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Introduction to NumPy
Selecting Elements from a 1-D Array

NumPy allows us to select elements from an array using their indices. Consider the one-dimensional array

a = np.array([5, 2, 7, 0, 11])

If we wanted to select the first element in this array, we would call:

>>> a[0] 5

In typical Python fashion, the indices for an array start at 0. This is known as zero-indexed numbering. In the array above, 5 is known as the zeroth element, a[0]. It follows that 2 is the first element, a[1].

We can also select negative indices, which count from opposite end of the array and start at -1. This is particularly useful when you want to access the last element or two of an array:

>>> a[-1] 11 >>> a[-2] 0

If we wanted to select multiple elements in the array, we can define a range, such as a[1:3], which will select all the elements from a[1] to a[3], including a[1] but excluding a[3].

>>> a[1:3] array([2, 7])

Similarly, if we wanted to select all elements before a[3] we would use:

>>> a[:3] array([5, 2, 7])

We can also use negative indices to select multiple elements. Let’s say we want to select the last 3 elements in an array:

>>> a[-3:] array([7, 0, 11])

Notice that when we select multiple elements, we get an array.

Instructions

1.

Let’s return to our student’s test scores. The following table shows all three test arrays aligned to the names of the students.

Tanya Manual Adwoa Jeremy Cody
test_1 92 94 88 91 87
test_2 79 100 86 93 91
test_3 87 85 72 90 92

Jeremy wants to know what he scored on the second test.

Select the score from the test_2 array and save it to the variable jeremy_test_2.

2.

You want to compare how Manual and Adwoa did on the first test.

Select both of their scores and save them in an array named manual_adwoa_test_1.

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