Control Flow

Programs aren't very interesting when they only do one thing. We like programs to be multi-purposed and to accept inputs. In order to accomplish this we need some sort of control that allows us to switch on data.

The if statement

If statements allow us to do something conditionally.

if (condition):
    print 'do something if our condition is true'

The if statement uses the reserved word if and then has parentheses which contain a condition. Only boolean values can go inside the parentheses. If that value is True, then the command(s) that are inside the if statement will be executed.

The if statement has a colon after the parentheses which indicates that a block is going to follow. The contents of the block are indented in Python. It is important that this indentation is consistent. If you indent your blocks by a single tab, then you must always do this. If you use four spaces then you must similarly always use 4 spaces. When you want to end the block then you simply go back to the indentation level you were at before the if.

if (True):
    print "I'm inside the if"
print "I'm outside the if"

Try creating your own if statement!


Sometimes we want to do something when a condition is True and something else when it is false. An else block allows you to switch on conditions like this:

happy = True
if (happy):
    print "good for you"
    print "that's too bad"

You can also switch on multiple conditions using an elif (else if) statement. An elif is just like an if but comes after an initial if.

mood = 'Grumpy'
if (mood == 'Morose');
    print "We should be friends"
elif (mood == 'Grumpy'):
    print "Don't be coming around me"
    print "I guess I don't recognize your mood"

Complex Examples

You can imagine more complex conditions where multiple variables come into play

age = 11
mood = 'Grumpy'
family_member = True
if (mood == 'Morose' and age > 10 or family_member);
    print "We should be friends"

The above condition uses and and or to combine multiple conditions. When you say the whole statement out loud you can tell what should happen. In this case mood must be equal to 'Morose' and age must be greater than 10. Since that combined condition is False we would not enter the if block. We have a third condition though. The family_member condition is also part of this and since there is an or combining the previous two conditions and this one, only one or the other is necessary to enter the block. Since family_member is True we will enter.

Try collecting some data from the user (raw_input) and doing an if statement based on their response!


You might look at the = and == and wonder how they are different. The single = is used as the assignment operator. This operator assigns variables on the left of the sign to the value of whatever expression is on the right. The == operator is used to express boolean conditions. The two operands in this case must be exactly equal to be evaluated as True. 42 == "42" may look like they are equal but they have different types. One is an int and the other is a str. Things can never be equal if they are of different types.

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