虽然循环在 Python 中连续运行，输入好还是坏？

Question

我正在编写一个向用户询问百分比的代码，并一直询问直到用户输入可接受的输入。但是，当我运行它时，无论我输入哪种输入，while 循环都不会中断。

这是我的代码：

import math

while True:
    try:
        entered = float(raw_input("Please enter the velocity (as a percentage of the speed of light): "))
    except ValueError:
        print("Sorry, an acceptable input was not entered. Try again.")
        continue

    if entered > 100:
        print("Sorry, a velocity greater than the speed of light cannot be used. Try again.")
        continue
    elif entered <= 0:
        print("Sorry, a percent cannot be negative. Try again.")
        continue
    else:
        #the percent entered is valid, break out of while loop
        break

print("Ship is traveling at ", entered, "% the speed of light.")
print("  ")

speedOfLight = 299792458                         #speed of light constant
percentage = entered / 100                       #turn entered percent into decimal
speed = speedOfLight * percentage                #actual speed (in m/s)         
denominator = math.sqrt(1 - (percentage ** 2))   #denominator of factor equation
factor =  1 / denominator                        #solve for given factor equation

shipWeight = 70000 * factor                      #given ship weight * factor
alphaCentauri = 4.3 / factor                     # given times divided by the factor
barnardsStar = 6.0 / factor
betelgeuse = 309 /factor
andromeda = 2000000 / factor

print("At this speed: ")
print("    Weight of the shuttle is ", shipWeight)
print("    Perceived time to travel to Alpha Centauri is ", alphaCentauri, " years.")
print("    Perceived time to travel to Barnard's Star is ", barnardsStar, " years.")
print("    Perceived time to travel to Betelgeuse is ", betelgeuse, " years.")
print("    Perceived time to travel to Andromeda Galaxy is ", andromeda, " years.")

Answer 1

您正在检查except 中的数据输入。除非对float 的强制转换引发ValueError，否则你永远不会进入你的内部。

您只是想将条件移出except 块，以便检查通过float 转换的数据：

while True:
    try:
        entered = float(input("Please enter the velocity (as a percentage of the speed of light): "))
    except ValueError:
        print("Sorry, an acceptable input was not entered. Try again.")
        continue

    if entered > 100:
        print("Sorry, a velocity greater than the speed of light cannot be used. Try again.")
        continue
    elif entered <= 0:
        print("Sorry, a percent cannot be negative. Try again.")
        continue
    else:
        #the percent entered is valid, break out of while loop
        break

Answer 2

您的缩进有点不稳定，代码永远不会到达break 语句，因为您在此之前continue 循环。幸运的是，您可以使用和 else 关键字使其工作：

while True:
    try:
        entered = float(raw_input("Please enter the velocity (as a percentage of the speed of light): "))
    except ValueError:
        print("Sorry, an acceptable input was not entered. Try again.")
        continue
    else: # no exception
        if entered > 100:
            print("Sorry, a velocity greater than the speed of light cannot be used. Try again.")
            continue
        elif entered <= 0:
            print("Sorry, a percent cannot be negative. Try again.")
            continue
        else:
            #the percent entered is valid, break out of while loop
            break

Answer 3

您甚至不需要继续/中断来完成这项工作。您还需要“导入数学”，一旦您最终跳出 while 循环，这将很重要。

您需要做的是观察缩进。 try/except 不在位置 - 如果这反映了代码的实际编写方式，这将解释您永无止境的 while 循环。

只有缩进修复和“导入数学”

import math

valid = False

while not valid:
    try:
            entered = float(raw_input("Please enter the velocity (as a percentage of the speed of light): "))
    except ValueError:
            print("Sorry, an acceptable input was not entered. Try again.")
            continue

    if entered > 100:
        print("Sorry, a velocity greater than the speed of light cannot be used. Try again.")
    elif entered <= 0:
        print("Sorry, a percent cannot be negative. Try again.")
    else:
        #the percent entered is valid, break out of while loop
            valid = True

print("Ship is traveling at ", entered, "% the speed of light.")
print("  ")

speedOfLight = 299792458                         #speed of light constant
percentage = entered / 100                       #turn entered percent into decimal
speed = speedOfLight * percentage                #actual speed (in m/s)         
denominator = math.sqrt(1 - (percentage ** 2))   #denominator of factor equation
factor =  1 / denominator                        #solve for given factor equation

shipWeight = 70000 * factor                      #given ship weight * factor
alphaCentauri = 4.3 / factor                     # given times divided by the factor
barnardsStar = 6.0 / factor
betelgeuse = 309 /factor
andromeda = 2000000 / factor

print("At this speed: ")
print("    Weight of the shuttle is ", shipWeight)
print("    Perceived time to travel to Alpha Centauri is ", alphaCentauri, " years.")
print("    Perceived time to travel to Barnard's Star is ", barnardsStar, " years.")
print("    Perceived time to travel to Betelgeuse is ", betelgeuse, " years.")
print("    Perceived time to travel to Andromeda Galaxy is ", andromeda, " years.")