CODEWITHSUNDEEP
pythonpython-3.5
Neon
Neon

Reputation: 135

Change the signs of the elements in a matrix if all rows contain at least one negative element

I want to check if all the rows of the matrix contain at least one negative element. If so, change the signs of all the elements of the matrix.

Here is my attempt, but this way just changes the sign of the negative elements:

matrix = [[5, -6, 2], [7, -2, 3], [8, 4, -9]]

for i in range(len(matrix)):
    for j in range(len(matrix[i])):
        if matrix[i][j] < 0:
             print(matrix[i][j]*(-1))

Upvotes: 1

Views: 1163

Answers (7)

Stefan Pochmann
Stefan Pochmann

Reputation: 28606

A nice way to check whether all rows contain a negative number (assuming your matrix is at least 1x1, not empty):

   max(map(min, matrix)) < 0

So overall:

if max(map(min, matrix)) < 0:
    matrix[:] = [[-x for x in row] for row in matrix]

Upvotes: 0

ᴀʀᴍᴀɴ
ᴀʀᴍᴀɴ

Reputation: 4528

The easiest way that comes to my mind :

m = [[5,-6, 2], [7, -2, 3], [8, 4, -9], [2 , 4 ,6]]

for i in range(len(m)):
    flag = False
    for y in m[i]:
        if(y<0):
            flag = True
            break
    if (flag == True):
        for j in range(len(m[i])):
            m[i][j] = -1 * m[i][j]


print (m)           # [[-5, 6, -2], [-7, 2, -3], [-8, -4, 9], [2, 4, 6]]

Upvotes: 0

bercik
bercik

Reputation: 916

EDIT:

So first you can check if matrix contains at least one negative element, then change sign of all elements.

matrix = [[5, -6, 2], [7, 2, 3], [8, 4, -9]]

hasNegativeElement = False

for i in range(len(matrix)):
    for j in range(len(matrix[i])):
        if matrix[i][j] < 0:
            hasNegativeElement = True
            break

if hasNegativeElement:
    for i in range(len(matrix)):
        for j in range(len(matrix[i])):
            matrix[i][j] *= (-1)

Upvotes: -1

John Sensebe
John Sensebe

Reputation: 1396

Try this:

allRowsNegative = True;
for i in range(len(matrix)):
    foundNegative = False;
    for j in range(len(matrix[i])):
        foundNegative = (foundNegative or matrix[i][j] < 0);
    allRowsNegative = (allRowsNegative and foundNegative);

if allRowsNegative:
    for i in range(len(matrix)):
        for j in range(len(matrix[i])):
            print(matrix[i][j]*(-1));
else:
    for i in range(len(matrix)):
        for j in range(len(matrix[i])):
            print(matrix[i][j]);

Upvotes: 0

timgeb
timgeb

Reputation: 78740

non numpy solution:

>>> matrix = [[5, -6, 2], [7, -2, 3], [8, 4, -9]]
>>> if all(any(x < 0 for x in sub) for sub in matrix):
...     result = [[-x for x in sub] for sub in matrix]
... 
>>> result
[[-5, 6, -2], [-7, 2, -3], [-8, -4, 9]]

numpy solution

>>> import numpy as np
>>> matrix = [[5, -6, 2], [7, -2, 3], [8, 4, -9]]
>>> m_arr = np.array(matrix)
>>> if np.all(np.any(m_arr < 0, axis=1)):
...     result = (m_arr*(-1)).tolist()
... 
>>> result
[[-5, 6, -2], [-7, 2, -3], [-8, -4, 9]]

Upvotes: 0

percusse
percusse

Reputation: 3106

You can check every row with np.any for a negative element which would give an array and you can then check if all of them turned out to have a negative element via np.all. If so reverse the sign of the matrix.

import numpy as np
A = np.array([[5, -6, 2], [7, 2, 3], [8, 4, -9]])

if np.all(np.any(A<0,axis=1)):
    A *= -1

Upvotes: 0

Anton Protopopov
Anton Protopopov

Reputation: 31682

You could do it very easy with numpy:

import numpy as np
In [108]: np.array(matrix) * -1
Out[108]: 
array([[-5,  6, -2],
       [-7, -2, -3],
       [-8, -4,  9]])

Or if you want a list at the end you could use tolist method:

In [124]: (np.array(matrix) * -1).tolist()
Out[124]: [[-5, 6, -2], [-7, -2, -3], [-8, -4, 9]]

Upvotes: 2

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