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Branching using Conditional Statements and Loops in Python


This tutorial covers the following topics:

  • Branching with if, else and elif
  • Nested conditions and if expressions
  • Iteration with while loops
  • Iterating over containers with for loops
  • Nested loops, break and continue statements

How to run the code

This tutorial is an executable Jupyter notebook hosted on Jovian. You can run this tutorial and experiment with the code examples in a couple of ways: using free online resources (recommended) or on your computer.

Option 1: Running using free online resources (1-click, recommended)

The easiest way to start executing the code is to click the Run button at the top of this page and select Run on Binder. You can also select "Run on Colab" or "Run on Kaggle", but you'll need to create an account on Google Colab or Kaggle to use these platforms.

Option 2: Running on your computer locally

To run the code on your computer locally, you'll need to set up Python, download the notebook and install the required libraries. We recommend using the Conda distribution of Python. Click the Run button at the top of this page, select the Run Locally option, and follow the instructions.

Jupyter Notebooks: This tutorial is a Jupyter notebook - a document made of cells. Each cell can contain code written in Python or explanations in plain English. You can execute code cells and view the results, e.g., numbers, messages, graphs, tables, files, etc., instantly within the notebook. Jupyter is a powerful platform for experimentation and analysis. Don't be afraid to mess around with the code & break things - you'll learn a lot by encountering and fixing errors. You can use the "Kernel > Restart & Clear Output" menu option to clear all outputs and start again from the top.

Branching with if, else and elif

One of the most powerful features of programming languages is branching: the ability to make decisions and execute a different set of statements based on whether one or more conditions are true.

The if statement

In Python, branching is implemented using the if statement, which is written as follows:

if condition:

The condition can be a value, variable or expression. If the condition evaluates to True, then the statements within the if block are executed. Notice the four spaces before statement1, statement2, etc. The spaces inform Python that these statements are associated with the if statement above. This technique of structuring code by adding spaces is called indentation.

Indentation: Python relies heavily on indentation (white space before a statement) to define code structure. This makes Python code easy to read and understand. You can run into problems if you don't use indentation properly. Indent your code by placing the cursor at the start of the line and pressing the Tab key once to add 4 spaces. Pressing Tab again will indent the code further by 4 more spaces, and press Shift+Tab will reduce the indentation by 4 spaces.

For example, let's write some code to check and print a message if a given number is even.

a_number = 34