Python trouble with lists

Question

I have a set of initial variables from a->i, a full list of variables, a separate list of integer values and a set of lists made from subsets of the initial variables:

a=0
b=0
...
i=0
exampleSubList_1 = [c, e, g]
fullList = [a, b, c, d, e, f, g, h, i] #The order of the list is important
otherList = [2, 4, 6, 8, 10, 12, 14, 16, 18] #The order of the list is important

I want my program to read an input exampleList_X and find its corresponding index entry in fullList, and use that index number to output the corresponding value in otherList. For example...

exampleList_1[0]
#Which is c
#It should find index 2 in fullList
#Entry 2 in otherList is the value 6.
#So it should output
6

Is this possible? I would be willing to use tuples/dictionary is required.

In the interest of clarity, this is for a Raspberry Pi noughts and crosses game project using LEDs. c, e and g correspond to the win condition of the diagonal from top right to bottom left, and otherList corresponds to the pin on the Raspberry Pi which sends out a current to light up the LED.

• Dietrich: Have you considered from collections import OrderedDict? ``
• Pingk: @GWW, I've experimented with various bits and pieces, but I keep getting stuck when trying to compare fullList to otherList.
• Pingk: @Dietrich, I've tried Dict, but I've not heard of OrderedDict. I'll have a look and come back to you
• Adam Smith: OP: currently exampleSubList_1 is [0,0,0], so fullList.index(exampleSubList_1[0]) == 0 since all your variables a through i are 0, so there's no telling them apart. Do you mean exampleSubList_1 = ['c','e','g'], fullList = ['a','b','c', ... , 'i'] ?
• Pingk: @Dietrich, I've had a look at the documentation, but it's not particularly clear. Do I initiate ordereddict in the same way as dict, and do the same commands for keys/values still work?

Answer 1

List comprehension:

results = [otherList[fullList.index(c)] for c in exampleSubList_1]

and results will return:

[6, 10, 14]

Or a simple for loop:

for c in exampleSubList_1:
 print otherList[fullList.index(c)]

Should print

6
10
14

• Pingk: Hmm, I'll have a look and get back to you

Answer 2

>>> symbol_pos_map = {v:k for k,v in enumerate(fullList)}
>>> otherList[symbol_pos_map[exampleSubList_1[0]]]
6

Don't use list.index, because it does a linear search everytime, first map fullList to a dictionary at linear cost, then subsequent lookups are a constant time.

• Two-Bit Alchemist: This is a useful optimization for anyone looking for what the OP seemed to be asking. (OP said in comments the expected output was the actual variable names. Your answer is still good against a valid reading of the question.)

Answer 3

You really should consider using a dictionary.

Consider:

l1 = ['c', 'e', 'g']
l2 = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i']
l3 = [2, 4, 6, 8, 10, 12, 14, 16, 18]
results = [l3[l2.index(c)] for c in l1]
print "results 1:", results
d1 = {'a': 2, 'b': 4, 'c': 6, 'd':8, 'e': 10, 'f': 12, 'g': 14, 'h': 16, 'i': 18}
results = [d1[c] for c in l1]
print "results 2:", results

Those give you the same results. Let the dictionary do what it was designed to do: lookup a key, and return a value.

Note that the key value can be almost anything: a number, a letter, a full string, a tuple of values...

If you already have your two "lookup lists" (l2 and l3 in my example), then you can use the dict() and zip() functions to create the dictionary for you:

d2 = dict(zip(l2, l3)) # this creates a dictionary identical to d1
results = [d2[c] for c in l1]
print "results 3:", results

• Pingk: I've tried, but Dict doesn't preserve the order in which you input the variables: dft.ba/-N_lists2
• Dan H: @pingk Why do you need the order preserved? In your post, you seem to use the order to keep two lists "organized" so that the values at same indexes are related; but with the dict, you just use the dict itself to relate the two data sets. My proposal produces the results you say you need without keeping the list ordered!

Answer 4

After reading around dictionaries, I've found that using OrderedDict works perfectly for what I need, see Python select ith element in OrderedDict.

@Dietrich thanks for the idea.