Step 2 Branching

Table of contents

1. What is Branching?
2. Task 2.1 Chose beverage
3. Task 2.2 Chained conditional
4. Task 2.3 Choose Addons

What is Branching?

A core function of the vending machine is to select between the beverages to serve. The computer chooses to do one of two actions (e.g. coffee or tea). We call this behaviour ‘branching’.

To create a branch, we need three elements:

• condition the definition of what is true or false
• action what to do when the condition is true
• alternative action what to do when the condition is false

Branching algorithm

The branching algorithm looks like this:

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If [condition] is true
then do [action];
otherwise do [alternative action]

Branching flow chart

The flow chart is also a convenient, graphical way to represent branches. As branching is a key element of flow charts, it has its specific shape, the diamond, representing a decision.

Branching Python syntax

In Python, the syntax for branches relies on the keywords if and else and looks as follows:

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if condition:
    # then action
else:
    # otherwise alternative action

Let’s pause on this syntax for a moment. It is the first Python syntax we encounter that requires several lines of code. We call this a compound statement. This structure involves two essential Python elements:

• The colons : ending the if and else lines tells the computer that the expression is not yet complete. It involves a sub-expression;
• The indentation – 4 spaces or a TAB in front of the action and alternative action – are required in front of each sub-expression.

Finally, the result of the condition is True or False. There is a data type for this, boolean. The variable of type boolean can only take the value True or False. It adds to our collection of types including int, float, char and string.

Task 2.1 Chose beverage

The branching algorithm gives us many possibilities to improve the functionalities of the vending machine. For instance, we can tell the users what they selected instead of repeating their selection number.

Choice Beverage Algorithm

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Tell the user 'Welcome to IDE Hot Beverage service!'
Tell the user 'Here is what we offer:'
Tell the user '1) Coffee'
Tell the user '2) Tea'

Create a variable called choice of type number with the initial value 0
Ask the user 'Type in your choice: ' and store the answer in choice

Convert choice to integer and store in choice
If choice is equal to 1
    Then tell the user "Here is your coffee."
Else serve a tea by default
    Otherwise tell the user "Here is your tea."

Tell the user "Have a great day!"

We can see that we have a conversion similar to the first assignment again to compare the user input to a number. Then, we recognise the if and else with their respective action and alternative action.

The text ‘Here is your coffee.’ appears for the following condition: choice is equal to 1. The Python syntax uses the double-equal == for this condition.

Attention! The (single) equal = assigns a value to a variable.

In Replit, add the new lines of the algorithm to your project as comments. Then, add the Python translation for each line.

Execute the code to check that it behaves the way you expect.

Check the code on Replit

Task 2.2 Chained conditional

So far, any choice that is not a coffee would lead to serving tea. Thus, we need to check the option twice for coffee and tea. Chained conditionals allow checking for multiple conditions, leading to a different action. If the first condition is False, the computer evaluates the following until a condition is True. If none are true, it uses the default action (else).

In Python, the keyword elif is used for chaining additional conditions with their associated actions.

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if condition:
    # action 1
elif condition2:
    # action 2
elif condition3:
    # action 3
else:
    # default action

Thus, we can add an alternative to check if the choice is 2 (for tea). We also adapt the default alternative to tell the user that this is not a valid option.

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...

If choice is equal to 1
    Then tell the user "Here is your coffee."
Else if choice is equal to 2
    Then tell the user "Here is your tea."
Else this is not a known choice
    Tell the user "Sorry, I do not know this choice."

Tell the user "Have a great day!"

In Replit, add the new lines of the algorithm to your project as comments, then add the Python translation for each line.

Execute the code to check that it behaves the way you expect.

Check the code on Replit

Task 2.3 Choose Addons

We now want to ask the user if they want sugar (on a scale of 0 to 5) or milk (y for yes and n for no) in their beverage. Then, we want to tune the final message accordingly.

Choice addons Algorithm

We could extend the algorithm with the following lines:

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Ask the user "Sugar (0-5): " and store the answer in sugar
Ask the user "Milk (y/n): " and store the answer in milk

...

Tell the user whether they take their beverage with or without sugar
Tell the user whether they take their beverage with or without milk

In Replit, add the new lines of the algorithm to your project as comments. Then, add the Python translation for each line. Note that there will be four lines of codes for each “Tell … whether”, as they involve a branch with if and else.

Execute the code to check that it behaves the way you expect.

Check the code on Replit

Next: Step 3 - Operator