Stage 1 · 25 minutes · Free

Conditionals: if / elif / else

Let a program decide: comparisons, if, elif, else, and/or/not operators and indentation rules.

Objectives

• Use comparison operators ==, !=, <, >, <=, >=
• Write if / elif / else
• Combine conditions with and, or, not
• Indent correctly

Why learn this

A smart program must branch: enough points means pass, too few means fail; the right password gets in, the wrong one gets an error.

Concept

if dieu_kien: runs the indented block below when the condition is true.

elif (else if) checks the next condition when the earlier ones are false. else runs when all are false. Python stops at the FIRST true branch.

Comparison uses ==, while = is assignment. Combine conditions: and (both true), or (one of them true), not (negation).

After the : you must go to a new line and indent 4 spaces. Lines in the same block must be indented equally.

Syntax

if dieu_kien_1:
    lenh
elif dieu_kien_2:
    lenh
else:
    lenh

Example

diem = float(input())
if diem >= 8:
    print("Giỏi")
elif diem >= 5:
    print("Đạt")
else:
    print("Chưa đạt")

Input data:

6.5

Output:

Đạt

Line-by-line walkthrough

1. Line 1: read the score 6.5.
2. Line 2: 6.5 >= 8 is false, skip line 3.
3. Line 4: 6.5 >= 5 is true, run line 5 printing Đạt.
4. Lines 6-7: do not run, because a branch was already true.

Common mistakes

if x = 5:

if x == 5:

if x > 3
    print(x)

if x > 3:

if x > 3:
print(x)

Thêm 4 dấu cách trước print(x).

Summary

• if / elif / else for branching.
• == compares, = assigns.
• and, or, not combine conditions.
• The : and 4-space indentation are mandatory.