Python ValueError:无法将字符串转换为浮点：

我在程序中使用了两次此语句。第二次失败

output=""
pitcherName=input("Enter name of the next contestant, or nothing to quit: ")
pitcherTime=input("Enter time for " +str(pitcherName)+ " in milliseconds: ")
highestSpeed=pitcherTime
lowestSpeed=pitcherTime
fastestPitcher=pitcherName
slowestPitcher=pitcherName
while pitcherName!="":
    pitcherName=input("Enter name of the next contestant, or nothing to quit: ")
    pitcherTime=float(input("Enter time for " +str(pitcherName) +" in milliseconds: "))
    pitcherSpeed=round(40908/pitcherTime, 2)
    output=output +str(pitcherName)+ "\t" +str(round(pitcherTime, 2)) + "\t"  +str(round(pitcherSpeed, 2)) + "\n"
    if fastestPitcher==pitcherName and pitcherSpeed>highestSpeed:
        fastestPitcher=pitcherName
        highestSpeed=pitcherSpeed
    elif slowestPitcher==pitcherName and pitcherSpeed>lowestSpeed:
        slowestPitcher=pitcherName
        lowestSpeed=pitcherSpeed
print("Name" + "\t" +"Time" +"\t" +"Speed" + "\n" + "===========================" + "\n")
print(output)
print("Slowest pitcher was " +str(slowestPitcher) +" at " +str(round(lowestSpeed, 2)) +" miles per hour")
print("Fastest pitcher was " +str(fastestPitcher) +" at " +str(round(highestSpeed, 2)) +" miles per hour")
exit=input("Press nothing to`enter code here` exit")

pitcherTime=float(input("Enter time for " +str(pitcherName) +" in milliseconds: "))

收到错误：

pitcherTime=float(input("Enter time for " +str(pitcherName) +" in milliseconds: "))
ValueError: could not convert string to float: 

---

我知道这可能是一个基本问题，但我想知道为什么它在

while

循环之外工作，而不是在循环内部工作。在已经完成后是否不需要转换为float

这样做，尝试并处理异常raise VALUERROR

while pitcherName!="":
    try:
        pitcherName=input("Enter name of the next contestant, or nothing to quit: ")
        pitcherTime=float(input("Enter time for " +str(pitcherName) +" in milliseconds: "))
    except ValueError:
        print "input error"

---

考虑到它，pitcherName在while之前有一些价值，而这不起作用的原因几乎肯定与您的

while

循环无关。如果没有执行奇怪操作的未包含代码，那么它最可能失败的原因是用户提供的输入无法转换为

float

。（例如，如果他们在您的

输入中键入
1.0fzjfk
，在这种情况下，使用
float（）
实际上是在调用
float（“1.0fzjfk”）
，这是不可能的。）

不过，问题的实质几乎完全取决于用户输入，因此很难准确指出问题在哪里以及如何失败。
当用户输入的值不能转换为浮点值时，就会出现这种情况。您可以通过将其包装在一个
try…中来检测是否发生这种情况，但以下情况除外：

try:
    pitcherTime=float(input("Enter time for " +str(pitcherName) +" in milliseconds: "))
except ValueError:
    continue # Start the loop over if they enter an invalid value.

实际上，仅仅将其放入while循环并不能改变错误。考虑到你没有给出太多的上下文，我不完全确定你的意思

希望这有帮助，祝你好运
 

def get_time():
    pitcherName = input("Enter name of the next contestant, or nothing to quit: ")
    goodinput = False
    while not goodinput:
        try:
            pitcherTime = float(input("Enter time for " + str(pitcherName) + " in milliseconds: "))
            goodinput = True
        except ValueError:
            goodinput = False
            print("Invalid Input")



get_time()

你之前说过

我在程序中使用了两次此语句。第二次失败

output=""
pitcherName=input("Enter name of the next contestant, or nothing to quit: ")
pitcherTime=input("Enter time for " +str(pitcherName)+ " in milliseconds: ")
highestSpeed=pitcherTime
lowestSpeed=pitcherTime
fastestPitcher=pitcherName
slowestPitcher=pitcherName
while pitcherName!="":
    pitcherName=input("Enter name of the next contestant, or nothing to quit: ")
    pitcherTime=float(input("Enter time for " +str(pitcherName) +" in milliseconds: "))
    pitcherSpeed=round(40908/pitcherTime, 2)
    output=output +str(pitcherName)+ "\t" +str(round(pitcherTime, 2)) + "\t"  +str(round(pitcherSpeed, 2)) + "\n"
    if fastestPitcher==pitcherName and pitcherSpeed>highestSpeed:
        fastestPitcher=pitcherName
        highestSpeed=pitcherSpeed
    elif slowestPitcher==pitcherName and pitcherSpeed>lowestSpeed:
        slowestPitcher=pitcherName
        lowestSpeed=pitcherSpeed
print("Name" + "\t" +"Time" +"\t" +"Speed" + "\n" + "===========================" + "\n")
print(output)
print("Slowest pitcher was " +str(slowestPitcher) +" at " +str(round(lowestSpeed, 2)) +" miles per hour")
print("Fastest pitcher was " +str(fastestPitcher) +" at " +str(round(highestSpeed, 2)) +" miles per hour")
exit=input("Press nothing to`enter code here` exit")

pitcherTime=float(input("Enter time for " +str(pitcherName) +" in milliseconds: "))

如果你没有更改代码，那就不是真的

# case 1
pitcherTime=input("Enter time for " +str(pitcherName)+ " in milliseconds: ")

#later...

#case 2
pitcherTime=float(input("Enter time for " +str(pitcherName) +" in milliseconds: "))

这是有区别的

input
从
stdin
中读取一行，并将其作为字符串返回。在第一种情况下，存储在
pitchrtime
中的结果（字符串）

在第二种情况下，您编写一个提示符，获取字符串，然后尝试将其转换为
float

在错误发生的那一刻。Python确切地说是出了什么问题：

could not convert string to float: 

，它只是意味着您提供的字符串不能转换为
float

因此，问题不在代码中。问题在于您或任何人向程序提供的输入。
如果用户的输入不能转换为
浮点值，则无法工作。这取决于用户输入的内容。我们可以有更多的上下文吗？您提供了什么作为输入？记住，一行中只能有一个浮点数。