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Python and nested lists problem

I have a matrix with 5 rows and 4 cols containing some numbers. I need to take each row, do an average from the first and second col, and another average from the third and fourth col, subtract these numbers from a constant and put these in another matrix, which will then have 5 rows and 2 cols.
Here's the code that I coined up, but it counts nonsenses:

Code:

vrch=624

pruz=[

    [511,512,392,394],

    [504,503,424,424],

    [469,468,439,440],

    [472,473,446,447],

    [517,517,496,496]]

delky=[[0,0],[0,0],[0,0],[0,0],[0,0]]

ins=[0,0]



i=0

# I count the final numbers

for row in pruz:

    ins[0]=vrch-(row[0]+row[1])/float(2)

    ins[1]=vrch-(row[2]+row[3])/float(2)

    delky[i]=ins # and put them into the second matrix (delky)

    print i,delky[i] # now I print it and it looks fine

    i+=1



# However when I try to print it again, I get wrong numbers

print

for row in delky:

    print row

The first cycle seems to be working fine, however the problem must be there. I print the information twice, first it's allright (from the firs cycle) and the second time I get only the last row of the new matrix five times.

Here's the output:

Code:

0 [112.5, 231.0]

1 [120.5, 200.0]

2 [155.5, 184.5]

3 [151.5, 177.5]

4 [107.0, 128.0]



[107.0, 128.0]

[107.0, 128.0]

[107.0, 128.0]

[107.0, 128.0]

[107.0, 128.0]

Any ideas where is the mistake? Why does delky[0:3] contain the same information as delky[4]?

In the loop you are assigning the list ins to all rows in delky.

Since a list is an object (not a primitive type, like e.g. an integer), all delky[i] will become a reference to ins. But the contents of ins change each time the loop runs. So all delky[i] will refer to the same ins!

A bit difficult to explain. But try this smaller program and see what happens in the last part:

Code:

delky = [[0,0], [0,0], [0,0]]

ins = [1,2]



delky[0] = ins

delky[1] = ins

delky[2] = ins

print delky



print "now changing 'ins' by index (only once):"

ins[0] = 555

ins[1] = 666

print delky

On solution is to make sure a new list ins is created each time the loop runs ins is assigned to delky[i]:

Code:

delky = [[0,0], [0,0], [0,0]]

ins = [1,2]



delky[0] = list(ins)

delky[1] = list(ins)

delky[2] = list(ins)

print delky



print "now changing 'ins' by index (only once):"

ins[0] = 555

ins[1] = 666

print delky

So, to fix your code:

Code:

vrch=624

pruz=[

    [511,512,392,394],

    [504,503,424,424],

    [469,468,439,440],

    [472,473,446,447],

    [517,517,496,496]]

delky=[[0,0],[0,0],[0,0],[0,0],[0,0]]

ins=[0,0]



i=0

# I count the final numbers

for row in pruz:

    ins[0]=vrch-(row[0]+row[1])/float(2)

    ins[1]=vrch-(row[2]+row[3])/float(2)

    delky[i]=list(ins) # and put them into the second matrix (delky)

    print i,delky[i] # now I print it and it looks fine

    i+=1



# However when I try to print it again, I get wrong numbers

print

for row in delky:

    print row

However, I would prefer to write your program like this (no need for ins and i):

Code:

vrch=624

pruz=[

    [511,512,392,394],

    [504,503,424,424],

    [469,468,439,440],

    [472,473,446,447],

    [517,517,496,496]]

delky=[]



for row in pruz:

    AAA = vrch-(row[0]+row[1])/float(2)

    BBB = vrch-(row[2]+row[3])/float(2)

    delky.append([AAA,BBB])



for row in delky:

    print row

Very well, thank you for the explanation! Now it works fine..