Loops and Conditional Statements

Loops and conditional statements are very common elements of scripts. These structures allow us to specify what should happen when a particular condition is satisfied or not.

1. Formulating Conditional Statements

Python supports many of the usual logical conditions from mathematics:

  • Equals: a == b
  • Not Equals: a != b
  • Less than: a < b
  • Less than or equal to: a <= b
  • Greater than: a > b
  • Greater than or equal to: a >= b

In these examples, a and b can take different values and they can be of different data-types. However, be aware that certain data types are not suitable for particular logical conditions. For example, if we have a piece of text “University”, which is a string, it will be impossible to evaluate if this is greater than another value. Although this may seem obvious in this example, it could be possible that you stored two numbers (e.g 2 & 4), but you stored one of them as a string instead of an integer. In this case, you will also receive an error. Please see here for more on data-types such as strings and integers.

Exercise 1: Understanding Conditional Statements

Below are some exercises to help you understand conditional statements in Python. Although they may seem simple, please make sure you fully understand how to apply these statements, because you will need this knowledge in the following steps.

  1. Start by specifying two variables: x=5, y=10, z=10 at the start of your script.
  2. What happens if you now run "x == y" in the console?
  3. What datatype is the output that you get? If you are not sure, store the output to a variable (e.g. var) by running "var= x==y" and run "type(var)".
  4. Write a conditional statement to confirm that x is not equal to y.
  5. What happens if you run "y < z" and "y <= z" ? Explain why the results are different.

2. If-Statements

An if-statement allows you to check whether a particular statement is true or not. Depending on the boolean evaluation (true versus false), a particular piece of code (or nothing) is executed.

The header line of the if statement begins with the keyword if followed by a boolean expression and ends with a colon (:). The indented statements that follow are called a block. This block contains the code that should be executed when the condition is satisfied. Each statement inside the block must have the same indentation. The first unindented statement marks the end of the block.

if <conditional statement>:
    <code to be executed when conditional statement is TRUE>

Are you not sure what indentation is and why it is so important in Python? Please check out the section on “Indentation” on the help page. You will find everything you need to know about indentation there!


If you would like to specify what happens when the conditional statement is not true, you can use a if else.

if <conditional statement>:
    <code to be executed when conditional statement is TRUE>
else:
    <code to be executed when conditional statement is FALSE>

We can also chain conditional statements to be checked using a if elif else structure as shown below.

if <first conditional statement>:
    <code to be executed when first conditional statement is TRUE>
elif <second conditional statement>:
    <code to be executed when second conditional statement is TRUE>
elif <third conditional statement>:
    <code to be executed when third conditional statement is TRUE>

...

else:
    <code to be executed when all conditional statements above are FALSE>

In the case that you do not want to execute any code in a particular situation, you can use pass.

Important things to keep in mind when writing if statements:

  • Be aware of the indentation. That is, the code that must be executed is indented by one level. Click here to learn more about indentation.
  • Do not forget the colon (:), it separates the header of the statement from its body.
  • Several forms of if statements are possible, as shown below. They differ in the number of conditions that are checked.

Below you can find several examples of if statements.

2.1 One condition

The most basic form of an if statement checks one condition. If this condition is true, the code below it will be executed. If the condition is not true, it will always perform the second piece of code.

If this is true, do that. Otherwise, do this.

a = 12
b = 5
if a > b:
    print("a is greater than b")
else:
    print("a is not greater than b")

_images/12.png

2.2 One condition, pass

As addressed above, we can use pass to specify that we do not want any code to be executed in a particular situation. In the example below, this is the case when the if statement (a > b) is not true.

If this is true, do that. Otherwise, do nothing.

a = 12
b = 5
if a > b:
    print("a is greater than b")
else:
    pass

_images/22.png

Exercise 2: Writing an IF Statement with one Condition

In this exercise, you will develop a simple IF-statement yourself. Imagine you are writing a very basic program to control the heating in your room. You want to heater to turn on only when the temperature is below 18 degrees celsius. Assume the program will run every time the temperature is measured.

  1. How many conditions will you need to specify?
  2. First, create a variable that stores the temperature threshold value.
  3. Next, create a variable that stores the value of the actual room temperature. You can choose any value to represent the hypothetical current temperature.
  4. Formulate a statement that captures the rule (*when the actual temperature is below the threshold*).
  5. Formulate what should happen if the condition is satisfied. As means of simulation, print "heating" when the heater is turned on.
  6. What did you use to specify that nothing should happen if the condition is not satisfied?
  7. Test your program for several different actual room temperature values. Does it behave as expected?
  8. You would like to tell from the output of your program if the heater is not turned on (instead of simply not getting any output). How can you do this? Try to implement this and test your program again.

2.3 Multiple conditions

As mentioned before, we can chain multiple statements using a if elif else structure. In this way we can capture all situations where the program should perform a specific behavior.

If this is true, do that. Else check if this is true, and do that. Else do this.

a = 3
b = 5
if a > b:
    print("a is greater than b")
elif a == b:
    print("a is equal to b")
else:
    print("b is greater than a")

_images/31.png

Exercise 3: Writing a an IF Statement with Two Conditions

Recall the situation described in Exercise 1. Now imagine you also have an airconditioner. If the temperature is below 18, you still want the heater to turn on. Additionally, you want the airconditioner to activaten whenever the temperature is above 25 degrees. If the temperature is between 18 and 25 degrees, you want both systems to be turned off. Again, simulate what happens by printing it.

  1. How many conditions will you need to specify?
  2. First, create the variables that store the actual temperature value and the threshold values.
  3. Formulate all conditional statements that are applicable to the situation.
  4. For each of the conditions, print what should happen in terms of the activation of either the heating or airco systems.
  5. Test your program for several different actual room temperature values. Does it behave as expected?

3. While-Loops

In a While-loop, a certain script is performed as long as a particular boolean statement evaluates to True.

In the example below, we introduce an indexing variable. Every time the script is executed (the value of i is printed), the index (i) is increased by 1. That is, we are counting how many times the script has been executed.

(Note that we can use i += x to specify a step increase of x units. Hence, i += x is a short way of writing i = i + 1).

The boolean statement i < 10 ensures that when the script has been ran 10 times (so when our index is equal to 10: i = 10), the script will no longer be executed again.

While this is true, do …

i = 1
while i < 10:
    print(i)
    i += 1 #i = i + 1

A while-loop can be interrupted or loops in a while-loop can be skipped using the break and continue statements.

i = 1
while i < 10:
    print(i)
    if i == 5:
        break
    i += 1

i = 1
while i < 10:
    print(i)
    if i == 5:
        continue
    i += 1

Exercise 4: Understanding While-Loops

  1. What happens when you do not increment the index variable? That is, when we leave out the final line of the code (i += 1) in the first example.
  2. Try to code a while-loop that prints each letter in your name separately to the console window.
  3. Draw a diagram like those shown in the earlier section on IF-statements that describes your while-loop.

4. For-Loops

A for loop iterates over a sequence of values and executes a particular piece of code for each of these items. As you might notice, it can actually often replace a while loop.

Recall that lists, tuples, sets and dictionaries contain multiple items. For example ["Volvo", "Ford", "Mercedes"] is a list of three string items, each of which represents the name of a car brand. We can use a for-loop to iterate over these items and print each of them or perform any other kind of operation.

Likewise, we can also iterate over characters in a string.

For every element in this entire sequence, do …

cars = ["Volvo", "Ford", "Mercedes"]
for carName in cars
    print(carName)
        
city = "Eindhoven"
for letter in city
    print(letter)

Exercise 5: Understanding For-Loops

  1. Reconsider the while-loop you developed in the previous exercise, which printed each character of your name. Would you prefer performing this action with a while-loop or a for-loop? Why? (Are there multiple reasons?)
  2. Develop a for-loop to print each character of your name.
  3. Create a list of the names of three of your friends. Use a for-loop to print a sentence saying “<Friend’s name> is my friend.” for each of your friends.