Going further · 18

2D lists (grids)

Grids of rows and columns: reading them, row and column sums, and neighbours inside the bounds.

A 2D list is a list of lists.
grid[i][j] means column j of row i.
The example below reads a grid, finds the row and column sums, and prints the sum of each cell's neighbours.

r, c = map(int, input().split())grid = [list(map(int, input().split())) for _ in range(r)]print(*[sum(row) for row in grid])print(*[sum(grid[i][j] for i in range(r)) for j in range(c)])moves = [(-1, 0), (1, 0), (0, -1), (0, 1)]near = [[0] * c for _ in range(r)]for i in range(r):    for j in range(c):        for di, dj in moves:            ni, nj = i + di, j + dj            if 0 <= ni < r and 0 <= nj < c:                near[i][j] += grid[ni][nj]for row in near:    print(*row)

const [[r, c], ...grid] = require("fs").readFileSync(0, "utf8").trim().split("\n").map((l) => l.trim().split(/\s+/).map(Number));console.log(grid.map((row) => row.reduce((a, b) => a + b, 0)).join(" "));const colSums = Array.from({ length: c }, (_, j) => grid.reduce((s, row) => s + row[j], 0));console.log(colSums.join(" "));const moves = [[-1, 0], [1, 0], [0, -1], [0, 1]];const near = Array.from({ length: r }, () => new Array(c).fill(0));for (let i = 0; i < r; i++) {  for (let j = 0; j < c; j++) {    for (const [di, dj] of moves) {      const ni = i + di, nj = j + dj;      if (ni >= 0 && ni < r && nj >= 0 && nj < c) near[i][j] += grid[ni][nj];    }  }}for (const row of near) console.log(row.join(" "));

import { readFileSync } from "node:fs";const [[r, c], ...grid]: number[][] = readFileSync(0, "utf8").trim().split("\n").map((l) => l.trim().split(/\s+/).map(Number));console.log(grid.map((row) => row.reduce((a, b) => a + b, 0)).join(" "));const colSums: number[] = Array.from({ length: c }, (_, j) => grid.reduce((s, row) => s + row[j], 0));console.log(colSums.join(" "));const moves: [number, number][] = [[-1, 0], [1, 0], [0, -1], [0, 1]];const near: number[][] = Array.from({ length: r }, () => new Array<number>(c).fill(0));for (let i = 0; i < r; i++) {  for (let j = 0; j < c; j++) {    for (const [di, dj] of moves) {      const ni = i + di, nj = j + dj;      if (ni >= 0 && ni < r && nj >= 0 && nj < c) near[i][j] += grid[ni][nj];    }  }}for (const row of near) console.log(row.join(" "));

import java.util.Scanner; public class Main {    static void print(int[] a) {        StringBuilder sb = new StringBuilder();        for (int i = 0; i < a.length; i++) sb.append(i > 0 ? " " : "").append(a[i]);        System.out.println(sb);    }     public static void main(String[] args) {        Scanner sc = new Scanner(System.in);        int r = sc.nextInt(), c = sc.nextInt();        int[][] grid = new int[r][c];        for (int i = 0; i < r; i++)            for (int j = 0; j < c; j++) grid[i][j] = sc.nextInt();        int[] rowSums = new int[r], colSums = new int[c];        for (int i = 0; i < r; i++)            for (int j = 0; j < c; j++) {                rowSums[i] += grid[i][j];                colSums[j] += grid[i][j];            }        print(rowSums);        print(colSums);        int[] di = {-1, 1, 0, 0}, dj = {0, 0, -1, 1};        for (int i = 0; i < r; i++) {            int[] near = new int[c];            for (int j = 0; j < c; j++)                for (int k = 0; k < 4; k++) {                    int ni = i + di[k], nj = j + dj[k];                    if (ni >= 0 && ni < r && nj >= 0 && nj < c) near[j] += grid[ni][nj];                }            print(near);        }    }}

using System;using System.Linq; class Program{    static void Main()    {        int[] rc = Console.ReadLine().Split(' ', StringSplitOptions.RemoveEmptyEntries).Select(int.Parse).ToArray();        int r = rc[0], c = rc[1];        int[,] grid = new int[r, c];        for (int i = 0; i < r; i++)        {            int[] row = Console.ReadLine().Split(' ', StringSplitOptions.RemoveEmptyEntries).Select(int.Parse).ToArray();            for (int j = 0; j < c; j++) grid[i, j] = row[j];        }        Console.WriteLine(string.Join(" ", Enumerable.Range(0, r).Select(i => Enumerable.Range(0, c).Sum(j => grid[i, j]))));        Console.WriteLine(string.Join(" ", Enumerable.Range(0, c).Select(j => Enumerable.Range(0, r).Sum(i => grid[i, j]))));        int[] di = { -1, 1, 0, 0 }, dj = { 0, 0, -1, 1 };        for (int i = 0; i < r; i++)        {            int[] near = new int[c];            for (int j = 0; j < c; j++)                for (int k = 0; k < 4; k++)                {                    int ni = i + di[k], nj = j + dj[k];                    if (ni >= 0 && ni < r && nj >= 0 && nj < c) near[j] += grid[ni, nj];                }            Console.WriteLine(string.Join(" ", near));        }    }}

#include <iostream>#include <vector> void print(const std::vector<int>& v) {    for (size_t i = 0; i < v.size(); i++) std::cout << (i > 0 ? " " : "") << v[i];    std::cout << '\n';} int main() {    int r, c;    std::cin >> r >> c;    std::vector<std::vector<int>> grid(r, std::vector<int>(c));    for (auto& row : grid)        for (int& x : row) std::cin >> x;    std::vector<int> rowSums(r, 0), colSums(c, 0);    for (int i = 0; i < r; i++)        for (int j = 0; j < c; j++) {            rowSums[i] += grid[i][j];            colSums[j] += grid[i][j];        }    print(rowSums);    print(colSums);    const int di[] = {-1, 1, 0, 0}, dj[] = {0, 0, -1, 1};    for (int i = 0; i < r; i++) {        std::vector<int> near(c, 0);        for (int j = 0; j < c; j++)            for (int k = 0; k < 4; k++) {                int ni = i + di[k], nj = j + dj[k];                if (ni >= 0 && ni < r && nj >= 0 && nj < c) near[j] += grid[ni][nj];            }        print(near);    }}

#include <stdio.h> #define MAX 100 int grid[MAX][MAX]; void print(const int *a, int n) {    for (int i = 0; i < n; i++) printf(i > 0 ? " %d" : "%d", a[i]);    printf("\n");} int main(void) {    int r, c, rowSums[MAX] = {0}, colSums[MAX] = {0};    scanf("%d %d", &r, &c);    for (int i = 0; i < r; i++)        for (int j = 0; j < c; j++) {            scanf("%d", &grid[i][j]);            rowSums[i] += grid[i][j];            colSums[j] += grid[i][j];        }    print(rowSums, r);    print(colSums, c);    const int di[] = {-1, 1, 0, 0}, dj[] = {0, 0, -1, 1};    for (int i = 0; i < r; i++) {        int near[MAX] = {0};        for (int j = 0; j < c; j++)            for (int k = 0; k < 4; k++) {                int ni = i + di[k], nj = j + dj[k];                if (ni >= 0 && ni < r && nj >= 0 && nj < c) near[j] += grid[ni][nj];            }        print(near, c);    }    return 0;}

rc = Console.ReadNumbers();r = rc[0];c = rc[1];grid = [];for (i = 0; i < r; i++){    grid.Add(Console.ReadNumbers());}rowSums = NewList(r, 0);colSums = NewList(c, 0);for (i = 0; i < r; i++){    for (j = 0; j < c; j++)    {        rowSums[i] += grid[i][j];        colSums[j] += grid[i][j];    }}Console.WriteLine(rowSums);Console.WriteLine(colSums);di = [-1, 1, 0, 0];dj = [0, 0, -1, 1];for (i = 0; i < r; i++){    near = NewList(c, 0);    for (j = 0; j < c; j++)    {        for (k = 0; k < 4; k++)        {            ni = i + di[k];            nj = j + dj[k];            if (ni >= 0 && ni < r && nj >= 0 && nj < c)            {                near[j] += grid[ni][nj];            }        }    }    Console.WriteLine(near);}

stdin

stdout

10 26 42
15 18 21 24
7 10 13 11
16 24 28 23
15 26 29 19

Key points

To visit every cell, put a loop over columns inside a loop over rows (a nested loop).
Row sums add up the cells of each row; column sums walk down each column.
You can also add into rowSums[i] and colSums[j] together in one pass.
For neighbours (up, down, left, right), keep the directions in lists such as di = [-1, 1, 0, 0] and dj = [0, 0, -1, 1] and loop over them.
The code gets short.
On the edge a neighbour falls outside.
Always check 0 <= ni < r and 0 <= nj < c before reading, including the negative side.
Each row must be created separately.
If the same row goes in several times, changing one cell changes every row.

How each language writes it

Python	`[[0] * c for _ in range(r)]`. `grid[-1]` is not an error but the last row
JavaScript	`Array.from({ length: r }, () => new Array(c).fill(0))`. A cell outside is `undefined`; reading a cell of a row outside throws `TypeError`
TypeScript	Same as JavaScript. The type is `number[][]`
Java	`new int[r][c]` — filled with zeros. Out of range: `ArrayIndexOutOfBoundsException`
C#	Two kinds: the rectangular array `int[r, c]` (read with `grid[i, j]`) and the jagged array of arrays `int[r][]`. Out of range: `IndexOutOfRangeException`
C++	`vector<vector<int>>(r, vector<int>(c))`. `[]` does not check the range
C	Fix the size in advance, like `int grid[100][100]`, and use only the top-left part. Reading out of range says nothing
Craykon	Build it row by row with `grid.Add(Console.ReadNumbers())`, and make a row of c zeros with `NewList(c, 0)`. Out of range stops with an error saying the list only has items 0 to 3

Common mistakes

Reading the top row's upper neighbour without a checkPython

grid = [[1, 2], [3, 4]]
i, j = 0, 0
print(grid[i - 1][j])

→ 3 — grid[-1] is the last row, so it quietly reads from the bottom row

Check the negative side too: 0 <= ni < r.

Skipping the range checkJavaScript · TypeScript · C · C++

const grid = [[1, 2], [3, 4]];
console.log(1 + grid[0][2]);
console.log(grid[2][0]);

→ Prints NaN, then the next line stops with a TypeError. C and C++ silently read the next row or unrelated memory

Check 0 <= ni && ni < r && 0 <= nj && nj < c before reading.
Java, C# and Craykon stop with an index error.

How languages differ
Python [[0] * c] * r makes r references to one and the same row: after g = [[0] * 3] * 2; g[0][0] = 5 you get [[5, 0, 0], [5, 0, 0]].
JavaScript new Array(r).fill(new Array(c).fill(0)) does the same, so always create each row separately.

2D lists (grids)

Key points

How each language writes it

Common mistakes

Reading the top row's upper neighbour without a checkPython

Skipping the range checkJavaScript · TypeScript · C · C++

Practice