mulitply a CSR matrix to a vector

Question

I was following this stackoverflow post on csr matrix multiplication to a vector and Implement it in python and getting list out of range error.

Here is my code:

def MatrixMultiplication(data,row_ptr,col_ptr,vec):
  ResultMatrix =[]
  vec_len = len(vec)
  for i in range(0,vec_len):
    ResultMatrix.insert(i,0)
  for i in range(0,vec_len):
    start, end = row_ptr[i], row_ptr[i + 1]
    for k in range(start, end):
      ResultMatrix[i] = ResultMatrix[i]+data[k]*vec[col_ptr[k]]
  return ResultMatrix

data = [2, 4, 7, 1, 3, 2]
row_ptr =  [2,3 ,5, 5 ,6]
col_ptr = [1 ,3, 4, 0, 3, 3]
vec = [2,3, 5, 4, 2]

MatrixMultiplication(data,row_ptr,col_ptr,vec)

Please help me out where I am going wrong .

Output should be: [ 22 14 14 0 8]

Error :

IndexError: list index out of range
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<command-338158343473691> in <module>()
----> 1 MatrixMultiplication(data,row_ptr,col_ptr,vec)

<command-3658506804172571> in MatrixMultiplication(data, row_ptr, col_ptr, vec)
      5     ResultMatrix.insert(i,0)
      6   for i in range(0,vec_len):
----> 7     start, end = row_ptr[i], row_ptr[i + 1]
      8     for k in range(start, end):
      9       ResultMatrix[i] = ResultMatrix[i]+data[k]*vec[col_ptr[k]]

IndexError: list index out of range

FYI:

the last element of row_ptr will be size of the data list

Answer 1

The error message is pretty self-explanatory: You're trying to access row_ptr[i + 1] in a for loop that goes up to vec_len, which is the length of your list. When you've reached the last iteration of your for loop and i = vec_len - 1, then i + 1 = vec_len, which is out of the range of your list (remember, Python lists are 0-initialised).

To prevent this error, your range should only go up to vec_len - 1 in your second for loop.