I am fairly new to python and I am trying to get around understanding the following code:
import numpy as np
n=4
matrix=np.zeros((n,n))
for j in range (0,n):
for i in range (n-1,n-j-2,-1):
matrix[i,j]=2*n-i-j-1
print (matrix)
I would greatly appreciate if someone could please help me understand how each line executes and how the code is revaluated with the loop.
Thanks in advance!
You can add the following print statement, and the loop will explain itself at each iteration:
n=4
matrix=np.zeros((n,n))
for i in range (0,n):
for j in range(0,i+1):
print(f'inserting {i-j+1} into the matrix at row index {i}, columns index {j}')
matrix[i,j]=i-j+1
When you run it, you get this output:
inserting 1 into the matrix at row index 0, columns index 0
inserting 2 into the matrix at row index 1, columns index 0
inserting 1 into the matrix at row index 1, columns index 1
...
inserting 3 into the matrix at row index 3, columns index 1
inserting 2 into the matrix at row index 3, columns index 2
inserting 1 into the matrix at row index 3, columns index 3
And your matrix is populated as before:
>>> matrix
array([[1., 0., 0., 0.],
[2., 1., 0., 0.],
[3., 2., 1., 0.],
[4., 3., 2., 1.]])
Just for reference:
>>> matrix
array([[1., 0., 0., 0.], #<- "row" index 0
[2., 1., 0., 0.], #<- "row" index 1
[3., 2., 1., 0.], #<- "row" index 2
[4., 3., 2., 1.]]) #<- "row" index 3
# ^ ... ^
# "col" 0 "col" 3
import numpy as np
n=4
We start by setting a 4x4 matrix with all coordinates set to null:
matrix=np.zeros((n,n))
We set new coordinate values by looping through rows and columns. First we loop through rows, from index 0 to n-1:
for i in range (0,n):
We next loop through columns. Now, notice that we only loop through those columns whose index is smaller than or equal to that of the current row (i.e., from 0 to i). This way we make sure that the values we set are on or below the diagonal of the matrix:
for j in range(0,i+1):
Finally, we set the desired value for the current coordinate:
matrix[i,j]=i-j+1
print(matrix)
i - j + 1whereiis the row index andjis the column index. You need to be more specific about what you don't understand if that does not solve the issue.forloop iterates over the row indices. The innerforloop iterates over the columns, but only up to the column number that is equal to the current row number.