Step 3 Operators
Table of contents
- Task 3.1 Count remaining cups
- Task 3.2 Compose messages
- Task 3.3 Shame greedy users
- Task 3.4 Combine Conditions
In the first assignment, we used the plus +. We can use this arithmetic operator the same way as the mathematic symbol. However, we debugged our programme, realising that we could also ‘add’ two strings (i.e. two pieces of text, two sequences of characters) together. It is called concatenation. Thus, operators can behave differently depending on the variable type.
In this step, we will explore other types of operators such as assignment, comparison and logical operators to extend the capabilities of the vending machine algorithm.
You can refer to the W3Schools list of operators for a complete list.
Task 3.1 Count remaining cups
Continuously upgrading our vending machine, we want to keep track of the number of remaining paper cups. Our algorithm will need a variable to keep this information in memory. We will decrease this number (removing one cup) for each served beverage. For the sake of debugging, we can also show the number of remaining cups before and after pouring the drink.
Counting cups algorithm
Counting cups could look as follows:
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Create a variable called number_remaining_cups of type int with the initial value 10
Tell the user "Welcome to IDE Hot Beverage service!"
Tell the user "There are " number_remaining_cups " cups left."
...
If choice is equal to 1
Then tell the user "Here is your coffee."
Decrease number of cups
Else if choice is equal to 2
Then tell the user "Here is your tea."
Decrease number of cups
...
Tell the user "BTW, there are " number_remaining_cups " cups left!"
Note that we initialise the number of cups to 10, but any positive number would work here.
What is new in this algorithm? Several times we concatenated (i.e. joined together) strings. Here this is a new case as we want to join a string, a number and another string: "There are " number_remaining_cups "left.". As we can only concatenate strings, we need to convert the number of cups into a string.
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string_variable = str(number_variable)
The second novelty of this algorithm is ‘Decrease’. Increasing or decreasing a variable is a typical algorithm operation. With the Python syntax we already know, we can do this with the assignment operator = in combination with the subtraction operator -.
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number_remaining_cups = number_remaining_cups - 1
As this is a common manipulation, there are assignment operators for increasing += and decreasing -=. In our example, it would be -=, which leads to the following:
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number_remaining_cups -= 1
It means that we take the current value of the variable, remove 1 and store the result in the variable.
With these elements, you can add the lines of the Counting Cups Algorithm to your project in Replit as comments. Then, translate each line into Python syntax. Finally, Execute the code to check that it behaves the way you expect.
Task 3.2 Compose messages
Concatenation of strings is helpful to build messages for the user. Instead of ‘telling’ each piece of information, we can construct a message and deliver it further in the algorithm. For instance, the choice of beverage with sugar and milk could form a single sentence. Let’s create a variable message and replace “Tell” with “Add” in our algorithm. We will only “tell” the message at the end. In Python, you can use the assignment operator += to add (or append) additional text to a string variable.
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message += "some additional text"
Our algorithm would look as follows:
Message concatenation algorithm
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Create a variable called message of type string with the initial value ""
If choice is equal to 1
Add to message "Here is your coffee"
Decrease number of cups
Else if choice is equal to 2
Add to message "Here is your tea"
Decrease number of cups
Otherwise this is not a known choice
Add to message "Sorry, I do not know this choice."
Add to message whether the user take their beverage with or without sugar
Add to message whether they take their beverage with or without milk
Tell the user message
...
In your Replit project, you can transform your algorithm to switch from “Tell” to “Add”. Do not forget to “Tell” the message at the end. You will need to adjust the punctuation and spaces to make it a proper sentence.
Execute the code to check that it behaves the way you expect.
Task 3.3 Shame greedy users
Our vending machine could be a bit biased and tune its message to promote healthy habits such as reducing sugar consumption. Of course, the best option would be not to offer sugar. Still, it gives us an excellent opportunity to explore comparison operators. We have seen ‘equal to’ with the double equal == earlier, which works with numbers and strings. Additionally, in Python you can use:
>and<for greater than and lower than, as in mathematics>=and<=for greater or equal to and lower or equal to!=for not equal to
For our biased vending machine, we could check whether the user takes three or more sugar units. Then, we would add a sneaky comment to the final message.
Two much Sugar algorithm
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Add to message whether the user take their beverage with or without sugar
Convert sugar into integer
If sugar greater or equal to 3
Add to message a 'healthy' comment
We now convert the sugar input into an integer to compare numbers. Otherwise, we would compare whether a string is greater or equal to another, leading to different behaviour.
Are you convinced that we should reward good behaviour instead of shaming bad behaviour? Then, tweak the algorithm to make it the way you want: you are the designer!
In your Replit project, you can transform your algorithm to add the biased message.
Execute the code to check that it behaves the way you expect.
Task 3.4 Combine Conditions
In this final task on operators, we will tune our vending machine to detect when the user wants a black coffee: no sugar, no milk. We can use logical operators to combine multiple conditions in this case. Such a combination could look as follows
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If choice is coffee without sugar nor milk
Add to message "Here is your black coffee."
Else if choice is equal to 1
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This algorithm checks three variables: choice, sugar, and milk. In Python, we can use and between each condition, enforcing that all conditions much be True. We can also use or, ensuring that at least one is True.
In your Replit project, you can transform your algorithm to add the black coffee branch.
Execute the code to check that it behaves the way you expect.
That’s it for our operator exploration. As mentioned in the introduction, there are many more which you can explore on the W3School website. However, the way to use them is the same. Thus, feel free to explore on your own.