it. {{% /notice %}}

Using not With if Statements

If you only want your code to run if the expression is False, use the not keyword.

>>> b = False
>>> if not b:
...     print("Negation in action!")
...
Negation in action!

if Statements and Truthiness

if statements also work with items that have a "truthiness" to them.

For example:

• The number 0 is False-y, any other number (including negatives) is Truth-y
• An empty list, set, tuple or dict is False-y
• Any of those structures with items in them are Truth-y

>>> message = "Hi there."

>>> a = 0
>>> if a:   # 0 is False-y
...     print(message)
...

>>> b = -1
>>> if b:  # -1 is Truth-y
...     print(message)
...
Hi there.

>>> c = []
>>> if c:  # Empty list is False-y
...     print(message)
...

>>> d = [1, 2, 3]
>>> if d:  # List with items is Truth-y
...     print(message)
...
Hi there.

>>> e = ""
>>> if e:
...     print(message)
...

if Statements and functions

You can easily declare if statements in your functions, you just need to mindful of the level of indentation.

Notice how the code belonging to the if statement is indented at two levels.

>>> def modify_name(name):
...    if len(name) < 5:
...         return name.upper()
...    else:
...         return name.lower()
...
>>> name = "Nina"
>>> modify_name(name)
'NINA'

Nested if Statements

Using the same technique, you can also nest your if statements.

>>> def num_info(num):
...    if num > 0:
...        print("Greater than zero")
...        if num > 10:
...            print("Also greater than 10.")
...
>>> num_info(1)
Greater than zero
>>> num_info(15)
Greater than zero
Also greater than 10.

How Not To Use if Statements

Remember, comparisons in Python evaluate to True or False. With conditional statements, we check for that value implicitly. In Python, we do not want to compare to True or False with ==.

{{% notice warning %}} Warning - pay attention, because the code below shows what you shouldn't do. {{% /notice %}}

# Warning: Don't do this!
>>> if (3 < 5) == True: # Warning: Don't do this!
...     print("Hello")
...
Hello

# Warning: Don't do this!
>>> if (3 < 5) is True: # Warning: Don't do this!
...     print("Hello")
...
Hello

{{% notice tip %}} Do this instead: {{% /notice %}}

>>> if 3 < 5:
...     print("Hello")
...
Hello

If we want to explicitly check if the value is explicitly set to True or False, we can use the is keyword.

>>> a = True        # a is set to True
>>> b = [1, 2, 3]   # b is a list with items, is "truthy"
>>>

>>> if a is True:   # we can explicitly check if a is True
...     print("Hello")
...
Hello
>>> if b:  # b is a list with items, is "truthy"
...     print("Hello")
...
Hello
>>> if b is True:   # b does not contain the actual value of True.
...     print("Hello")
...
>>>

else

The else statement is what you want to run if and only if your if statement wasn't triggered.

An else statement is part of an if statement. If your if statement ran, your else statement will never run.

>>> a = True
>>> if a:
...     print("Hello")
... else:
...     print("Goodbye")
...
Hello

And vice-versa.

>>> a = False
>>> if a:
...     print("Hello")
... else:
...     print("Goodbye")
...
Goodbye

In the REPL it must be written on the line after your last line of indented code. In Python code in a file, there can't be any other code between the if and the else.

{{% notice info %}} You'll see SyntaxError: invalid syntax if you try to write an else statement on its own, or put extra code between the if and the else in a Python file. {{% /notice %}}

>>> if a:
...     print("Hello")
...
Hello
>>> else:
  File "<stdin>", line 1
    else:
       ^
SyntaxError: invalid syntax

elif Means Else, If.

elif means else if. It means, if this if statement isn't considered True, try this instead.

You can have as many elif statements in your code as you want. They get evaluated in the order that they're declared until Python finds one that's True. That runs the code defined in that elif, and skips the rest.

>>> a = 5
>>> if a > 10:
...     print("Greater than 10")
... elif a < 10:
...     print("Less than 10")
... elif a == 10:
...     print("exactly 10")
... else:
...     print("Dunno")
...
Less than 10