Stage 1 · 30 minutes · Free

for and while loops

Repeat work with for and range(), while, accumulation, break and continue.

Objectives

• Use for with range(start, stop, step)
• Use while with a stopping condition
• Write accumulation patterns
• Use break and continue

Why learn this

Computers are best at doing the same thing again and again without getting tired. Printing 100 lines, adding 1000 numbers, going through a class list: all of it needs loops.

Concept

for i in range(n): repeats n times with i = 0, 1, ..., n-1. range(a, b) goes from a to b-1. range(a, b, buoc) steps by buoc.

while dieu_kien: repeats while the condition stays true. You must have a statement that gradually makes the condition false, otherwise it loops forever (the grader stops after a few seconds).

Accumulation pattern: create tong = 0 before the loop, each round tong = tong + gia_tri.

break exits the loop immediately; continue skips the rest of the current round.

Syntax

for i in range(1, 6):
    print(i)

while n > 0:
    n = n - 1

Example

n = int(input())
tong = 0
for i in range(1, n + 1):
    tong = tong + i
print("Tổng 1 đến", n, "là", tong)

Input data:

5

Output:

Tổng 1 đến 5 là 15

Line-by-line walkthrough

1. Line 1: n = 5.
2. Line 2: tong starts at 0.
3. Line 3: i runs 1, 2, 3, 4, 5 (range stops before n + 1 = 6).
4. Line 4: each round adds i to tong: 1, 3, 6, 10, 15.
5. Line 5: printed after the loop (this line is NOT indented).

Common mistakes

for i in range(1, 5):  # muốn 1..5

range(1, 6) hoặc range(1, n + 1)

while i < 5:
    print(i)

Thêm i = i + 1 trong vòng lặp.

for i in range(5):
    tong = 0
    tong = tong + i

Đặt tong = 0 trước vòng for.

Summary

• for + range to repeat a known number of times.
• while repeats by condition; remember to make the condition gradually false.
• An accumulation variable is initialized before the loop.
• break exits, continue skips the current round.