2. 定义一个数组(二维)，按列计算数组的最大值 max，最小值 min，均值 mean，要求每种特征值均为一种函

数的实现方式。

假设二维数组为arr，有n行m列，其中每个元素为arr[i][j]。

1. 计算最大值max

def get_max(arr):
    n = len(arr)    # 行数
    m = len(arr[0]) # 列数
    max_list = []
    for j in range(m):
        max_val = arr[0][j]
        for i in range(1, n):
            if arr[i][j] > max_val:
                max_val = arr[i][j]
        max_list.append(max_val)
    return max_list

2. 计算最小值min

def get_min(arr):
    n = len(arr)    # 行数
    m = len(arr[0]) # 列数
    min_list = []
    for j in range(m):
        min_val = arr[0][j]
        for i in range(1, n):
            if arr[i][j] < min_val:
                min_val = arr[i][j]
        min_list.append(min_val)
    return min_list

3. 计算均值mean

def get_mean(arr):
    n = len(arr)    # 行数
    m = len(arr[0]) # 列数
    mean_list = []
    for j in range(m):
        sum_val = 0
        for i in range(n):
            sum_val += arr[i][j]
        mean_val = sum_val / n
        mean_list.append(mean_val)
    return mean_list