CODEWITHSUNDEEP
pythonpython-3.x
Bart1K
Bart1K

Reputation: 23

python string input to integer conversion

I need to create a script to ask a user to input a string containing round numbers separated by a space. Then I need to convert the string to integers.

Any pointers on where to start? This is what I've tried:

a=str(input('Voer een door spatie gescheiden lijst met getallen in:').split())
b=int(a)
print(b)

but I keep getting:

ValueError: invalid literal for int() with base 10: "['50', '60']"

Upvotes: 1

Views: 1478

Answers (4)

gboffi
gboffi

Reputation: 25093

The Lengthy Answer

Oversymplifyng the issue, the int builtin (follow the link to know the whole story) requires a single argument, either a numeric literal or a string.

Your code

>>> a=str(input('Voer een door spatie gescheiden lijst met getallen in: ').split())

is creating the textual representation of a list, because you use the str builtin that returns a textual representation of a python object:

>>> a
"['2', '3', '4', '5']"

The above is not the textual representation of a number, as required by int, but the textual representation of a list, so when you pass it to int it complains 

ValueError: invalid literal for int() with base 10: "['2', '3', '4', '5']"

What if we omit the call to str?

>>> a = input('Voer een door spatie gescheiden lijst met getallen in: ').split()
>>> a
['2', '3', '4', '5']
>>> int(a)
TypeError: int() argument must be a string or a number, not 'list'

Note that the error message is different: no more a ValueError but a TypeError. We have a list, whose items are strings representing numbers, but we're not allowed to use the list as is as an argument to int.

Enter list comprehension, the contemporary idiom to manipulate the contents of a list (or of a generic iterable).

>>> [int(elt) for elt in a]
[2, 3, 4, 5]

The syntax is easy, [...] you have the opening and closing bracket, as you would write when using a list literal, and inside you have an inside-out loop, first the body of the loop and then the for specification. The results of the evaluation of the body are sequentially stored in a list, that the interpreter outputs for you, as you can see above we have no more strings representing numbers but integers. Assign the result of the list comprehension to a variable

>>> b = [int(elt) for elt in a]

and you're done:

>>> print(b)
[2, 3, 4, 5]
>>>

Upvotes: 1

Padraic Cunningham
Padraic Cunningham

Reputation: 180540

You are trying to cast a string representation of a list, just split the contents into a list and use map to cast to ints:

inp = input('Voer een door spatie gescheiden lijst met getallen in:').split())
a,b = map(int,inp)

This presumes the user always enters two numbers separated by a space.

Safer use a while loop and a try/except:

while True:
    try:
        inp = input('Voer een door spatie gescheiden lijst met getallen in:').split()
        a, b  = map(int, inp)
        break
    except ValueError:
        print("Invalid input")
print(a,b)

If you have an unknown amount of ints entered:

  while True:
        try:
            inp = input('Voer een door spatie gescheiden lijst met getallen in:').split()
            nums = map(int, inp)
            break
        except ValueError:
            print("Invalid input")
    for num in nums:
       print(num)

Using python3, map returns a map object so it is an efficient way to assign.
[int(x) for x in inp] will also work but unless you need a list there is no point creating one.

Upvotes: 3

Pankaj Parashar
Pankaj Parashar

Reputation: 10232

You need to convert each element of the list into int using the following way,

a = [int(each) for each in a]

This will give you a list of integers that you could use in your program. In case if it is an invalid integer, then you can use try-except,

b = []
for each in a:
    try:
      b.append(int(each))
    except ValueError:
      print('Invalid number')

Upvotes: 0

Cory Kramer
Cory Kramer

Reputation: 118021

After split you have a list of str, so you need to convert each of the individual items to int.

You could use a list comprehension

b = [int(i) for i in a]

Or use map

b = list(map(int, a))

Upvotes: 0

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