Bi-dimensional arrays in Python

Question

I have a vector with int values like:

v=[10,8,6]

and what I want is to create an m*m matrix that stores the distance between these elements, i.e. take each element of the vector and substract it from all the other ones, so at the end I will end up with:

 m[3][3]=10-10   10-8   10-6
         8-10    8-8    8-6
         6-10    6-8    6-6

I want to implement it into Python, but without using NumPy. I have done this so far:

def main():
    v=[10,8,6]
    l=len(v)
    m=[]
    #filling the matrix
    for i in range(0,l-1):
        for j in range(0,l-1):
            m[i][j]=abs(v[i]-v[j])

    #visualize the matrix
    for i in range(0,l-1):
        for j in range(0,l-1):
            print m[i][j]

But I am getting some error that does not recognize with the bounds of m. Why is that?

Vader · Accepted Answer · 2011-11-13 21:06:29Z

4

v= [10,8,6] 
m = [[abs(y-x) for y in v] for x in v]

EDIT:

For pretty printing you can use something like:

for i in m:
 print '%s '*len(i) % tuple(i)

edited Nov 13, 2011 at 21:06

answered Nov 13, 2011 at 20:23

Vader

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3 Comments

Platinum Azure Over a year ago

One of these days, I'll be good enough at Python to rattle off two or more list comprehensions at once. (+1)

Layla Over a year ago

it seems nice solution, but I need to store those results in one matrix so I can used it later

Vader Over a year ago

@Manolo Not sure that I understood your comment. This code does the same things as the one from your question suppose to do, but in shorter way.

Justin Peel · Accepted Answer · 2011-11-13 20:47:17Z

1

You need to make a list for each row inside of the enveloping list. I used a double list comprehension to make m and then made the formatting a little prettier for printing out the matrix. Also, watch your indices: remember that range goes from the first index to one minus the second index passed to it. You could also do print ' '.join(row) for each row in m to print it out nicely.

def main():
    v=[10,8,6]
    l=len(v)
    #filling the matrix
    m=[[abs(x - y) for y in v] for x in v]

    #visualize the matrix
    for i in range(0,l):
        for j in range(0,l):
            print m[i][j],
        print
main()

result:

0 2 4
2 0 2
4 2 0

answered Nov 13, 2011 at 20:47

Justin Peel

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Comments

Winston Ewert · Accepted Answer · 2011-11-13 21:00:38Z

1

Your list starts out empty. You can't index the element in the list. One way to solve this would be to create the lists and then append them.

m=[]
#filling the matrix
for i in range(0,l-1):
    x = []
    for j in range(0,l-1):
        x.append( abs(v[i]-v[j]) )
    m.append(x)

Or you could create the matrix and then fill it up

m=[[0] *l for x in range(l)]
#filling the matrix
for i in range(0,l-1):
    for j in range(0,l-1):
        m[i][j]=abs(v[i]-v[j])

better yet is the list comprehension other have shown

m = [[abs(y-x) for y in v] for x in v]

but then, I'd use numpy/scipy

m = scipy.spatial.distance.pdist(v)

answered Nov 13, 2011 at 21:00

Winston Ewert

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Comments

Platinum Azure · Accepted Answer · 2011-11-13 20:20:57Z

0

You need to initialize each object in the first dimension as an array, otherwise it's like you're basically trying to index on None.

#filling the matrix
for i in range(0,l-1):
    m[i] = []
    for j in range(0,l-1):
        m[i][j]=abs(v[i]-v[j])

answered Nov 13, 2011 at 20:20

Platinum Azure

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1 Comment

Winston Ewert Over a year ago

Your code is incorrect. m[i][j] will be an invalid index into the zero length list m[i] = []

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