Boolean Indexing and 2D Arrays - Exam 1

What does x > 0 produce when x is an array?

A Boolean array of True/False, one result per entry.

How do you select only the positive values of x?

x[x > 0]

How do you select the negative values of x?

x[x < 0]

How do you exclude all zeros from x?

x[x != 0]

For x=[-2,0,3], what is x[x>0]?

[3]

How do you select values between 0 and 3 in array x?

x[(x >= 0) & (x <= 3)]

What joins two NumPy array masks: and or &?

Use & for elementwise AND; use parentheses around comparisons.

In a 2D array A, what does A[1,2] mean?

Row index 1, column index 2 (second row, third column).

What does A[:,0] select for a 2D array?

Every row of the first column.

What does A[:2,:] select for a 2D array?

The first two rows and all columns.

What is the shape of a 4-row, 3-column array?

(4, 3)

For x=np.array([-2,0,3]), what mask is x>0?

[False False True]

For x=np.array([-2,0,3]), what is x[x!=0]?

[-2 3]

For x=np.array([-2,0,3]), what is np.abs(x[x!=0])?

[2 3]

What does (x>0).sum() count in a NumPy array?

How many entries are positive, since True counts as 1.

If x=[-1,0,2,4] as NumPy array, how many positives are there?

2; (x>0).sum() returns 2.

How do you select NumPy array entries with absolute value > 2?

x[np.abs(x) > 2]

What does x[(x<0)|(x>5)] keep?

Values less than 0 OR greater than 5.

Why use parentheses in (x>0)&(x<3)?

They ensure each comparison happens before combining masks.

For A=[[1,2,3],[4,5,6]] as NumPy array, what is A[1,0]?

4

For A=[[1,2,3],[4,5,6]] as NumPy array, what is A[0,2]?

3

For A=[[1,2,3],[4,5,6]] as NumPy array, what is A[0,:]?

[1 2 3] (entire first row).

For A=[[1,2,3],[4,5,6]] as NumPy array, what is A[:,1]?

[2 5] (second column).

For A=[[1,2,3],[4,5,6]] as NumPy array, what is A[:,:2]?

[[1 2], [4 5]] (first two columns).

Write code to copy the third row of array A

row = A[2,:].copy()

Write code to select the last column of a 2D array A

A[:,-1]

What is np.array([[1,2],[3,4]]).shape?

(2,2)

Why is 2D array indexing written A[row,column]?

The first index chooses a row, then the second a column.

What does x[x>0] = 0 do?

Sets all positive entries in x to zero.