# Hands-on Numpy(II): Performing basic operations with ndarrays

In the last article, we learned many different ways in which we can create ndarrays. Now that we know how to create NumPy arrays it's time to start playing around with them.

We will learn to perform basic operations, a task you can divide into 3 categories:

• Operations between arrays of the same size.
• Operations between an array and a scalar.
• Operations between arrays of different sizes.

The third scenario is known as broadcasting, and we won't deal with it in this article, instead, we will focus on the first two.

Great, let's get started.

## Operations using ndarrays of the same size

Operations between arrays of the same size are applied in an element-wise fashion. We will create two arrays and see how subtraction, addition, multiplication, and division work.

``````import numpy as np
arr_1 = np.array([1, 2, 3, 4, 5])
arr_2 = np.array([20, 30, 40, 50, 60])
``````
``````# Element-wise sum of two arrays
sum_result = arr_1 + arr_2
print(sum_result)
``````
``````[21 32 43 54 65]
``````
``````# Element-wise substraction of two arrays
sub_result = arr_2 - arr_1
print(sub_result)
``````
``````[19 28 37 46 55]
``````
``````# Element-wise multiplication of two arrays
mult_result = arr_1 * arr_2
print(mult_result)
``````
``````[ 20  60 120 200 300]
``````
``````# Element-wise division of two arrays
div_result = arr_1 / arr_2
print(div_result)
``````
``````[0.05       0.06666667 0.075      0.08       0.08333333]
``````
``````# Element-wise power
pow_result = arr_2 ** arr_1
print(pow_result)
``````
``````[       20       900     64000   6250000 777600000]
``````

You can also perform comparisons between two arrays. The result is a boolean array where every entry is the result of performing an element-wise comparison:

``````arr_1 = np.array([1, 10, 2, 20])
arr_2 = np.array([50, 5, 30, 3])

# Return True if the elements in the first array are larger than the ones in the second array
comparison_result = arr_1 > arr_2
print(comparison_result)
``````
``````[False  True False  True]
``````

Now, there are two additional things to note.

The first one is that you can chain operations with as many arrays as you want:

``````arr_1 = np.array([1,4,7])
arr_2 = np.array([2,5,8])
arr_3 = np.array([3,6,9])

sum_result = arr_1 + arr_2 + arr_3
print(sum_result)
``````
``````[ 6 15 24]
``````

The second one is that you can use the same array several times when defining the operation:

``````arr = np.array([1, 5, 10])
result = arr * arr - arr # This applies the operation x^2 - x to every entry
print(result)
``````
``````[ 0 20 90]
``````

Good, now let's see what happens when we perform these operations with an array and a scalar.

## Operations using ndarrays of the same size

Operations between an array and a scalar are very intuitive: they are performed over every entry in the array and the scalar, and the result is a boolean array. This is much easier to understand with examples:

``````arr = np.arange(1, 6) # Generate entries from 1 to 5
print(arr)
``````
``````[1 2 3 4 5]
``````
``````# Sum of an array and a scalar
sum_result = arr + 10
print(sum_result)
``````
``````[11 12 13 14 15]
``````
``````# Substraction between an array and a scalar
sub_result = arr - 1
print(sub_result)
``````
``````[0 1 2 3 4]
``````
``````# Like the rest of the operations, substractions works as you would expect if you invert the order
sub_result = 1 - arr
print(sub_result)
``````
``````[ 0 -1 -2 -3 -4]
``````
``````# Multiplication between an array and a scalar
mult_result = arr * 5
print(mult_result)
``````
``````[ 5 10 15 20 25]
``````
``````# Division between an array and a scalar
div_result = arr / 2
print(div_result)
``````
``````[0.5 1.  1.5 2.  2.5]
``````
``````# Again, the order of the elements is important, just as you would expect
div_result = 2 / arr
print(div_result)
``````
``````[2.         1.         0.66666667 0.5        0.4       ]
``````

Comparisons between a scalar work as you would expect. The comparison returns a boolean array with the result of evaluating the comparison over every element in the original array.

``````result = arr > 3
print(result)
``````
``````[False False False  True  True]
``````

## Pretty straightforward, isn't it?

One of the best things about NumPy is how consistent and usually obvious it is, and these basic operations are proof of that.

Knowing how to perform basic operations is the foundation you need to build more elaborate solutions, and now you got that covered. In the next article, we will learn about an extremely important topic: Indexing and slicing. See you then. 