Two-dimensional arrays

Python for IGCSE CS Lesson 9 2:03 English narration · English + 中文 subtitles burned in

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A table of marks, a seating plan, a game board — these have rows and columns, and one index is not enough. 一张成绩表、一份座位安排、一个棋盘—— 这些东西有行也有列，一个下标不够用。

A two-dimensional array is a list of lists: each item of the outer list is itself a whole row. 二维数组就是一个列表的列表：外层列表的每一项，本身就是一整行。

To reach one cell you give two numbers, and the order is fixed. 要取到一个格子，你要给两个数字，而且顺序是固定的。

This one reads as row one, column two, and it holds six. 这个读作"第 1 行、第 2 列"，它装的是 6。

Swap the two numbers and you are not being approximate — you are asking for a different cell entirely. 把这两个数字调换一下，那不是"差不多"—— 你要的是完全另一个格子。

On this grid there is no row two, so it errors, and you find out at once. 在这个网格上没有第 2 行，所以它会报错，你立刻就知道了。

On a square grid there would be a row two, and you would get the wrong cell in silence, with a plausible number that is simply not the one you asked for. 但在一个方形的网格上，第 2 行是存在的，你会悄无声息地拿到错误的那个格子，得到一个看起来合理、却根本不是你要的数字。

Row first, always. 永远是先行。

To touch every cell you need one loop inside another. 要碰到每一个格子，你需要一个循环套在另一个里面。

The outer loop picks one row at a time. 外层循环每次取出一行。

The inner loop walks along it, cell by cell, and only when it finishes does the outer one move down. 内层循环沿着这一行一格一格地走，只有等它走完，外层才往下移一行。

Count the passes: the body runs rows times columns times — six here — which is exactly the number of cells. 数一数执行次数：循环体执行了"行数乘以列数"次——这里是 6 次—— 正好等于格子的数量。

Here is the second task in both notations. 这是第二道题的两种写法。

The declarations look different — the exam writes one pair of brackets with a comma inside, and Python writes two separate pairs — but the walk through the grid is the same nested loop in both. 声明看起来不一样—— 考卷写的是一对方括号，里面用逗号隔开，而 Python 写的是两对分开的方括号—— 但遍历网格的方式，在两边都是同一个嵌套循环。

Read across, and the pseudocode stops being unfamiliar. 横着读，伪代码就不再陌生了。

Four things to take with you. 带走四点。

One: a two-dimensional array is a list whose items are lists. 第一：二维数组是一个"元素本身也是列表"的列表。

Two: the first index is the row and the second is the column. 第二：第一个下标是行，第二个是列。

Three: an outer loop for rows, and an inner one for columns. 第三：外层循环管行，内层循环管列。

Four: swapping the two indexes gives you a different cell, not a near miss. 第四：把两个下标调换，得到的是另一个格子，不是"差一点"。

Now do the three tasks. 现在去做那三道题。