for 迴圈是 Python 遍歷序列（列表、元組、字串等）的核心工具。本文涵蓋基本語法、流程圖、直接遍歷元素、索引遍歷、搭配 else 子句、以及尋找質數的實戰範例。

基本語法

for iterating_var in sequence:
    statement(s)

流程圖

範例 1：直接遍歷字串與列表

#!/usr/bin/python
for letter in 'Python':
    print 'Current Letter :', letter

fruits = ['banana', 'apple', 'mango']
for fruit in fruits:
    print 'Current fruit :', fruit

print "Good bye!"

執行結果

Current Letter : P
Current Letter : y
Current Letter : t
Current Letter : h
Current Letter : o
Current Letter : n
Current fruit : banana
Current fruit : apple
Current fruit : mango
Good bye!

範例 2：索引遍歷

fruits = ['banana', 'apple', 'mango']
for index in range(len(fruits)):
    print 'Current fruit :', fruits[index]

範例 3：for-else 尋找質數

for num in range(10, 20):
    for i in range(2, num):
        if num % i == 0:
            j = num / i
            print '%d equals %d * %d' % (num, i, j)
            break
    else:
        print num, 'is a prime number'

執行結果

10 equals 2 * 5
11 is a prime number
12 equals 2 * 6
13 is a prime number
14 equals 2 * 7
15 equals 3 * 5
16 equals 2 * 8
17 is a prime number
18 equals 2 * 9
19 is a prime number

關鍵點： else 區塊在迴圈正常結束（未遇 break）時執行。