Walkthrough: Making Decisions (if / else)¶

This guide walks through the thinking process for this exercise. It does NOT give you the complete solution. For that, see SOLUTION.md.

Before reading this¶

Try the exercise yourself first. Spend at least 15 minutes. If you have not tried yet, close this file and open the exercise file.

Understanding the problem¶

Programs need to make choices. "If something is true, do this. Otherwise, do something else." This exercise shows you how Python decides which code to run using if, elif, and else.

The exercise file has four examples: 1. A temperature checker (hot or not?) 2. A grade calculator (A, B, C, D, or F) 3. A password checker (right or wrong?) 4. Combining conditions with and, or, and not

Planning before code¶

Before writing any if statement, ask yourself:

What am I checking? (a number, a string, a True/False value?)
How many possible outcomes are there? (two? five?)
What order should I check them? (does it matter?)

Step 1: Simple if/else -- two outcomes¶

The simplest decision has two paths: yes or no.

temperature = 75

if temperature > 80:
    print("It is hot outside.")
else:
    print("It is not too hot.")

The indented line under if runs when the condition is True. The indented line under else runs when it is False. Indentation matters -- use 4 spaces.

Predict before you scroll¶

If temperature is 75, which message prints? What if you change it to 85?

Step 2: Multiple conditions with elif¶

When there are more than two outcomes, use elif (short for "else if"):

score = 85

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")

Python checks each condition from top to bottom and runs the first one that is True. Once it finds a match, it skips the rest.

Predict before you scroll¶

If score is 85, which grade prints? Why does it not also print "Grade: C" even though 85 is also >= 70?

Step 3: The = vs == trap¶

This is the single most common mistake for beginners:

# WRONG -- this tries to store 5 in x
if x = 5:

# RIGHT -- this checks if x equals 5
if x == 5:

One = means "store this value." Two == means "are these equal?"

Common mistakes¶

Mistake	Why it happens	How to fix
Using `=` instead of `==`	Looks similar, easy to forget	`=` stores, `==` compares -- always use `==` in `if`
Forgetting the colon `:`	New syntax to remember	Every `if`, `elif`, `else` line ends with `:`
Wrong indentation	Python is picky about spaces	Use exactly 4 spaces for each level
Checking conditions in wrong order	`if score >= 70` before `if score >= 90` catches 90+ first	Check the most specific condition first

What to explore next¶

Build an age checker that uses input() to ask the user their age, then prints whether they are an adult or a minor
Change the temperature and score values in the exercise file, predict the output, then run it -- were you right?