为什么`letter ==" A"或“a”始终评估为真？ [英] Why does `letter=="A" or "a"` always evaluate to True?

问题描述

请查看代码。我正在使用机器人汽车来画一个字母，在这个代码中，当我输入b时，它仍然会画小案例a。

Please look at the code. I'm using a robot car to draw a letter and in this code, when I type b, it will still draw small case a.

import create

# Draw a:
def drawa():
 #create robot
 robot = create.Create(4)
 #switch robot to full mode
 robot.toFullMode()
 for i in range(1280):
  robot.go(20,30)
 robot.stop()
 robot.move(-40,20)

# Draw b:
def drawb():
 #create robot
 robot = create.Create(4)
 #switch robot to full mode
 robot.toFullMode()
 robot.move(-100,20)
 for i in range(1270):
  robot.go(20,-30)
 robot.stop()

# Draw c:
def drawc():
 #create robot
 robot = create.Create(4)
 #switch robot to full mode
 robot.toFullMode()
 for i in range(700):
  robot.go(20,30)
 robot.stop()

# Define Main Function
def main():
 # While loop
 while(True):
  # Prompt user to enter a letter
  letter = raw_input("Please enter the letter you want to draw: ")
  # If user enters the letter a, draw a
  if letter=="A" or "a":
   drawa()
  # If user enters the letter b, draw b
  elif letter=="B" or "b":
   drawb();
  # If user enters the letter c, draw c
  elif letter=="C" or "c":
   drawc();
  # If user enters anything other than a letter from a-z,
  # ask them to enter a valid input
  else:
   print("Please enter a letter from a-z.")

main()

请帮忙。

推荐答案

这是因为你的条件。当你说...

It's because of your conditions. When you say...

if letter == "A" or "a"

...你实际上在说......

...you are actually saying...

if it's true that 'letter' equals 'A', or is true that 'a'

...和a，作为非空字符串，总是计算为true。您不是在或右侧的 letter 中询问任何内容。这样做：

... and "a", as a non-empty string, evaluates always to true. You are not asking anything from letter in the right-hand side of the or. Do this:

if letter == "A" or letter == "a"

或者，因为我们在python中：

Or, since we're in python:

if letter in ["A", "a"]

干杯！

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