Loops and Logic

Last updated on 2026-06-27 | Edit this page

Overview

Questions

How do I use loops to repeat code in Python?
What are the different types of loops in Python and when should I use each one?
What are the main logical operators in Python and how do they work?

Objectives

Learn the different types of loops in Python and when to use each one
Understand and use the main logical operators in Python

Scaling Up with Loops

One of the most common and fun things to do in Python is to write loops of code. Loops make Python iterate over a certain section of code (usually over a list or a set of numbers). There are two main types of loops, the for loop and the while loop.

`for` the Record

The for statement in Python iterates over the items of a sequence (e.g., a list, a string, or range of values) in the order that the items appear in the sequence. The syntax of a for loop is

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letters = ["a", "b", "c"]
for letter in letters:
    # Code to execute for each item in the sequence
    print(letter)

Discussion

The body of a “for” loop is separated from the rest of the code using indentation. Indentation is Python’s way of grouping statements. The official coding conventions for Python dictate using 4 spaces for indentation.

In general follow two rules when using indentation in Python: 1. Use 4 spaces per indentation level. 2. Never mix tabs and spaces.

In actual English, you can think of the line for letter in letters: as meaning

“For every letter in letters, print the letter”.

You should have a file called loops_01.py in the scripts directory that contains the code,

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# loops_01.py

# create a list to loop over
loop_list = ['cat', 'dog', 'hamster', 'bird']

# use a "for" loop to loop over our list
# print out each entry "i" in "loop_list"
for i in loop_list:
    print(i)

Try running the code in loops_01.py and see what happens.

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OUTPUT

cat
dog
hamster
bird

This type of loop can also be used to iterate over a range of values, which is extremely helpful for indexing and slicing in a loop. To iterate over a range of values, you can use the range function, which takes a start value, a stop value, and an optional step value,

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range(start, stop, step)

Just like with indexing, the start value is inclusive while the stop value is exclusive.

To see how this works in practice, open the file loops_02.py in the scripts directory and take a look at the code.

Run the code in loops_02.py and see what happens.

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OUTPUT

Example 2.1: range(0, 10, 1)
0
1
2
3
4
5
6
7
8
9

Example 2.2: range(0, 10, 2)
0
2
4
6
8

Example 2.3: range(4)
0
1
2
3

Example 2.4: range(length of x)
O
L
C
F

Example 2.5: enumerate(x)
0 O
1 L
2 C
3 F

`while` We’re Here…

In contrast with for loops, while loops keep iterating until a certain condition is no longer true. The syntax of a while loop is

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while some_condition:
    # Code to execute while the condition is true
    print("This code will keep running until some_condition is no longer true.")

In natural English, you can think of the line while some_condition: as meaning

While some condition is true, Python will execute the code in the “while” block.

Because the while loop constantly checks to see if a certain statement is “true”, the loop is often used with comparison operators. The standard comparison operators are: < (less than), > (greater than), == (equal to), <= (less than or equal to), >= (greater than or equal to) and != (not equal to).

This means

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# will never stop
while True:
    print("This loop will run forever!")

and

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# will never start
while False:
    print("This loop will never run!")

To see how this works in practice, open the file loops_03.py in the scripts directory and take a look at the code.

Run the code in loops_03.py and see what happens.

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OUTPUT

Example 6.5: while x < y
x equals 0
x equals 1
x equals 2
x equals 3
x equals 4

Example 6.6: while z != 6
Am I 6 yet? Nope, I am 0
Am I 6 yet? Nope, I am 1
Am I 6 yet? Nope, I am 2
Am I 6 yet? Nope, I am 3
Am I 6 yet? Nope, I am 4
Am I 6 yet? Nope, I am 5

while loops can be dangerous at times, as it is very easy to accidentally get stuck in an infinite loop if it is not coded properly. In the above examples, if we did not modify either x or z in our loops, their value never would have changed and we would have gotten stuck in the loop (as both conditions would always remain True). In general, if you can convert a while loop into a for loop, it is recommended to use the for loop version instead.

Discussion

The preference for for over while goes beyond just avoiding infinite loops. while loops almost always require mutation of a variable (e.g., x or z in the above examples), which is a common source of bugs in code. for loops, on the other hand, can often be written without mutation, which makes them easier to reason about and less prone to bugs. Finally, for loops without mutation are immediately equivalent to functional programming constructs like map and filter, which are often objectively better in almost every aspect.

Caution

If you do get stuck in a loop, either close the the terminal window, or press CTRL + C to interrupt the execution.

Now that we’ve introduced conditions being either “True” or “False” in the context of loops, the natural next step is to introduce how to control the flow of your code based on “True” or “False” conditions.

Fork in the Code

If you want to execute a piece of code only if a specific condition is met, the if statement comes in handy. For example, maybe you only want to print something out if a specific variable is “True”, or when a math expression yields a specific value. No matter the reason, the syntax of the if statement is

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if condition:
    print("Execute this block if condition is True")

flowchart LR
    accTitle: {if control flow}
    A{"Condition"}
    B("Code")
    C([Continue])
    A -- True --> B
    A -- False --> C
    B --> C

Like with loops, the body of an if statement is indented. In the example above, Python first evaluates condition to determine whether it is “True” or “False”. Next, Python executes the body of the statement only if condition is “True”. In natural English, you can think of the line if condition: as meaning

If this line is True, Python will do the following: …“.

To see this in practice, open the file logic_01.py in the scripts directory and take a look at the code.

Run the code in logic_01.py and see what happens.

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OUTPUT

'x is 1'

Recall from the Types and Operators episode that the == operator checks if two things are equal. In Example 1.1, x did indeed equal 1, so the condition was “True” and Python executed the print function in the first if statement. In Example 1.2, x did NOT equal 2, so the condition was “False” and Python did not execute the if statement. Because we did not tell Python what to do if the condition was “False”, Python did nothing and skipped over it.

To explicitly tell Python what to do in the event that the condition is “False”, you need to add an else statement, as in

PYTHON

if condition:
    print("Execute this block if condition is True")
else:
    print("Execute this block if condition is False")

flowchart LR
    accTitle: {if-else control flow}
    A{"Condition"}
    B("Code")
    C("Code")
    D([Continue])
    A -- True --> B
    A -- False --> C
    B --> D
    C --> D

First, Python checks to see if the condition is “True”. If the condition is “True”, then Python executes the body of the if statement and ignores the body of the else statement. However, if Python found that the condition was “False”, it would execute the body of the else statement instead.

To see this in practice, open the file logic_02.py in the scripts directory and take a look at the code.

Run the code in logic_02.py and see what happens.

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OUTPUT

'x is not 1'

We see that Python only executed one of the print statements because of our if and else statements. The x variable was equal to 2, so the if block x == 1 condition was “False” and Python executed the else statement instead.

Extending things further, perhaps you want to tell Python to check for multiple conditions, instead of just one. In that case, you need to add an elif statement (short for “else-if”), as in

PYTHON

if condition:
    # A
    print("Execute this block if condition is True")
elif another_condition:
    # B
    print("Execute this block if another_condition is True")
else:
    # C
    print("Execute this block if condition is False")

flowchart LR
    accTitle: {if-elif-else control flow}
    X{"Condition"}
    Y{"Another<br/>Condition"}
    B1("A")
    D1("B")
    E1("C")
    F([Continue])
    X -- True --> B1
    X -- False --> Y
    Y -- True --> D1
    Y -- False --> E1
    B1 --> F
    D1 --> F
    E1 --> F

First, Python checks to see if condition is “True”. If it finds that condition is “True”, then Python will execute the body of the if statement and skip the rest (regardless of the remaining conditions). However, if it finds that condition is “False”, then Python then checks to see if another_condition is “True”. If it finds that another_condition is “True”, then Python will execute the body of the elif statement and skip the rest. And, if all conditions are “False”, then Python will execute the else statement as a last resort.

Because Python skips the remaining if-elif-else code once it finds a “True” condition, only ONE of the statements will be executed across an entire if-elif-else section of code. Even if multiple statements are “True”, it will only execute whichever one it finds first, starting from the top.

We only used one elif line in the above syntax example, but you can include as many elif statements as you like. However, note that you can only ever have one if and else line in a given if-elif-else block of code.

To see this in practice, open the file logic_03.py in the scripts directory and take a look at the code.

Run the code in logic_03.py and see what happens.

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OUTPUT

'x is less than 2'

Key Points

for loops are used to iterate over a sequence of items, while while loops are used to repeat code until a certain condition is no longer true.
The main logical operators in Python are == (equal to), != (not equal to), < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to).
The if statement allows you to execute a block of code only if a specific condition is met, while the else statement allows you to execute a block of code if the condition is not met.
The elif statement allows you to check for multiple conditions in an if-else block, and only the first “True” condition will be executed.
Logical operators can be combined to create more complex conditions in if and elif statements.

Loops and Logic

Overview

Questions

Objectives

Scaling Up with Loops

for the Record

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OUTPUT

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while We’re Here…

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Fork in the Code

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`for` the Record

`while` We’re Here…