How do I apply a conditional statement to a 2D array in python?-CodePudding

I have a 2D array in python with numbers ranging from positive values to negative ones. However I need to set an 'if' statement that says something like: if an element of the 2D array is negative (less than zero) set the value to zero.

Here's a snippet of the code I've used:

final_array = np.random.randint(-1,1, size=(4,4))

for i in final_array:
    if final_array[i] < 0:
        final_array[i] == 0

When running this code I get the error: IndexError: arrays used as indices must be of integer (or boolean) type

Any help would be massively appreciated

CodePudding user response：

You can set the value of elements in a Numpy array using conditions. For example, the code below sets every element less than 0 equal to 0.

final_array[final_array<0] = 0

CodePudding user response：

With your code I get a different error:

In [91]: final_array = np.random.randint(-1,1, size=(4,4))
    ...: 
    ...: for i in final_array:
    ...:     if final_array[i] < 0:
    ...:         final_array[i] == 0
    ...: 
Traceback (most recent call last):
  Input In [91] in <cell line: 3>
    if final_array[i] < 0:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

In [92]: final_array
Out[92]: 
array([[ 0,  0, -1,  0],
       [-1, -1,  0,  0],
       [ 0,  0,  0, -1],
       [ 0, -1,  0,  0]])
In [93]: i
Out[93]: array([ 0,  0, -1,  0])
In [94]: final_array[i]
Out[94]: 
array([[ 0,  0, -1,  0],
       [ 0,  0, -1,  0],
       [ 0, -1,  0,  0],
       [ 0,  0, -1,  0]])

Notice that i is a row of final_array. While it works in this case, it doesn't make sense to use it as an index, does it? It seems that you haven't read enough basic Python to do a simple iteration correctly.

For example with a list (of lists):

In [95]: alist = [[1],[3,4]]
    ...: for i in alist:
    ...:     print(i)
[1]
[3, 4]

i is one of the sublists, not an index! Trying to use it as an index produces an error:

In [96]: alist = [[1],[3,4]]
    ...: for i in alist:
    ...:     print(alist[i])
Traceback (most recent call last):
  Input In [96] in <cell line: 2>
    print(alist[i])
TypeError: list indices must be integers or slices, not list

Use for i in range(len(arr)): if you want to generate indices. Or for i,v in enumerate(arr): to get indices and values.

But the error you got suggests that final_array was an array of floats, not integers that the code sample produces.

The ambiguity error that I got was the result of using

In [98]: Out[94]<0
Out[98]: 
array([[False, False,  True, False],
       [False, False,  True, False],
       [False,  True, False, False],
       [False, False,  True, False]])

the if statement. if requires a simple True/False. It doesn't do any sort of iteration.

Even proper indexing of a row of final_array would give this ambiguity error

In [100]: final_array[0]<0
Out[100]: array([False, False,  True, False])

The if only works if you are testing scalar elements of the array. For example if the array was 1d:

In [101]: x = np.arange(-3,4)
In [102]: for i in range(len(x)):
     ...:     if x[i]<0:
     ...:         x[i] = 0
     ...: 
In [103]: x
Out[103]: array([0, 0, 0, 0, 1, 2, 3])

But usually in numpy we don't need to iterate (or rather prefer not to). Instead we can test all the values at once. Sticking with the 1d array for simplicity:

In [104]: x = np.arange(-3,4)
In [105]: mask = x<0
In [106]: mask
Out[106]: array([ True,  True,  True, False, False, False, False])
In [107]: x[mask]
Out[107]: array([-3, -2, -1])
In [108]: x[mask] = 0
In [109]: x
Out[109]: array([0, 0, 0, 0, 1, 2, 3])