BANA 6620 - Midterm Cards 42-61

Main purpose of NumPy?

Efficient storage and computation with arrays/matrices

NumPy array vs. Python list?

NumPy array → regular structure/ same data type

Python list → can contain different data types

.shape vs .size vs .ndim?

shape → dimensions

size → total elements

nidim → number of dimensions

A NumPy array has 3 rows and 4 columns.

What are:

shape, size, nidm?

shape = (3, 4)

size= 12

nidim= 2

ROWS FIRST

what does:

[::-1]

do?

Reverses the array

What does:

[::2]

do?

Selects every second element, beginning at index 0

What does np.ones((2,3)) create?

A 2-row x 3-column array filled with 1.0

What does np.zeros((4,2)) create?

A 4-row x 2-column array filled with 0.0

What does:

np.linspace (start, stop, num)

do?

Creates num evenly spaced values from start through stop

Stop is included by default.

np.random.rand(5)

5 random floats in:

[0,1)

Argument order:

np.random.normal(…)

normal(mean, std, size)

Argument order:

np.random.randint(…)

randint(low, high, size)

integers in:

[low, high)

How does NumPy arithmetic normally operate?

ELEMENT-WISE

arr*2

→ multiply every element by 2

Given:

arr = np.array([2,5,8])

What is:

arr*3

[6 15 24]

What does np.mean(arr) calculate?

Arithmetic average.

ADD divide HOW MANY

What does np.median(arr) calculate?

The middle value after ordering.

Even number of values → average the two middle values

axis=0 vs axis=1?

axis=0 → COLUMNS

axis=1 → ROWS

For a 3×4 array, how many answers does:

np.mean(arr, axis=0)

return?

One mean for each COLUMN

What does:

arr > 10

return?

A Boolean array:

True/Fales

What does:

arr[arr > 10 ]

return?

The actual VALUES greater than 10.

Comparison → mask

Boolean indexing → values