The purpose of this post is to introduce the loops, if statement and conditional logic. Why? Because a lot of programming involves deciding between multiple paths and handling collection of data.

By the end of this post you can

• understand how an if-statement is constructed
• understand how to use a for loop in simple scenario
• begin to develop boolean conditions in an if-statement

You must have Python installed and be able to start IDLE to follow along. You should be familiar with how to create modules/script and run a Python script, either from the terminal or the GUI window.

A good starting point is this link: How To Install Python On Your Computer

Additionally you should have finished Python data structures part 1 – a look at variables and lists

If you found anything difficult in that part, please let us know! Contact Us

A Python If-statement gives you more control over a program. With an if-statement, we use the information of a variable to make a choice in your program. For example, if you want to display all exercises when the user responds “Yes,” and show a different message if the user responds “No,” you can use an if-statement.

Let’s see what they look like!

The if-statement in previous listing highlights three branches:

1. Condition with if: If the user types “Yes,” the code block following the if statement is executed. We call this the first conditional block. The boolean expression (response == ‘Yes’) evaluates to either True or False, depending on what the user typed.
2. Alternative with elif: If the user types “No,” a different message is displayed.
3. Invalid response: If the response is something other than “Yes” or “No,” the program shows a message explaining that only those answers are valid.

It’s plain English, yet so important to get it right with Python syntax and indentation. Notice the colon, and the indentation for each block. Otherwise you will get a syntax error when you try to run the program.

Run the code and try typing “Yes,” “No,” and an invalid response (like “Maybe”) to see what happens. You’ll notice that the program becomes more interactive and adapts to the user’s input now. Pretty neat, right?

When we write an if-statement, the code block or the branch to enter is evaluated according to a boolean expression. It’s the same as asking a Yes/No question, but in the computer world we call this True/False result.

The if-statement anatomy consists of the conditional block and the boolean expression

if True:
    print("This will always be printed")

When we compared the user’s answer of “Yes” or “No”, the result of that comparison would either be True or False internally.

On the other hand, if we’d write

if False:
    print("This will never be printed")

Then we’d never enter that block. Why?

Here are more examples of boolean expressions, where we use the comparion operator greater than >. The first evaluates whether 5 is greater than 10 (False obviously). The second checks if the length of the variable some_list exceeds 100.

if  5 > 10: # Evaluate to False
    print("5 is greater than 10, is it really?")

if len(some_list) > 100: # Test if the list has more than 100 elements
    print("You have more than 100 elements in the list")

The purpose is to only execute part of the code when the boolean expression is True. In the above example it’s simply just one print statement. This is something we will see more of and learn about as we move along.

This type of logic becomes important as you want develop the program, also when when you want to put constraints in your program.

Imagine a scenario to never allow more than 5 exercises per workout day, or where you can limit the number of sessions total per week. All these things require some kind of conditional logic to exist in your program. Therefore as you gain experience and develop the program, it’s only you as the designer of the program that can decide what is valid in your program or not.

Finally, let’s look at how to combine multiple expressions, to check several conditions at the same time.

if Expression1 or Expression2:
    print("we use the or-condition")

if answer == "Yes" or answer == "yes":
    print("You entered Yes...")

The second example becomes handy when you want to let a user be more flexible, you will allow “Yes” and “yes” as an answer to a question, you’ll try to implement this in your program too with the exercises.

Now let’s get back to our program, and look at the first branch i.e. the boolean expression where the user type “Yes” – we’ll introduce the concept of loops now. Remember, the program should display all exercises, according to the question presented to the user

response = input('Would you like to see all exercises in the training schedule? (Yes/No) ')

if response == "Yes":
  print("Here are all the exercises:")
  for exercise in exercises:
    print(exercise)

Notice the construct: keyword for followed by a variable, followed by keyword in followed by a variable (a list or tuple).

for variable1 in variable2

This tells the program to loop through each element in a data structure (like a list or tuple) and execute the code in the loop for each individual element. For our program, it will print each exercise on a separate line, and the output should look like the following. Run the program and confirm you see the same.

Bench press
Deadlift
Squat
Biceps curl

On each iteration print(exercise) works because exercise is bound or assigned the value from the tuple, one value at a time.

In this example:

1. Starting the loop: for exercise in exercises means that Python starts going through each element in exercises.
2. Assignment of exercise : For each iteration (loop), exercise becomes the name of one exercise in turn, from ‘Bench press’ to ‘Biceps curl.’
3. Executing instructions: The code in the loop, print(exercise), is executed each time the loop repeats. So each exercise is printed on its own line.

Insert another statement inside the for-loop block, right above print(exercises) and see the result. Don’t forget to add same indentation.

Python has a neat operator we can use called in. Let’s demonstrate how to use it in our program. Extend the program by adding a new question asking the user which exercise details to display. In programming terms, we want to search the tuple exercises for a value, namely the exercise the user type into the program dialogue.

As a result, we’ll just print a generic message for each exercise. Add the following after the code above:

selected_exercise = input('Which exercise would you like to know more about? ')

if selected_exercise in exercises:
    print(f'You chose {selected_exercise}. This is a fundamental exercise for building strength.')
else:
    print('That exercise is not in the schedule.')

Here, we use in to check if the exercise entered by the user exists in exercises. If it does, we display information about the exercise; otherwise, we inform the user that the exercise isn’t in the schedule.

1. Extend the exercises tuple with more exercises.
2. Improve the exercise details, by creating a different messages for each exercise. You’d need to write an if-statements inside an if-statement.
3. Extend the if-statement to for the word No, with both uppercase and lowercase “N”. The user should be able to enter either “no” or “No”.
4. Extend the if-statement in previous exercise to allow more alternatives, e.g. “Nope”, “Nah” or whatever you can think of.
5. In the next part, we’ll use a dictionary to store descriptions for each exercise in a cleaner way.
6. Another loop is the while-loop, go to docs.python.org to find out more about it. Use the function len() to determine the condition.

Review these terms

• tuple
• list
• if-statement
• for-loop
• boolean expression

In this part, you’ve seen how to use the if-statements in order to handle different scenarios in your program. It allow your program to become more interactive, but also handle things in the code to work according to your plan. Being able to create different options and decision paths in a program is an important part of programming. It makes your programs more dynamic and user-friendly.

In the next part, we’ll dive deeper into working with loops and explore dictionaries.

If you’ve missed the previous post, check it out Python if-statements – beginning the training app

See you in the next section!

The purpose is to become familiar of basic building blocks of programming and Python. Storing information is where it all begins in a computer program, you must start here and understand how to make the best use out of them.

By the end of this post you can

• create variables and understand why they are necessary
• use variables of different types: primitive single-value variables, tuple and list
• start to work on the training app we introduce, and which we’ll continue to improve over the course

You need to have Python installed and be able to start IDLE to follow along. You should be familiar with how to create modules/script and run a Python script, either from the terminal or the GUI window.

A good starting point is this link: How To Install Python On Your Computer

Store variables is necessary because a program need to keep track of information while the program is running. The logic in a program is based on logic and information. Since the amount of information is usually more than what the programmer can keep track of in short-term memory, we use variables.

Everyone starts to learn about variables when they write their first computer program.

On a basic level, just remember this:

• We use variables as storage in our programs
• Variables makes the program easier to read
• Variables off-load the programmer’s short-term memory
• Variables can contain primitive values like numbers or more complex structures, e.g. like lists of numbers

In order to enhance learning, we will introduce the project for managing workout programs.

Here we will lay the foundation for the program by reusing our knowledge of variables in a real project.

For now, we will focus on tracking which exercises to perform, how many times you’ll train per week, and similar tasks.

Here’s how you can start building the foundation for a real app to create a workout schedule, we will introduce the tuple now.

1. Add a variable frequency that indicates how many times per week you train. Remember, you only have four exercises, a valid value could be something between 1–4—you decide!
2. Add new line containing the input function, see code block below. This allows the user to enter a response to the program. Run the program, enter a response, and press Enter. What do you see?
3. Add a new question to the user, again use input with the question: Which exercise do you want to know more about?. Print the result (the value) from that input. You can use the code block below, remember to change the content between the quotation marks only.

Use this code block where the exercise asks for it

response = input('Do you want to see all the exercises in the workout plan?')
print(response)

When the program runs, it will pause and wait for the user’s input, i.e. press a key. If you don’t press a key, the program could wait indefinitely. So try pressing Enter and see the program finish. In the code, we’ve added a variable response to the left of the input expression.

week = 1
response = input('Do you want to see all the exercises in the workout plan?')

If we compare these two right-hand sides, `1` is called a constant value, and something we as the programmer have assigned to the variable week.

What we need to understand is that input will generate a value, technically it’s the return value of the function. While it doesn’t appear as a number in the code, it will guarantee a value will be be stored in a variable response.

The return value of input will be stored in the variable response. Remember this!

Since we cannot see the value, it’s up to the programmer to either further examine the value or leave it as is. As part of this learning, we decide to print the value.

For now and for a good while ahead, it’s enough to understand and assume that what stands to the right of the equal sign must be an expression that results in a value.

You’ve used input and print as part of Python. Both are called a built-in Python function. Their purpose is to handle communication between the user and the program, they help make the program interactive.

Functions are packaged solutions to common problems for a programmer – as matter of fact, they exist in all languages.

Relying on built-in Python functions can take you a long way, and later we will write our own functions too.

Imagine a carpenter cutting a piece of wood, that could be done with a common tool such as a saw (function). An electric saw would be even better, both are examples of functionality the carpenter can use to help him perform his task. Both are better than trying to cut the piece with bare hands only.

Likewise, the reason print and input are built-in functions is that it’s complex enough for the programmer to handle how a computer “communicates” with a user under-the-hood, and it would be a waste of time to program that logic yourself. Functions hide complex logic in a simple format so that the programmer can have a more enjoyable and easier life.

This principle is called abstraction and something we will explore later.

week = 1
schedule = 'Workout A'
exercises = ('Bench press', 'Deadlift', 'Squat', 'Biceps curl')

response = input('Do you want to see all the exercises in the workout plan? (Yes/No)')
print(response)  # Prints the user’s response

The program flow is

1. The program asks the user if it wants to view all the exercises of the workout plan
2. The user answers the question
3. The program stores the response (return value) from the function in the variable response.

When we build a program, it’s normal to think about the relevant questions needed for the program to function the way you want. This will be a recurring theme in your job as a programmer, it’s important to have an idea and work towards that. It’s important to think this way early to avoid building the “wrong” program, or rather to build the program that suits the user as best as possible.

Remember, we haven’t been precise about what a workout plan should look like, so you must decide how the program should be used as well. This is called requirements analysis in industry language. Translating requirements into code is a major part of writing good programs. The best way to learn is to do it with a real project in mind, that’s the motivation we start with examples early on.

Review these terms

• string
• functions
• abstraction
• tuple
• variable
• return value

In this section, we’ve gone through concepts that can be used to build many different programs. You’ve practiced Python’s built-in functions and saved information in a program.

We have also gone through a simplified process for how a program can be developed. We started with an introductory text and expanded it in several steps by asking the user a question. We also printed the user’s response. This is how programming begins—different parts evolve at different pace.

It’s not easy to do everything at once, so it’s good to think of this process as normal. If someone wants to do everything at once, avoid these personalities at work (hah).

In the next section, we’ll see how to control the flow of a program in different ways and use more Python functions to read text files.

Do you have any ideas right now of how you want to develop your program? After the user enters a response, you can, for example, make the program more user-friendly and ask for details about an exercise.

See you in the next section!