How to compare two arrays in python based on a value

Question

I am doing classification. I have two arrays, the first is 'Actual', and the second is 'Predicted'. I want to compare these two arrays. Suppose the first array is:

Actual = [1, 1, 2, 3, 1]

this tells us that the the first, second, and the last indexes are corresponding to class 1.

The 'Predicted' array is:

Predicted = [1, 1, 1, 3, 3]

this tells us that the first and second indexes have been predicted accurately.

I want the output the tells us just those indexes that accurately predicted as 1, like this:

output = [True, True, False, False, False]

Update I want to evaluate just based on value 1. If you see, the forth predicted value is accurately predicted by 3, but I do not want that, because I want evaluate 1 value.

What have you tried? What is the expected behavior when the lengths are different? — Michael Aaron Safyan
– Michael Aaron Safyan, Commented Jul 26, 2015 at 2:50
You are comparing two lists. What if one is longer than the other? — Michael Aaron Safyan
– Michael Aaron Safyan, Commented Jul 26, 2015 at 2:53
yes, but that is only one example input. Surely if you were only processing a single example, you wouldn't write a program to do it. Assuming you process many different kinds of inputs, dealing with cases such as differing sizes is something that one must be capable of. — Michael Aaron Safyan
– Michael Aaron Safyan, Commented Jul 26, 2015 at 3:01

Ozgur Vatansever · Accepted Answer · 2015-07-26 03:02:25Z

6

Presuming length of two lists are same:

>>> [(x == y == 1) for x, y in zip(Actual, Predicted)]
[True, True, False, False, False]

To feel safe;

>>> from itertools import izip_longest
>>> [(x == y == 1) for x, y in izip_longest(Actual, Predicted, fillvalue=0)]
[True, True, False, False, False]

edited Jul 26, 2015 at 3:02

answered Jul 26, 2015 at 2:54

Ozgur Vatansever

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Comments

Anand S Kumar · Accepted Answer · 2015-07-26 02:59:59Z

2

If you do not mind using numpy library, then this can be done very easily -

In [10]: import numpy as np

In [11]: Actual=[1,1,2,3,1]

In [12]: ActualNp = np.array(Actual)

In [13]: Predicted=[1,1,1,3,3]

In [15]: PredictedNp = np.array(Predicted)

In [20]: (ActualNp == PredictedNp) & (PredictedNp == 1)
Out[20]: array([ True,  True, False, False, False], dtype=bool)

If not, assumming that you only want to check till the length of the smallest list (If they are of different lengths), you can use zip -

>>> Actual=[1,1,2,3,1]
>>> Predicted=[1,1,1,3,3]
>>> output = [a == b == 1 for a,b in zip(Actual,Predicted)]
>>> output
[True, True, False, False, False]

edited Jul 26, 2015 at 2:59

answered Jul 26, 2015 at 2:54

Anand S Kumar

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2 Comments

mar Over a year ago

In you simulation, the forth element is True. It is by the value 3. I do not want that. I want evaluate just based on 1.

Anand S Kumar Over a year ago

Updated the answer for that.

helado · Accepted Answer · 2015-07-26 02:56:00Z

0

A one liner simple approach would be

def get_prediction_results(prediction, actual, predicted):
 return [a == predicted[i] == prediction for i, a in enumerate(actual)]

>>> get_prediction_results(1, [1,1,2], [1,1,2])
[True, True, False]

answered Jul 26, 2015 at 2:56

helado

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Comments

Michael Aaron Safyan · Accepted Answer · 2015-07-26 03:00:13Z

First, some array basics:

To get the number of elements in an array, use len:
```
 x = ['a', 'b', c']
 y = len(x)  # y == 3
```

To access the ith element of an array, use []:

 x = ['a','b', 'c']
 y = x[1]  # y == 'b'

To obtain an iterator with values 0, 1, ..., n-1 ,use range(n):
```
 x = list(range(3))  # x = [0, 1, 2]
```

To iterate through the values of an array, use for ... in:

 x = ['a', 'b', 'c']
 for value in x:
    process(value)   # called for 'a', 'b', and 'c'

To compare items for equality, use == (or != for inequality).

Putting this altogether,now:

def ComputeArrayDifference(a, b):
   alen = len(a)
   blen = len(b)
   if alen != blen:
      raise DifferingSizesException('Inputs have different sizes', a, b)
   result = []
   for i in range(alen):
      result.append(a[i] == b[i])
   return result

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