Conditionals

Is That So?¶

Now that can correctly determine how many groups our alien population will form, we can take a look on how the food situation develops. There are three possible options:

The population will grow
The population will be stable
The population will shrink

When we have such statements, what we are interested in is the truth of it (i.e. Is this statement True or False). It is possible to express these statements in Python.

will_grow = excess_food > 0
will_shrink = excess_food < 0

Note: it is sufficient to express the two “interesting” statements here, should both turn out to be False, it has to be the third option.

The data type that our two variables will_grow and will_shrink employ is called a boolean (bool for short).

Logically…¶

While boolean logic forms its own branch of mathmatics, it is sufficient to understand a few basic ideas. It only allows the two values: True and False. Instead of your regular mathmatical operators you have

not: Inverts the truth value, so not True becomes False and vice versa.
and: Combines two values and decides whether both are True.
or: Combines two values and decides whether at least one of them is True

Checking Conditions¶

In Python we can use a boolean value to make a decision.

from constants import …, GROWTH_PER_GROUP

…

if will_grow:
    current_population = current_population + number_groups * GROWTH_PER_GROUP
    print("Population grows to", current_population, "individuals")

The if-keyword initiates the checking of a following condition. Any action that should be taken if the contition is True is written in the following lines, also known as a branch. This section is indented by one level ( = 4 spaces) to signify that it belongs to the if above.

What else?¶

Now we have also a second condition that we have to check, because there are still two alternatives left

from constants import …, GROWTH_PER_GROUP, DECLINE_PER_GROUP

if will_grow:
    current_population = current_population + number_groups * GROWTH_PER_GROUP
    print("Population grows to", current_population, "individuals")
else:
    if will_shrink:
        current_population = current_population - number_groups * DECLINE_PER_GROUP
        print("Population shrinks to", current_population, "individuals")
    else:
        print("Population is stable at", current_population, "individuals")

The else-keyword allows us to specify a branch with an alternative should the original condition not have been met. We can also nest these conditionals inside each other as often as we like, however it can become very unwieldy very quick. There is a better way to write it down though.

if will_grow:
    current_population = current_population + number_groups * GROWTH_PER_GROUP
    print("Population grows to", current_population, "individuals")
elif will_shrink:
    current_population = current_population - number_groups * DECLINE_PER_GROUP
    print("Population shrinks to", current_population, "individuals")
else:
    print("Population is stable at", current_population, "individuals")

It is allowed to contract an else with a following if into the elif-keyword, which allows to specify a secondary condition. You can have as many elif-segments as you like, but there are some rules to keep in mind:

The if always comes first.
The else always comes last.
- If there is nothing to do in the else-branch, it can be ommitted.
if and elif get checked in the order they are written down.
The first condition that was True decides which branch gets evaluated.
- All following conditions and blocks are then ignored.
- Should no condition be True, the else-branch is evaluated.

Now you

We must not forget to adjust the food count for the next day. If the excess food was negative, we can not make that up retroactively :( But luckily the food does not go bad that fast, so if there is any left over we can leave it for the next day.

Assign the value of food_per_day to the variable called current_food. If there was excess food left over, add it to the current_food as well.

Spoilers

…

current_food = food_per_day
if excess_food > 0:
    current_food = current_food + excess_food

Note that there are other correct solutions!

Now we have all we need to simulate one day of our population 😀

Key Points

A condition can be checked with an if-clause
- A branch is an indented section of code following an if, elif or else
- Should the condition be True the branch after it will be executed
- Otherwise, the branch will be skipped
Alternatives are specified with an optional else-clause
An else and a following if can be combined into an elif for better code structure
The clauses always follow the order if - elif - else

Code Checkpoint

This is the code that we have so far:

📁 alien_growth_model