Comparing list values by index in Python

Question

I need to see if 2 items from a list appears in another list, and if they do, compare the items by their position in the other list. Example in pseudo code:

j=0
for x in mylist #loop through the list
    i=0
    for y in mylist #loop through the list again to compare items
        if index of mylist[j] > index of mylist[i] in list1 and list2:
            score[i][j] = 1 #writes the score to a 2d array(numpy) called score
            i=i+1
        else: 
            score[i][j]=0
            i=i+1
j=j+1

Sample Narrative Description:

Names = [John, James, Barry, Greg, Jenny]
Results1 = [James, Barry, Jenny, Greg, John]
Results2 = [Barry, Jenny, Greg, James, John]

loop through Names for i
    loop through Names for j
        if (index value for john) > (index value for james) in results1 and 
           (index value for john) > (index value for james) results2:
            score[i][j] = 1

Can someone please point me in the right direction? I've been looking at numerous list, array and .index tutorials but nothing seems to answer my question

This will be rather easier to understand if you provide a narrative description of exactly what you want to do, such as "for every i,j less than length of list1, I want to ..." — Marcin
– Marcin, Commented Jul 10, 2012 at 22:16
@Marcin done, hope it's clear enough. I understand it is hard to explain what I want — adohertyd
– adohertyd, Commented Jul 10, 2012 at 22:25
@AshwiniChaudhary No I need to loop through twice to compare each pair of names i.e.names[0][1]=[john, james] — adohertyd
– adohertyd, Commented Jul 10, 2012 at 22:38
your narrative would be easier to understand with consistent names. Is name list the same as Names? Which list is index value referring to? — dawg
– dawg, Commented Jul 10, 2012 at 22:41
@drewk apologies I've fixed that. index value is the index value for that person in both results1 and results2. I'm comparing their index values for probability purposes — adohertyd
– adohertyd, Commented Jul 10, 2012 at 22:44

Mark Ransom · Accepted Answer · 2012-07-11 15:12:22Z

3

Convert your list2 to a dictionary that encodes the position given an item:

dic2 = dict((item,i) for i,item in enumerate(list2))

Now you can test for something being in the list by using x in dic2 and y in dic2 and use dic2[x] to get it's index in the list.

Edit: It goes against my better instincts, but here's the complete code. The first part is using what I showed above, turning a simple list into a lookup for the index. Next comes the standard if unintuitive way of initializing a 2D list. This is followed by your loops, using the ever handy enumerate function to assign an index to each name in the list.

Names = ['John', 'James', 'Barry', 'Greg', 'Jenny']
Results1 = ['James', 'Barry', 'Jenny', 'Greg', 'John']
Results2 = ['Barry', 'Jenny', 'Greg', 'James', 'John']

Order1 = dict((name,order) for order,name in enumerate(Results1))
Order2 = dict((name,order) for order,name in enumerate(Results2))

score = [[0]*len(Names) for y in range(len(Names))]

for i,name1 in enumerate(Names):
    for j,name2 in enumerate(Names):
        if name1 in Order1 and name2 in Order1 and Order1[name1] > Order1[name2] and name1 in Order2 and name2 in Order2 and Order2[name1] > Order2[name2]:
            score[i][j] = 1

edited Jul 11, 2012 at 15:12

answered Jul 10, 2012 at 22:24

Mark Ransom

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4 Comments

adohertyd Over a year ago

Mark, could you give me an example of that based on the narrative description that I added above?

Mark Ransom Over a year ago

@adohertyd, your example in the edited question is still very unclear.

adohertyd Over a year ago

The names list is a list of athletes. Results1 and Results2 are the respective positions of each athlete in 2 races. What I want to do is check if e.g. john beat james in both races and repeat this comparison for every pair of names. If so, I want to write the score 1 to score; this is for probability calculations later.

adohertyd Over a year ago

Thanks Mark that's a great answer. Don't worry about posting the whole code this isn't a homework assignment or anything it's part of a project that I'm working on and I'm exploring different things. Thanks for the help!

Ashwini Chaudhary · Accepted Answer · 2012-07-10 22:16:23Z

2

lis1=[1,2,3,4,5,6,7,8]
num1=lis1[1]
num2=lis1[4]
lis2=[11,12,13,14,2,7,5,34]
if num1 in lis2 and num2 in lis2:
    if lis2.index(num1)>lis2.index(num2):
        #do something here
    else:
        #do something else

answered Jul 10, 2012 at 22:16

Ashwini Chaudhary

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2 Comments

adohertyd Over a year ago

Ashwini, would you mind redoing that using the narrative description I gave? I'm still not sure exactly what you did there.

abarnert Over a year ago

+1 for directly mapping the pseudocode into Python in an obvious way. But it might be worth explaining that using an appropriate data structure (as the other answers do) allows both simpler and more efficient code, so you generally don't actually want this code (or the original pseudocode, for that matter).

dawg · Accepted Answer · 2012-07-10 22:53:27Z

1

IF I understand what you are trying to do, here is an approach:

score={}

Names = ["John", "James", "Barry", "Greg", "Jenny"]
Results1 = ["James", "Barry", "Jenny", "Greg", "John"]
Results2 = ["Barry", "Jenny", "Greg", "James", "John"]

r1dict={name:i for i,name in enumerate(Results1)}
r2dict={name:i for i,name in enumerate(Results2)}

for i, ni in enumerate(Names):
    for j, nj in enumerate(Names):
        if r1dict[ni] > r2dict[nj]:
            score[(i,j)]=1

print(score)

Prints:

{(0, 1): 1, (3, 2): 1, (4, 4): 1, (3, 3): 1, (2, 2): 1, 
 (4, 2): 1, (0, 3): 1, (0, 4): 1, (3, 4): 1, (0, 2): 1}

answered Jul 10, 2012 at 22:53

dawg

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2 Comments

adohertyd Over a year ago

Thank you! It's very similar to Mark's answer below. The only difference is score is a 2d matrix (created using numpy). It appears that I have no choice but to create a dictionary from a list and then convert to a 2d array. How would this affect the program computationally, especially if the names list is large?

dawg Over a year ago

@adohertyd: If your numpy 2d array is already allocated with proper bounds, you can use a[i][j] in the same way that I am using the tuple. The array bounds must be in range however. (Python arrays do not autovivify)

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