Python : Getting an Indexing error when trying to append data to a list

Question

I have data in a list: ls1 which prints fine when giving the print function

[5, 2, 7, 4, 3, 9, 8, 6, 10]

However, I am getting an error when trying this:

P81=[]  
P81.append(ls1[5])

Is there anything wrong with the code? Here is a full copy for reference. The code is just a key generation function which accepts a 10 element list and performs some permutations and shifts. leftShift is a function which just performs a shift operation on the list.

def keyGen(key):
    import numpy
    #3 5 2 7 4 10 1 9 8 6
    P10=[]
    P10.append(key[2])
    P10.append(key[4])
    P10.append(key[1])
    P10.append(key[6])
    P10.append(key[3])
    P10.append(key[9])
    P10.append(key[8])
    P10.append(key[7])
    P10.append(key[5])

    #Now, P10 contains the keys after initial permutation

    #Take 2 halves and perform left shift
    ls1a=leftShift(P10[0:5])
    ls1b=leftShift(P10[5:10])
    ls1=ls1a+ls1b

    P81=[]
    #6 3 7 4 8 5 10 9

    print ls1
    P81.append(ls1[5])
    P81.append(ls1[2])
    P81.append(ls1[6])
    P81.append(ls1[3])
    P81.append(ls1[7])
    P81.append(ls1[4])
    P81.append(ls1[9])
    P81.append(ls1[8])

    #For the second set of keys perform the second shift
    ls2a=leftShift(ls1a)
    ls2b=leftShift(ls1b)
    ls2=ls2a+ls2b

    P82=[]
    P82.append(ls2[5])
    P82.append(ls2[2])
    P82.append(ls2[6])
    P82.append(ls2[3])
    P82.append(ls2[7])
    P82.append(ls2[4])
    P82.append(ls2[9])
    P82.append(ls2[8])

    return([P81,P82])

Martijn Pieters · Accepted Answer · 2013-10-03 13:03:29Z

2

The index error is for indexing into ls1, not the .append() call.

Your ls1 does not have 10 elements, yet you try to index that many:

P81.append(ls2[9])
P81.append(ls2[8])

You only ever appended 9 elements to P10 (you ignored key[0]), so your assumptions already fall apart there. As a result, provided leftShift doesn't lose any more elements, ls1 is 9 elements long, so:

P81.append(ls1[9])

will fail. Even if it doesn't, you ignore ls1[0] and ls1[1]. ls2 suffers from the same problem; there are 9 elements in that list, not 10, provided leftShift doesn't drop any elements.

edited Oct 3, 2013 at 13:03

answered Oct 3, 2013 at 12:57

Martijn Pieters

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