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simple question

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max.yevs

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simple question

Jun 8th, 2009

yet another ridiculously simple question, yet i feel compelled to ask because i've been stuck here for like an hour without it working

ok so say you have one list, a, and you need the user of the program to input a list of several more numbers, call that list b, then take out the numbers in b from a.

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
>>>a = [1,2,3,4,5,6,7,8,9,10,10,10]
>>>b = input ("Enter numbers: ")
[3,4,10]
>>>#something here
>>>print (a)
[1,2,5,6,7,8,9,10,10]
>>>a = [1,2,3,4,5,6,7,8,9,10,10,10]
>>>b = input ("Enter numbers: ")
[3,4,10]
>>>#something here
>>>print (a)
[1,2,5,6,7,8,9,10,10]

so i try a bunch of stuff from the tutorial in like 100 different ways: a = a - b, for i in b: a.remove(i), a.remove(b)... yet i always get an error

it works with sets but i can't use sets because they keep throwing out the nonunique numbers (so like if you have [10,10,10] it would just make it [10])

Last edited by max.yevs; Jun 8th, 2009 at 1:57 am.

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shadwickman shadwickman is offline

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Re: simple question

Jun 8th, 2009

How about:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
for number in b:
    if number in a:
        a.remove(number)
for number in b:
    if number in a:
        a.remove(number)

That should go through each number in b and remove it from a if it appears in a...

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Re: simple question

Jun 8th, 2009

Use simple boolean statements.
Here is my python interaction:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
>>> a = [1,2,3,4,5,6,7,8,9,10,10,10]
>>> b = list(input ("Enter numbers: "))
Enter numbers: [3,4,10]
>>> a= [x for x in a if x not in b]
>>> a
[1, 2, 5, 6, 7, 8, 9]
>>>
>>> a = [1,2,3,4,5,6,7,8,9,10,10,10]
>>> b = list(input ("Enter numbers: "))
Enter numbers: [3,4,10]
>>> a= [x for x in a if x not in b]
>>> a
[1, 2, 5, 6, 7, 8, 9]
>>>

Last edited by siddhant3s; Jun 8th, 2009 at 3:20 am.

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Re: simple question

Jun 8th, 2009

siddhant3s, you may notice that your option has in fact removed the three 10s from list a, when you only input one 10. In the question, the other two 10s are left in list a after only one 10 is input.

Last edited by shadwickman; Jun 8th, 2009 at 3:26 am.

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max.yevs

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Re: simple question

Jun 8th, 2009

shadwick man's version works great, but i think my real problem is inputting the list

if the list is already defined it works great

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
>>>a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10]
>>>b = [3,4,10]
>>>for number in b:
>>>    if number in a:
>>>        a.remove(number)
>>>print (a)
[1,2,5,6,7,8,9,10,10]
>>>a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10]
>>>b = [3,4,10]
>>>for number in b:
>>>    if number in a:
>>>        a.remove(number)
>>>print (a)
[1,2,5,6,7,8,9,10,10]

but i can't find a way for the user to input it

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
>>>a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10]
>>>b = input("enter numbers: ")
[3,4,10]
b= list(b) #i would think this should convert it from string to list but it doesn't
>>>for number in b:
>>>    if number in a:
>>>        a.remove(number)
>>>print (a)
[1,2,5,6,7,8,9,10,10]
>>>a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10]
>>>b = input("enter numbers: ")
[3,4,10]
b= list(b) #i would think this should convert it from string to list but it doesn't
>>>for number in b:
>>>    if number in a:
>>>        a.remove(number)
>>>print (a)
[1,2,5,6,7,8,9,10,10]

if i say print (b) it does not define b as [3,4] but rather as '[3, 4]'

Last edited by max.yevs; Jun 8th, 2009 at 3:38 am.

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Re: simple question

Jun 8th, 2009

If you want, you can have it take input and split it at every space, or comma, etc:

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 python Syntax (Toggle Plain Text) 
b = raw_input("Enter numbers separated by a space: ").split(" ")
b = raw_input("Enter numbers separated by a space: ").split(" ")

It works like this:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
>>>Enter numbers separated by a space: 3 4 10
>>>b
>>>[3,4,10]
>>>Enter numbers separated by a space: 3 4 10
>>>b
>>>[3,4,10]

Last edited by shadwickman; Jun 8th, 2009 at 3:53 am.

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max.yevs

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Re: simple question

Jun 8th, 2009

that's exactly what i'm looking for shadwickman... but when i do it it doesn't work...

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
b = raw_input("Enter numbers separated by a space: ").split(" ")
b = raw_input("Enter numbers separated by a space: ").split(" ")

what i get is

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
Enter numbers separated by a space: 3 4 5
>>> b
['3', '4', '5']
Enter numbers separated by a space: 3 4 5
>>> b
['3', '4', '5']

it does not see the numbers as integers for some reason... and it does not remove them from list "a" either

Last edited by max.yevs; Jun 8th, 2009 at 3:54 am.

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Re: simple question

Jun 8th, 2009

Oh wow, I'm sorry. I completely forgot about raw_input returning them as strings. You can run each index through the int() function, changing it to an integer, but a problem arises if someone enters something other than digits. Let me give two solutions...

This one is simple, yet throws an error if the user enters any non-digit characters.

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
b = raw_input("Enter numbers separated by a space: ").split(" ")
for i, n in enumerate(b):
    b[i] = int(n)
 
# can also be written as:
# b = [int(i) for i in raw_input("Enter numbers separated by a space: ").split(" ")]
b = raw_input("Enter numbers separated by a space: ").split(" ")
for i, n in enumerate(b):
    b[i] = int(n)

# can also be written as:
# b = [int(i) for i in raw_input("Enter numbers separated by a space: ").split(" ")]

This one loops if there are any non-digits, and handles the error they present.

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
while 1:
    end = True  # end loop unless error
    b = raw_input("Enter numbers separated by a space: ").split(" ")
    for i, n in enumerate(b):
        try:
            b[i] = int(n)  # will cause error if i can't be converted to int
        except:
            print "Non-numerical input found. Only enter numbers!"
            end = False
    if end:
        break
while 1:
    end = True  # end loop unless error
    b = raw_input("Enter numbers separated by a space: ").split(" ")
    for i, n in enumerate(b):
        try:
            b[i] = int(n)  # will cause error if i can't be converted to int
        except:
            print "Non-numerical input found. Only enter numbers!"
            end = False
    if end:
        break

Last edited by shadwickman; Jun 8th, 2009 at 4:04 am.

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max.yevs

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Re: simple question

Jun 8th, 2009

thanks, that first one works perfectly!

not sure how it works but i need digit only so its perfect.

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Re: simple question

#10

Jun 8th, 2009

All it does is split the input into a list and save it as b like we did before. As of then, we have a list of numbers, but they are actually strings (e.g. ['3', '4', '10']).

From there, we use a for loop to cycle though list b (i is the index number, and n is the index value from the enumerate function). All we do then is tell it to replace b[i] (the current index it's on) with the old value of it (the string, n) converted to an integer through the int function.

I hope that clarifies it a bit!

Last edited by shadwickman; Jun 8th, 2009 at 4:15 am.

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