I need assistance in the logic of the code to rotate the matrix 90 clockwise

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Write a function rotate(m) that takes a list representation m of a square matrix as input, and returns the matrix obtained by rotating the original matrix clockwize by 90 degrees. For instance, if we rotate the matrix above, we get

the matrix is n*n

>>> rotate([[1,2,3],[4,5,6],[7,8,9]])
[[7, 4, 1], [8, 5, 2], [9, 6, 3]]

>>> rotate([[1,1,1],[2,2,2],[3,3,3]])
[[3, 2, 1], [3, 2, 1], [3, 2, 1]]

What I have tried:

Python

def rotate(m):
    l1=m[::]
    n=len(m)-1 
    count=n
    for i in range(n+1):
        for j in range(n+1):
            l1[j][count]=l1[i][j]
        count-=1
    return l1

Posted 12-Aug-19 20:52pm

Pranav Sharma

Updated 12-Aug-19 22:11pm

v2

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Pranav Sharma 13-Aug-19 2:53am

all the loops have proper indentations, they are not showing up in the code

2 solutions

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OriginalGriff · Accepted Answer · 2019-08-12T21:16:00

Solution 1

So ... imagine you were the matrix. What do you think happens to you when you run that code on yourself?

Let's make it easier: you are a one dimensional array, and you want to reverse yourself:

1, 2, 3, 4

When you do this:

Python

for i in range(4)
   me[3-i] = me[i]

What do you get?
is it

4, 3, 2, 1

Or

1, 2, 2, 1

To reverse the array, don't you need to swap the elements?

Python

for i in range(4)
   temp = me[3-i]
   me[3-i] = me[i]
   me[i] = temp

Posted 12-Aug-19 21:16pm

OriginalGriff

Comments

Richard Deeming 13-Aug-19 11:32am

Except your final code will reverse the array twice, leaving you with the original array.

Or was than intended as an "exercise for the reader"? :)

OriginalGriff 13-Aug-19 11:45am

You gotta leave 'em something to do! :laugh:

Patrice T · Accepted Answer · 2019-08-12T22:11:00

Your code replace elements of matrix on itself:

Python

l1[j][count]=l1[i][j]

this means that

123 121 121 121
456 456 452 452
789 789 789 781

So first you kill the 3, then you kill the 6, and you kill the 9 with the 1 that was 3 at beginning.
Try something like

Python

l1[j][count]=m[i][j]

at least, it will not kill some elements.

I need assistance in the logic of the code to rotate the matrix 90 clockwise

2 solutions

Solution 1

Solution 2

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