How to make sure that a user inputs a number?

Question

I've written a BMI calculator but I'd like there to be a way to ensure the user only types numbers so, if something else was input the question was to be asked again. Here's some of my code.

#Ask whether metric or imperial is to be used and ensures no typing errors
measure = input("Would you like to use Metric or imperial measurements? Please type i for imperial or m for metric  \n")

while(measure != "i") and (measure != "m"):
    measure =input("Please type a valid response. Would you like to use Metric or imperial measurements? Please type i for imperial or m for metric")

#Asks the users weight
if(measure == "i"):
    weights = input("Please type in your weight in stones and pounds- stones=")
    weightlb = input("- pounds=")
    weights = int(weights)
    weightlb = int(weightlb)
    weight = (weights*14)+weightlb

elif(measure == "m"):
    weight = input("Please type in your weight in kilograms=")

No. The asker just wants to know how to make sure the user types in a number. That's all. — Anthony Pham, Jan 31 '15 at 16:47

Anthony Pham · Accepted Answer · 2015-01-31T15:01:29.780

3

You can simply use a try, except loop plus an while loop. Here's what I mean:

intweight = 0
while True:
    try: 
        weight = float(input())
    except ValueError:
        print "Please enter a number"
    else:
        break
        intweight = weight

The while loop will force the user to enter a string until it only has numbers. The program will try to convert the string into a number. If there are letters, the except part will be activated. If the conversion is successful, the else part will activate, breaking the loop. I hope this helps you!

edited Jan 31 '15 at 15:01

answered Jan 31 '15 at 14:27

Anthony Pham

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This goes well with the Python philosophy: [Easier to ask for forgiveness than permission](https://docs.python.org/2/glossary.html#term-eafp). I would wrap that in a function as this is used in several part of the program, though. – Sylvain Leroux Jan 31 '15 at 14:48
Sorry, bit of a newbie, do I just paste this under the kilograms part? thank you for your help :) – nitrogirl Jan 31 '15 at 14:49
this goes in place of your `while`-loop where you ask for `input("Please ... metric")` – RvdBerg Jan 31 '15 at 14:57
You can use `while True`, and instead of `integer = 1` use `break` after `intweight = weight` – user Jan 31 '15 at 14:59
But that while loop ensures a metric or imperial is chosen, its not anything to do with the numbers? I want it to re-ask for the users weight if for example the user types a letter or symbol. – nitrogirl Jan 31 '15 at 15:02
The while loop insures a number is chosen. If there is a letter, the program will ask the user to re-try. – Anthony Pham Jan 31 '15 at 15:19
You can use this code wherever you need the user to only type in numbers – Anthony Pham Jan 31 '15 at 15:21

score 1 · Answer 2 · answered Jan 31 '15 at 14:21

1

Its what that str.isdigit is for :

while( not measure.isdigit()) :
     measure =input("Please type numbers only ")

answered Jan 31 '15 at 14:21

Mazdak

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How to make sure that a user inputs a number?

2 Answers2