Reading in a list python

Question

Im writing a program that reads in a list, then displays the length of the list, and displays the list with one number displayed per line. This is what I have so far:

from List import *

def main():
   numbers = eval(input("Give me an list of integers: "))
   strings = ArrayToList(numbers)
   length = len(strings)
   print("The length of the list: ", length)

This is what Im expecting the result to be like:

Enter a list of integers: [3, [5, [1, [6, [7, None]]]]]
The length of the list:
5
The list:
3
5
1
6
7

Can anybody help me out? Im getting the length of the list to show as 2 instead of 5 which is what it should be.

Do you come from the world of PROLOG? it's very odd to handle lists the way you do, in python — inspectorG4dget, May 01 '14 at 18:35
A list of those five integers would be written `[3, 5, 1, 6, 7]` in Python. What you've entered is a two-element list in which the second element is another two-element list etc. — jwodder, May 01 '14 at 18:37
@JoranBeasley: it's PROLOG lingo. It's how list concatenation is handled, which is how larger lists are made. Interestingly, to port over PROLOG, that last `None` should be a `[]` — inspectorG4dget, May 01 '14 at 18:38
a/ don't use eval. b/ don't use prolog notation c/ yes, your list has a size of 2, not 5, you'll have to flatten it first — njzk2, May 01 '14 at 18:41
What's ArrayToList? Is this another [CS 150](http://stackoverflow.com/questions/23006458/command-line-input-extreme-and-map-patterns-python) question? — Kevin, May 01 '14 at 18:43
@inspectorG4dget why do you keep all-capsing Prolog? :) It seems to me to have its roots in a term algebra representation of a list, but it would make more sense with list(item1,list(item2,nil)) so it thus can be a straightforward representation of a list. — C.B., May 01 '14 at 18:47
@C.B.: I thought the all-capsing was an artifact of nomenclature. Apparently, I was wrong — inspectorG4dget, May 01 '14 at 19:10

dckrooney · Answer 1 · 2014-05-01T18:47:49.853

Without seeing the implementation of ArrayToList, this is just a guess.

That said, I imagine your problem is that you aren't entering a list of integers, you're entering a list composed of an integer and another list... which itself is a list containing an integer and another list, all the way down. Hence len(strings) is 2, because len is not recursive.

You're could try inputting the list like [1, 2, 3, 4, 5] instead.

Alternatively, you could build your list in a loop, asking for user input for each character until an "end of input" event (of your choosing) is hit. This would let you avoid eval entirely, which is often a good idea.

score 1 · Answer 2 · answered May 01 '14 at 18:44

def main():
    numbers = input("Give me a list of space-separated integers: ").split()
    print("length of the list:", len(numbers))
    print("The list:", *numbers, sep='\n')

Output

In [14]: main()
Give me a list of space-separated integers: 3 5 1 6 7
length of the list: 5
The list:
3
5
1
6
7

score 0 · Answer 3 · answered May 01 '14 at 18:44

I am assuming you are in Python3, because in Python2 you would not need the eval around input.

numbers = eval(input("Give me an list of integers: "))
length = len(numbers)
print("The length of the list: %d" % length)
print("The list: ")
for i in numbers:
  print(i)

Here, you would have to enter the list in standard Python fashion though:

[1,2,3,4,5]

The equivalent in Python2 would just be a one-line change.

numbers = input("Give me an list of integers: ")

Maikflow · Answer 4 · 2014-05-01T19:23:10.983

0

In Python 2 I would do it like this:

def main():
    numbers = []
    integers = []
    numbers.append(raw_input("Give me an list of integers: "))
    for integer in numbers[0]:
        print integer
        if integer.isdigit():
            integers.append(integer)
    print ("The length of the list: ", len(integers))

main()

edited May 01 '14 at 19:23

answered May 01 '14 at 18:58

Maikflow

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Reading in a list python

4 Answers4