C语言二维数组元素下落异常：1向左偏移后下落问题排查

在C语言中定义了20×20的二维数组，期望将值为1的元素垂直下落至数组底部，但运行代码后元素会先向左偏移再向下移动。预期逻辑是将当前位置设为0，下方位置设为1，重复此操作实现直落效果，核心逻辑在fallDown()函数中。

原代码 #

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
#ifdef __unix__
# include <unistd.h>
#elif defined _WIN32
# include <windows.h>
#define sleep(x) Sleep(1000 * (x))
#endif

int sizeX = 20;
int sizeY = 20;

int grid[20][20] = {{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0}};

int fallDown(int grid[sizeX][sizeY], int x, int y){
    grid[x][y]=0;
    grid[x][y+1]=1;
}

int main(void){
    int neighbour_count[sizeX][sizeY];
    int x,y,iterations;

    for(iterations=0;iterations<500;iterations++){
        system("cls"); //Clear screen
        for(x=0;x<sizeX;x++){
            printf("\n");
            for(y=0;y<sizeY;y++){
                if(grid[x][y]==1){
                    printf("@");
                }
                else{
                    printf(" ");
                }
            }
        }
        for(y=0;y<sizeY;y++){
            for(x=0;x<sizeX;x++){
                if(grid[x][y] == 1){
                    fallDown(grid, x, y);
                }
            }
        }
        printf("\n");
        sleep(1);
    }
}

问题原因 #

1. 坐标逻辑错误：根据打印逻辑，x代表行（每轮x循环输出一行），y代表列。垂直下落应该是行号增加（向下移动一行），但原代码中fallDown()函数修改的是y+1（列号增加，即向右移动），这是偏移的根本原因。
2. 遍历顺序错误：主函数中从x=0到x=sizeX-1遍历行，当处理完(x,y)的元素后，将(x,y+1)设为1，后续循环会处理到这个新设置的1，导致同一轮内元素被多次移动，出现向左偏移的异常。
3. 无边界判断：当元素处于最后一行时，x+1会超出数组范围，引发越界访问。

修复方案 #

1. 修正fallDown()函数的坐标逻辑，改为修改行号x+1，并添加边界判断，确保元素不会越界。
2. 调整遍历顺序：从下往上遍历行（x从sizeX-2到0），同一列内先处理下方的元素，避免同一轮循环中重复处理刚下落的元素。
3. 修正遍历循环的顺序，确保每一轮只处理一次当前位置的元素。

修复后的代码 #

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
#ifdef __unix__
# include <unistd.h>
#elif defined _WIN32
# include <windows.h>
#define sleep(x) Sleep(1000 * (x))
#endif

int sizeX = 20;
int sizeY = 20;

int grid[20][20] = {{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0}};

// 修正下落逻辑，添加边界判断
void fallDown(int grid[sizeX][sizeY], int x, int y){
    // 确保不是最后一行，才能下落
    if(x < sizeX - 1){
        grid[x][y] = 0;
        grid[x + 1][y] = 1;
    }
}

int main(void){
    int x,y,iterations;

    for(iterations=0;iterations<500;iterations++){
        system("cls"); // 清屏
        for(x=0;x<sizeX;x++){
            printf("\n");
            for(y=0;y<sizeY;y++){
                if(grid[x][y]==1){
                    printf("@");
                }
                else{
                    printf(" ");
                }
            }
        }
        // 从下往上遍历行，避免同一轮重复处理
        for(y=0;y<sizeY;y++){
            for(x=sizeX - 2;x >= 0;x--){
                if(grid[x][y] == 1){
                    fallDown(grid, x, y);
                }
            }
        }
        printf("\n");
        sleep(1);
    }
    return 0;
}

关键修改点 #

• 将fallDown()的返回类型改为void（原代码无返回值，不符合语法），并修改坐标为x+1，添加x < sizeX -1的边界判断。
• 遍历行时从sizeX-2（倒数第二行）开始向上遍历到0，这样处理当前元素时，下方的位置还未被本轮循环修改，不会出现重复移动的问题。

内容的提问来源于stack exchange，提问作者Saw