使用 Linkbot/决策学习 Python 3

if 语句

我一直认为我应该用热身打字练习来开始每一章，所以这里有一个简短的程序来计算整数的绝对值

n = int(input("Number? "))
if n < 0:
   print("The absolute value of", n, "is", -n)
else:
   print("The absolute value of", n, "is", n)

这是我运行此程序两次的输出

Number? -34
The absolute value of -34 is 34

Number? 1
The absolute value of 1 is 1

那么当计算机看到这段代码时它会做什么呢？首先，它使用语句 "n = int(input("Number? "))" 提示用户输入一个数字。接下来，它读取行 "if n < 0:"。如果 n 小于零，Python 运行行 "print("The absolute value of", n, "is", -n)"。否则，它运行行 "print("The absolute value of", n, "is", n)"。

更正式地说，Python 会查看表达式 n < 0 是真还是假。一个 if 语句后面跟着一个缩进的块语句，当表达式为真时会运行这些语句。可选地在 if 语句之后是一个 else 语句和另一个缩进的块语句。如果表达式为假，则运行第二个块语句。

表达式可以有许多不同的测试。以下是一张所有测试的表格

运算符	函数
`<`	小于
`<=`	小于或等于
`>`	大于
`>=`	大于或等于
`==`	等于
`!=`	不等于

if 命令的另一个特性是 elif 语句。它代表 else if，意思是如果原始 if 语句为假，但 elif 部分为真，则执行 elif 部分。如果 if 或 elif 表达式都不为真，则执行 else 块中的内容。以下是一个示例

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")

以及输出

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 语句不为真时才会被测试。可以有多个 elif 表达式，允许在单个 if 语句中进行多个测试。

移动 Linkbot 的电机

在上一章谁在那里？中，我们看到可以使用 moveJoint() 函数移动 Linkbot 的电机。它通常像这样使用

myLinkbot.moveJoint(1, 180)

其中第一个参数是要移动的电机，第二个参数是要移动的角度（以度为单位）。上面的代码将以正方向将电机 "1" 移动 180 度。

如果你想一次移动多个电机怎么办？Linkbot 还具有一个 move() 函数，它可以做到这一点。它看起来像这样

myLinkbot.move(90, 180, 270)

move() 函数希望在括号内包含三个数字。第一个数字是移动电机 1 的角度，第二个数字是移动电机 2 的角度，第三个数字是移动电机 3 的角度。如果你的 Linkbot 只有两个电机，则没有电机的面的值会被忽略。例如，如果我有一个 Linkbot-I 并运行了上面的示例代码，则 "180" 会被忽略，因为电机 2 不是可移动电机。相反，Linkbot 会将电机 1 向前移动 90 度，并将电机 3 向前移动 270 度。

当 move() 函数执行时，Linkbot 将同时移动其任何/所有电机。

command_linkbot.py

在这个例子中，我们想创建一个程序，我们可以用它来移动我们的 Linkbot。这个演示将使用 python break 命令。在上一章中，我们看到 Python 会陷入“无限循环”。break 命令可以用于退出循环，即使循环条件仍然为真。

让我们先从让 Linkbot 向前和向后移动开始。要运行这个演示，你需要一个 Linkbot-I 和两个轮子。也建议使用一个万向轮，但不是必需的。

import barobo
dongle = barobo.Dongle()
dongle.connect()
myLinkbot = dongle.getLinkbot('abcd') # Replace abcd with your Linkbot's serial ID

while True: # This is an infinite loop. Get out by using the "break" statement
    command = input('Please enter a robot command or "quit" to exit the program: ')
    if command == 'forward':
        # Rotate the wheels 180 degrees to move the robot forward
        myLinkbot.move(180, 0, -180) # Motor 3 has to spin in the negative direction to move the robot forward
    elif command == 'backward':
        myLinkbot.move(-180, 0, 180)
    elif command == 'quit':
        break
    else:
        print('I\'m sorry. I don\'t understand that command.')

print('Goodbye!')

关于字符串的说明

你可能已经注意到，在程序的最后，有一行代码是 print('I\'m sorry. I don\'t understand that command.')，代码中有一些奇怪的反斜杠。但是，当你运行程序时，它不会打印这些反斜杠。这些反斜杠实际上起着重要的作用。

当 Python 遇到单引号 ' 或双引号 " 时，它知道接下来将是一个以相同类型的引号结尾的字符串，该引号是它开始的引号。但是，如果我们想将一些引号包含在字符串中呢？请考虑以下代码

mystring = "Hello there. My dog says "woof!" when he's bored."

当 Python 在 "Hello th... 中看到第一个引号时，它会说：“好的，这将是一个字符串，一旦我看到另一个引号，这就是字符串的结尾”。很快，Python 在 ...says "woof! 中看到了另一个引号，并认为“我找到了字符串的结尾！”，但这不是我们字符串的结尾！我们希望 Python 一直走到 ...bored." 处的结尾。我们如何告诉 Python "woof!" 周围的引号是特殊的，并且我们的字符串实际上并没有在那里结束呢？

答案是反斜杠。如果在引号前面有一个反斜杠，那么对 Python 来说这是一个特殊指示，指示将该引号包含在字符串中，而不是终止字符串。因此，修正后的代码应该是

mystring = "Hello there. My dog says \"woof!\" when he's bored."

在这个例子中，he's 中的单引号会自动包含在字符串中，因为字符串以双引号开头，因此 Python 正在寻找一个双引号来终止字符串。

示例运行

Please enter a robot command or 'quit' to exit the program: forward
Please enter a robot command or 'quit' to exit the program: backward
Please enter a robot command or 'quit' to exit the program: aoeu
I'm sorry. I don't understand that command.
Please enter a robot command or 'quit' to exit the program: quit
Goodbye!

示例

equality.py

# This Program Demonstrates the use of the == operator
# using numbers
print(5 == 6)
# Using variables
x = 5
y = 8
print(x == y)

以及输出

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 = 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.")

示例运行

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.")

示例运行

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)

示例运行

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)

示例运行

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

练习

编写一个程序，该程序询问用户他们的姓名，如果他们输入你的姓名，则说“这是一个好名字”，如果他们输入“John Cleese”或“Michael Palin”，则告诉他们你对他们的感受；)，否则告诉他们“你有一个好名字”。

解决方案

name = input('Your name: ')
if name == 'Ada':
    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.')

修改本节中高或低的程序，以跟踪用户输入错误数字的次数。如果超过 3 次，则在最后打印“那一定很复杂”，否则打印“干得好！”。

解决方案

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!")

编写一个程序，该程序要求提供两个数字。如果这两个数字的和大于 100，则打印“这是一个大数字”。

解决方案

number1 = float(input('1st number: '))
number2 = float(input('2nd number: '))
if number1 + number2 > 100:
    print('That is a big number.')

修改前面介绍的 Linkbot 程序，以包含命令“turnright”、“turnleft”和“beep”。使“turnright”命令使机器人向右转，“turnleft”命令使机器人向左转，“beep”命令使机器人蜂鸣器响一秒钟。

解决方案

import barobo
import time # So we can use time.sleep() later
dongle = barobo.Dongle()
dongle.connect()
myLinkbot = dongle.getLinkbot('abcd') # Replace abcd with your Linkbot's serial ID

while True: # This is an infinite loop. Get out by using the "break" statement
    command = input('Please enter a robot command or "quit" to exit the program: ')
    if command == 'forward':
        # Rotate the wheels 180 degrees to move the robot forward
        myLinkbot.move(180, 0, -180) # Motor 3 has to spin in the negative direction to move the robot forward
    elif command == 'backward':
        myLinkbot.move(-180, 0, 180)
    elif command == 'turnright':
        myLinkbot.move(180, 0, 180)
    elif command == 'turnleft':
        myLinkbot.move(-180, 0, -180)
    elif command == 'beep':
        myLinkbot.setBuzzerFrequency(440)
        time.sleep(1)
        myLinkbot.setBuzzerFrequency(0)
    elif command == 'quit':
        break
    else:
        print('I\'m sorry. I don\'t understand that command.')

print('Goodbye!')

使用 Linkbot 学习 Python 3
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