【numpy學習筆記】陣列的切片，索引，迭代

1. 一維陣列

切片

a = np.arange(10)
# array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

a[5]
# 5

a[2:6] 
# array([2, 3, 4, 5])

a[0:3] = 100
a  # 原陣列發生改變
array([100, 100, 100,   3,   4,   5,   6,   7,   8,   9])

# 如果不想對原來的 array 進行修改，我們可以把選取的部分 copy 出來
a_slice = a[0:5].copy()
a_slice
# array([100, 100, 100,   3,   4])

2. 多維陣列

切片

def f(x,y):
    return 4 * x + y
b = np.fromfunction(f,(3,2),dtype=int)
b
# array([[0, 1],
        [4, 5],
        [8, 9]])
# derive from locations of the matrix
#[[00],[01],[10],[11],[20],[21]]
#通過函式來建矩陣

b[1]
# array([4, 5])

b[1][0] #等價於b[1,0]
# 4

b[:, 1] # 所有行&第二列
# array([1, 5, 9])

b[1:3,1] # 第二行到第三行&第二列
# array([5, 9])

迭代

c = np.zeros ([5,5])
column_length = c.shape[1] # 列的個數；shape[0]行的個數
# 5
for i in range(column_length):
    c[i] = i
    print(c[i])

#[0. 0. 0. 0. 0.]
 [1. 1. 1. 1. 1.]
 [2. 2. 2. 2. 2.]
 [3. 3. 3. 3. 3.]
 [4. 4. 4. 4. 4.]

索引

week_days=np.array(['Monday','Tuesday','Wednesday','Thursday','Friday'])

work_time=np.round(np.random.randn(5,5)+8.0, 2)  
# 小數點後保持兩位 
# round() 方法返回浮點數x的四捨五入值。
work_time
# array([[10.12,  6.86,  8.21,  8.17,  9.03],
        [ 8.64,  8.7 ,  9.13,  8.53,  7.78],
        [ 5.79,  8.12,  7.88,  8.59,  8.94],
        [ 8.46,  7.17,  7.1 ,  8.25,  7.06],
        [ 7.46,  6.69,  9.17,  6.97,  8.34]])
week_days == 'Tuesday'
# array([False,  True, False, False, False])

work_time.T[week_days == 'Tuesday']
# array([[6.86, 8.7 , 8.12, 7.17, 6.69]])