Can't return element index in an array Python

Question

I want to use the array.index(x) function multiple times to find the element index in a small list and its place within a larger list. For example, I have...

a=1
b=2
c=1
d=2

small_list = [a, b, c]
full_list = [d, c, b, a]

x=small_list.index(c)
y=full_list.index(x)
z=full_list[y]

When running, I get...

x=0
y=0
z=2

But I want to get...

x=2
y=1
z=1

How do I change my code to do this? I can see that it is looking up the value of c when I run the code. How do I stop this happening?

EDIT: My own answer to my problem is below.

"When running, I get..." You do? I get ValueError: 0 is not in list. Is this your actual code? — Kevin
– Kevin, Commented Mar 21, 2014 at 13:34
I'm not sure how your example input is meant to lead to your example output. Certainly the first index of 1 in your small_list is not 2. — kindall
– kindall, Commented Mar 21, 2014 at 13:39

atlasologist · Accepted Answer · 2014-03-21 13:44:46Z

1

You're using a list, not an array. The list.index() method returns the index of the first value specified. So, when you ask for

small_list.index(c)

You're getting the index of c, which are the values 0 and 1, respectively.

http://docs.python.org/2/tutorial/datastructures.html#more-on-lists

edited Mar 21, 2014 at 13:44

answered Mar 21, 2014 at 13:39

atlasologist

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Jem · Accepted Answer · 2014-03-21 13:39:15Z

0

I t looks like it is returning the correct value! Your list [a,b,c] is [1,2,1]. So the index(c) (or index(1)) is returning the first instance of c, which is spot 0.

answered Mar 21, 2014 at 13:39

Jem

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Pingk · Accepted Answer · 2014-03-21 13:55:00Z

0

Ahh, I've found what I need to do: Lists aren't the right basis for what I want to do, I think I should use tuples. That way, if I define...

tuple1 = (a, b, c)
tuple2 = (d, c, b, a)

print(tuple1[c]) # =2
print(tuple2[c]) # =1

Which is what I want. Essentially, I wanted the code to find the position of the variable c, not the position of the value of c.

Thank you to all answerers.

answered Mar 21, 2014 at 13:55

Pingk

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Christian Groleau · Accepted Answer · 2014-03-21 13:56:05Z

0

Calling small_list.index(c) is equivalent to calling small_list.index(1). The interpreter evaluates the value of c which is 1.

index() returns the first occurrence position of the specified value and that is at position 0.

See: http://docs.python.org/2/tutorial/datastructures.html

answered Mar 21, 2014 at 13:56

Christian Groleau

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