[Tutor] dealing with user input whose value I don't know

[David]

Hello Steve,

thanks for all your help and comments.

What happens, though, is that with

numbers = int(raw_input("Please type the numbers, separated by commas: "))

my code is still defunct (whereas input() works):

Please type the numbers, separated by commas: 1,2
Traceback (most recent call last):
  File "avgInput.py", line 8, in <module>
    numbers = int(raw_input("Please type the numbers, separated by 
commas: "))
ValueError: invalid literal for int() with base 10: '1,2'

**** End of process output ****


Here is the entire code:

print "This program takes the average of numbers you supply!!"
amount = raw_input("How many numbers do you want me to work with? ")
print "You want me to take the average of", amount, "numbers."
numbers = int(raw_input("Please type the numbers, separated by commas: "))
print "You want to know the average of the numbers:", numbers

add = 0
for i in numbers:
    add = add + i
print "The sum of your numbers is:", add
average = add / float(amount)
print "Therefore the average of your numbers is", average


David



Steve Willoughby wrote:
> On Fri, Oct 03, 2008 at 02:06:47AM +0800, David wrote:
>   
>> Cheers for the insights!
>>
>> However, I just found out that changing input() to raw_input() breaks my 
>> code:
>>     
>
> Recall that we told you raw_input() returns a string, while
> input() returns an integer if you typed an integer value.
>
> So you need to convert the string of characters the user typed
> into an integer value before using it as a number:
>
> numbers = int(raw_input(...))
>
>
>   
>> This program takes the average of numbers you supply!!
>> How many numbers do you want me to work with? 2
>> You want me to take the average of 2 numbers.
>> Please type the numbers, separated by commas: 1,2
>> You want to know the average of the numbers: 1,2
>> Traceback (most recent call last):
>>  File "avgInput.py", line 13, in <module>
>>    add = add + i
>> TypeError: unsupported operand type(s) for +: 'int' and 'str'
>>
>> **** End of process output ****
>>
>> The reason being, I take, that
>>
>> numbers = raw_input("Please type the numbers, separated by commas: ")
>>
>> also returns the comma (1,2) and thus the for loop can't cope...
>> So should I therefore retain
>>
>> numbers = input("Please type the numbers, separated by commas: ") ?
>>
>> Otherwise I don't know (yet) what to do....
>>
>> David
>>
>>
>> Bill Campbell wrote:
>>     
>>> On Thu, Oct 02, 2008, Steve Willoughby wrote:
>>>  
>>>       
>>>> On Fri, Oct 03, 2008 at 01:38:48AM +0800, David wrote:
>>>>    
>>>>         
>>>>> Does that mean input() is obsolete (after all, Zelle's book is not the 
>>>>> freshest on the shelf)? Or do they have different uses?
>>>>>      
>>>>>           
>>>> Depends on how you look at it.
>>>>
>>>> input() automatically evaluates whatever the user types as a Python
>>>> expression and returns the result.  So if they type 5, the integer
>>>> 5 is returned.  For your program, that's probably what you want, and
>>>> has the advantage of letting you type something like 2+3 so your user
>>>> can let Python evaluate math expressions.
>>>>
>>>> On the other hand, you'd think that you could ask a user for a text
>>>> response using input():
>>>>  name = input("What is your name? ")
>>>>  print "Hello, ", name
>>>>
>>>> But if they just type the answer, Python will crash with an error
>>>> because it's expecting a legal Python expression there (so a 
>>>> string value would have to be typed in quotes).
>>>>    
>>>>         
>>> Remember the cardinal rule NEVER TRUST USER INPUT!  Always check
>>> for validity, and use methods that prevent malicious strings from
>>> allowing the user to get unauthorized access or change things
>>> they shouldn't.
>>>
>>> Many of the common exploits of web pages are the result of poor
>>> checking of input resulting in sql injection attacks, and other
>>> breaches.
>>>
>>> Bill
>>>  
>>>       
>> _______________________________________________
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>>     
>
>