理论与实践

数据输入输出

任务

条件语句

任务

for循环

任务

字符串

任务

while循环

任务

列表

任务

二维数组

任务

字典

任务

集合

任务

函数与递归

任务

课程 7. Two -dimensional arrays

难度：

任务«月度支出»

💻 Python

你在一家小型家族企业中担任分析师，希望自动化计算每月食品杂货费用的流程。你有一个表格，它是一个矩阵，其中每一行对应一个月中的一周，每一列对应一周中的某一天。每个单元格表示当天在食品杂货上的花费。你的任务是编写一个程序，接收这个表格作为输入，并计算整个月食品杂货的总花费。

输入格式

第一行包含两个整数 n 和 m ，以空格分隔，其中 n 表示一个月中的周数（矩阵的行数），而 m 表示一周中的天数（矩阵的列数）。接下来的 n 行每行包含 m 个以空格分隔的整数，这些整数构成一个产品矩阵。

输出格式

一个数字 & ndash; 每月食品总支出

示例

输入

3 7
100 50 75 120 80 60 90
110 40 65 130 90 70 100
90 60 85 110 70 50 80

输出

1955

提示

min function

The min() function is your tool for quickly searching for the smallest element of several values or within a collection.

Syntax

min(iterable, *[, key, default])
min(arg1, arg2, *args[, key])

iterable: An iterable object (for example, a list, tuple, string, set).
arg1, arg2, *args: One or more arguments passed directly for comparison.
key (optional): A function that is applied to each element before comparison. The minimum will be searched based on the results of this function.
default (optional): The value that will be returned if iterable is empty. If default is not specified, an attempt to find the minimum in an empty collection will cause the error ValueError.

Usage examples

With a single iterable object:

numbers = [3, 1, 4, 1, 5, 9]
print(min(numbers))  # Output: 1

With multiple arguments:

print(min(3, 1, 4, 1, 5, 9)) # Output: 1

With the key parameter: Searching for an element by any of its properties.

words = ["apple", "banana", "cherry"]
# Finding the word with the shortest length
print(min(words, key=len))  # Output: 'apple'

With the default parameter: Safe handling of potentially empty collections.

empty_list = []
print(min(empty_list, default="empty"))  # Output: 'empty'

Useful tips and subtleties:

Comparing different types: Incomparable data types, such as a number and a string, cannot be compared. min(5, "hello") will cause the error TypeError.
How the key works: It is important to understand that key only changes the value for comparison, but returns the original element. In the example min(words, key=len), the lengths (5, 6, 6) are compared, but the word 'apple' itself is returned.
Using lambda functions with key: For complex comparisons, it is convenient to use anonymous lambda functions. For example, to find a dictionary with the minimum value by the key 'age':
```
people = [{'name': 'John', 'age': 25}, {'name': 'Jane', 'age': 22}]
print(min(people, key=lambda person: person['age']))  # Output: {'name': 'Jane', 'age': 22}
```
String comparison: The strings are compared lexicographically (in alphabetical order). min("a", "b", "A") returns 'A', since capital letters have a smaller code in the symbol table.

Function `max()`

The max() function works similarly to min(), but returns the largest element from the passed arguments or collection.

Syntax

max(iterable, *[, key, default])
max(arg1, arg2, *args[, key])

The parameters (iterable, arg1, arg2, *args, key, default) have the same value as in the min() function.

Usage examples

With a single iterable object:

numbers = [3, 1, 4, 1, 5, 9]
print(max(numbers))  # Output: 9

With multiple arguments:

print(max(3, 1, 4, 1, 5, 9)) # Output: 9

With the key parameter:

words = ["apple", "banana", "cherry"]
# Finding the word with the longest length
print(max(words, key=len))  # Output: 'banana'

With the default parameter:

empty_list = []
print(max(empty_list, default='N/A'))  # Output: 'N/A'

Useful tips and subtleties:

All the tips and subtleties applicable to min() are also relevant for max(): avoid mixing types, use lambda functions for complex keys and keep in mind the default parameter for empty collections.
If there are several identical maximum/minimum elements, the first one is returned. In the example, max(words, key=len) the words "banana" and "cherry" have the same length of 6, but max returns the first encountered 'banana'.

Function `sum()`

The sum() function is used to find the sum of numeric elements in an iterable object.

Syntax

sum(iterable, start=0)

iterable: An iterable object containing numbers (for example, a list, a tuple, a set).
start (optional): The initial value that is added to the total amount. The default value is 0.

Usage examples

Sum of list/tuple/set elements:

numbers_list = [1, 2, 3, 4, 5]
print(sum(numbers_list))  # Output: 15

Using the start parameter:

numbers = [1, 2, 3, 4, 5]
# We start summing not from 0, but from 10
total = sum(numbers, 10)
print(total) # Output: 25 (10 + 1 + 2 + 3 + 4 + 5)

Application to two-dimensional arrays (lists of lists)

sum() can be effectively used to process matrices, but with one important caveat.

Wrong approach: sum(matrix) will cause the error TypeError, as Python will try to add the initial value 0 with the list [1, 2, 3].

The right approach: You must first calculate the sum of each row, and then add these amounts.

# Two-dimensional array
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

# Sum of all array elements using a generator
total_sum = sum(sum(row) for row in matrix)
print(total_sum) # Output: 45

Useful tips and subtleties:

For numbers only: sum() only works with numeric types (int, float). An attempt to summarize a list of strings (sum(["a", "b"])) will result in a TypeError.
Summation True/False: True is interpreted as 1, and False as 0. This can be used to count the true values in the collection.
```
bools = [True, False, True, True] # 1 + 0 + 1 + 1
print(sum(bools)) # Output: 3
```
"Flattening" the list of lists: The start parameter can be cunningly used to combine lists, although it is less readable and efficient than other methods.
```
list_of_lists = [[1, 2], [3, 4], [5]]
flat_list = sum(list_of_lists, [])
print(flat_list) # Output: [1, 2, 3, 4, 5]
```
Warning: This method is not recommended for large lists due to poor performance.