Python 3 非程式設計師教程/決策
像往常一樣，我認為我應該用熱身打字練習開始每一章，所以這裡有一個簡短的程式來計算一個整數的絕對值
n = int(input("Number? "))
if n < 0:
print("The absolute value of", n, "is", -n)
else:
print("The absolute value of", n, "is", n)
以下是程式執行兩次的輸出結果
Number? -34 The absolute value of -34 is 34
Number? 1 The absolute value of 1 is 1
那麼，當計算機看到這段程式碼時它會做什麼呢？首先，它使用語句“n = int(input("Number? "))”提示使用者輸入一個數字。然後，它讀取行“if n < 0:”。如果n小於零，Python 將執行行“print("The absolute value of", n, "is", -n)”。否則，它將執行行“print("The absolute value of", n, "is", n)”。
更正式地說，Python 會檢視表示式n < 0 是真還是假。一個if語句後面跟著一個縮排的程式碼塊，當表示式為真時，這些語句就會被執行。可選地，if語句後面可以跟著一個else語句和另一個縮排的程式碼塊。如果表示式為假，這個第二個程式碼塊將被執行。
表示式可以進行許多不同的測試。這裡是一個所有測試的表格
| 運算子 | 函式 |
|---|---|
<
|
小於 |
<=
|
小於或等於 |
>
|
大於 |
>=
|
大於或等於 |
==
|
等於 |
!=
|
不等於 |
if命令的另一個特點是elif語句。它代表else if，意思是如果原始的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。這裡有一個例子
a = 0
while a < 10:
a = a + 1
if a > 5:
print(a, ">", 5)
elif a <= 3:
print(a, "<=", 3)
else:
print("Neither test was true")
以及輸出結果
1 <= 3 2 <= 3 3 <= 3 Neither test was true Neither test was true 6 > 5 7 > 5 8 > 5 9 > 5 10 > 5
注意，elif a <= 3 只有在if語句不為真時才會被測試。可以有多個elif表示式，允許在單個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)
以及輸出結果
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 = 7
guess = -1
print("Guess the number!")
while guess != number:
guess = int(input("Is it... "))
if guess == number:
print("Hooray! You guessed it right!")
elif guess < number:
print("It's bigger...")
elif guess > number:
print("It's not so big.")
示例執行
Guess the number! Is it... 2 It's bigger... Is it... 5 It's bigger... Is it... 10 It's not so big. Is it... 7 Hooray! You guessed it right!
even.py
# Asks for a number.
# Prints if it is even or odd
number = float(input("Tell me a number: "))
if number % 2 == 0:
print(int(number), "is even.")
elif number % 2 == 1:
print(int(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: 3.4895 3.4895 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 = float(input("Enter a number: "))
if number != 0:
count = count + 1
sum = sum + number
if number == 0:
print("The average was:", sum / count)
示例執行
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.
#Notice that we use an integer to keep track of how many numbers,
# but floating point numbers for the input of each number
sum = 0.0
print("This program will take several numbers then average them")
count = int(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 = float(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
編寫一個程式，提示使用者輸入他們的姓名，如果他們輸入你的姓名，則顯示“That is a nice name”，如果他們輸入“John Cleese”或“Michael Palin”，則告訴他們你對他們的感受；)，否則告訴他們“You have a nice name”。
解決方案
name = input('Your name: ')
if name == 'Bryn':
print('That is a nice name.')
elif name == 'John Cleese':
print('... some funny text.')
elif name == 'Michael Palin':
print('... some funny text.')
else:
print('You have a nice name.')
修改本節中更高或更低的程式，以跟蹤使用者輸入錯誤數字的次數。如果超過3次，在最後列印“That must have been complicated”，否則列印“Good job！”。
解決方案
number = 7
guess = -1
count = 0
print("Guess the number!")
while guess != number:
guess = int(input("Is it... "))
count = count + 1
if guess == number:
print("Hooray! You guessed it right!")
elif guess < number:
print("It's bigger...")
elif guess > number:
print("It's not so big.")
if count > 3:
print("That must have been complicated.")
else:
print("Good job!")
編寫一個程式，要求輸入兩個數字。如果兩個數字的和大於100，則列印“That is a big number”。
解決方案
number1 = float(input('1st number: '))
number2 = float(input('2nd number: '))
if number1 + number2 > 100:
print('That is a big number.')