面向非程式設計師的 Python 2.6 教程/決策
一如既往，我認為我應該用熱身打字練習來開始每一章，所以這裡有一個簡短的程式來計算一個數字的絕對值
n = int(input("Type in a number: "))
if n < 0:
print('The absolute value of', int(n), 'is: ', abs(-n))
else:
print('The absolute value of', int(n), 'is: ', abs(n))
以下是兩次執行該程式的輸出
Type in a number: -14 The absolute value of -14 is: 14
Type in a number: 24 The absolute value of 24 is: 24
那麼，當計算機看到這段程式碼時它做了什麼？首先，它使用語句 "n = input("Number? ")" 提示使用者輸入一個數字。然後，它讀取 "if n < 0:" 這行。如果 n 小於零，Python 就會執行 "print('The absolute value of', int(n), 'is: ', abs(-n))" 這行。否則，它將執行 "print('The absolute value of', int(n), 'is: ', abs(n))" 這行。
更正式地說，Python 會檢視表示式 n < 0 是真還是假。if 語句後面跟著一個縮排的塊，當表示式為真時，該塊中的語句會被執行。可選地，在 if 語句之後是 else 語句和另一個縮排的塊。如果表示式為假，則執行第二個語句塊。
表示式可以有許多不同的測試。以下是所有測試的表格
| 運算子 | 函式 |
|---|---|
<
|
小於 |
<=
|
小於或等於 |
>
|
大於 |
>=
|
大於或等於 |
==
|
等於 |
!=
|
不等於 |
<>
|
另一種表示不等於的方式（舊式，不推薦） |
if 命令的另一個功能是 elif 語句。它代表 else if，意思是如果原始 if 語句為假，但 elif 部分為真，則執行 elif 部分。如果 if 或 elif 表示式都不為真，則執行 else 塊中的內容。以下是一個示例
a = 0
while a < 10:
a = a + 1
if a > 5:
print a, ">", 5
elif a <= 7:
print a, "<=", 7
else:
print "Neither test was true"
以及輸出
1 <= 7 2 <= 7 3 <= 7 4 <= 7 5 <= 7 6 > 5 7 > 5 8 > 5 9 > 5 10 > 5
注意 elif a <= 7 只有在 if 語句不為真時才會被測試。可以有多個 elif 表示式，允許在一個 if 語句中進行多個測試。
# This Program Demonstrates the use of the == operator
# using numbers
print 5 == 6
# Using variables
x = 5
y = 8
print x == y
以及輸出
False False
High_low.py
# Plays the guessing game higher or lower
# This should actually be something that is semi random like the
# last digits of the time or something else, but that will have to
# wait till a later chapter. (Extra Credit, modify it to be random
# after the Modules chapter)
number = 78
guess = 0
while guess != number:
guess = input("Guess a number: ")
if guess > number:
print "Too high"
elif guess < number:
print "Too low"
print "Just right"
示例執行
Guess a number: 100 Too high Guess a number: 50 Too low Guess a number: 75 Too low Guess a number: 87 Too high Guess a number: 81 Too high Guess a number: 78 Just right
even.py
# Asks for a number.
# Prints if it is even or odd
number = input("Tell me a number: ")
if number % 2 == 0:
print number, "is even."
elif number % 2 == 1:
print number, "is odd."
else:
print number, "is very strange."
示例執行
Tell me a number: 3 3 is odd.
Tell me a number: 2 2 is even.
Tell me a number: 3.14159 3.14159 is very strange.
average1.py
# keeps asking for numbers until 0 is entered.
# Prints the average value.
count = 0
sum = 0.0
number = 1 # set to something that will not exit the while loop immediately.
print "Enter 0 to exit the loop"
while number != 0:
number = input("Enter a number: ")
if number != 0:
count = count + 1
sum = sum + number
print "The average was:", sum / count
示例執行
Enter 0 to exit the loop Enter a number: 3 Enter a number: 5 Enter a number: 0 The average was: 4.0
Enter 0 to exit the loop Enter a number: 1 Enter a number: 4 Enter a number: 3 Enter a number: 0 The average was: 2.66666666667
average2.py
# keeps asking for numbers until count numbers have been entered.
# Prints the average value.
sum = 0.0
print "This program will take several numbers then average them"
count = input("How many numbers would you like to average: ")
current_count = 0
while current_count < count:
current_count = current_count + 1
print "Number", current_count
number = input("Enter a number: ")
sum = sum + number
print "The average was:", sum / count
示例執行
This program will take several numbers then average them How many numbers would you like to average: 2 Number 1 Enter a number: 3 Number 2 Enter a number: 5 The average was: 4.0
This program will take several numbers then average them How many numbers would you like to average: 3 Number 1 Enter a number: 1 Number 2 Enter a number: 4 Number 3 Enter a number: 3 The average was: 2.66666666667
- 修改本節中的高低程式，以跟蹤使用者輸入錯誤數字的次數。如果超過 3 次，列印 "That must have been complicated." 請注意，程式不必在猜測數字之前停止詢問數字，它只需要在猜測數字後列印這條資訊。
- 編寫一個程式，要求使用者輸入兩個數字。如果兩個數字的和大於 100，列印 "That is a big number."。
- 編寫一個程式，要求使用者輸入他們的名字，如果他們輸入你的名字，就說 "That is a nice name"，如果他們輸入 "John Cleese" 或 "Michael Palin"，告訴他們你對他們的感受 ;)，否則就告訴他們 "You have a nice name."。
解決方案
修改本節中的高低程式，以跟蹤使用者輸入錯誤數字的次數。如果超過 3 次，列印 "That must have been complicated."。
number = 42
guess = 0
count = 0
while guess != number:
count = count + 1
guess = input('Guess a number: ')
if guess > number:
print 'Too high'
elif guess < number:
print 'Too low'
else:
print 'Just right'
break
if count > 2:
print 'That must have been complicated.'
break
編寫一個程式，要求使用者輸入兩個數字。如果兩個數字的和大於 100，列印 "That is a big number."。
number1 = input('1st number: ')
number2 = input('2nd number: ')
if number1 + number2 > 100:
print 'That is a big number.'
編寫一個程式，要求使用者輸入他們的名字，如果他們輸入你的名字，就說 "That is a nice name"，如果他們輸入 "John Cleese" 或 "Michael Palin"，告訴他們你對他們的感受 ;)，否則就告訴他們 "You have a nice name."。
name = raw_input('Your name: ')
if name == 'Ada':
print 'That is a nice name.'
elif name == 'John Cleese' or name == 'Michael Palin':
print '... some funny text.'
else:
print 'You have a nice name.'