c语言，为什么我定义了一个全局变量，总是会自动变为零？

Question

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#include<conio.h> 
int score=0,move=0,a[4][4]={0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},temp[5],highest_score=0;
void print(void)
{
    int i,j;
    system("cls");
    printf("                      ___________________2048________________\n");
    printf("                        ------------how to play?----------\n");
    printf("                    A---left,D-right,W---up,S---down,0---exit\n");
    printf("                        ----------------------------------\n");
    printf("                                   highest score:%d\n",highest_score);
    printf("                        ----------------------------------\n");
    printf("                              score:%d          move:%d     \n",score,move);    
    for(i=0;i<4;i++)
    {
        printf("                               |-------------------|\n");
        printf("                               "); 
        for(j=0;j<4;j++)
        {
            if(a[i][j]==0)
            printf("|    ");
            else
            printf("|%-4d",a[i][j]);
            if(j==3)
            printf("|\n");
        }
    } 
    printf("                               |-------------------|\n");
}
int add()
{
    int i,j,t=1,change=0;
    while(t<=3)
    {
        for(i=0;i<3;i++)    
            {
                if(temp[i]==0)
                {   
                    if(temp[i]!=temp[i+1])
                        change=1;
                    temp[i]=temp[i+1];
                    temp[i+1]=0;
                }
            }
        t++;
    }
    for(i=0;i<3;i++)
    {
        if(temp[i]==temp[i+1])
        {
            if(temp[i]!=0)
            {
                change=1;
                score=score+temp[i];
            }
            temp[i]=temp[i]*2;
            temp[i+1]=0;
        }
    }
    t=1;
    while(t<=3)
    {
        for(i=0;i<3;i++)
        {
            if(temp[i]==0)
            {
                temp[i]=temp[i+1];
                temp[i+1]=0;
            }
        }
        t++;
    }
    return change;
}
int main()
{
    int change=1,gameover=0,i,j;
    char input;
    srand((unsigned)time(NULL));
    while(gameover==0)
    {
        if(change>=1)
        {
            do
            {
                i=((unsigned)rand())%4;
                j=((unsigned)rand())%4;
            }while(a[i][j]!=0);
            if(((unsigned)rand())%2==0)
                a[i][j]=2;
            else 
                a[i][j]=4; 
        }
        print();
        input=getch();
        change=0;
        if(input=='0')
        {
            printf("Do you really want to exit this game?Y/N\n");
            input=getch();
            if(input=='y'||input=='Y')
            break; 
        }
        else if(input=='w'||input=='W')
        {
            for(j=0;j<4;j++)
            {
                for(i=0;i<4;i++)
                    temp[i]=a[i][j];
                temp[5]=0;
                change+=add();
                for(i=0;i<4;i++)
                    a[i][j]=temp[i];
            }
            move++;
        }
        else if(input=='s'||input=='S')
        {
            for(j=0;j<4;j++)
            {
                for(i=0;i<4;i++)
                    temp[i]=a[3-i][j];
                temp[5]=0;
                change+=add();
                for(i=0;i<4;i++)
                    a[i][j]=temp[3-i];
            }
            move++;
        }
        else if(input=='a'||input=='A')
        {
            for(i=0;i<4;i++)
            {
                for(j=0;j<4;j++)
                    temp[j]=a[i][j];
                temp[5]=0;
                change+=add();
                for(j=0;j<4;j++)
                    a[i][j]=temp[j] ;   
            }
            move++;
        }
        else if(input=='d'||input=='D')
        {
            for(i=0;i<4;i++)
            {
                for(j=0;j<4;j++)
                    temp[j]=a[i][3-j];
                temp[5]=0;
                change+=add();
                for(j=0;j<4;j++)
                a[i][j]=temp[3-j];
            }
            move++;
        }
        gameover=1;
        for(i=0;i<4;i++)
            for(j=0;j<4;j++)
                {
                    if(a[i][j]==0)
                gameover=0;
                }
        if(gameover==1)
        {
            for(i=0;i<3;i++)
                for(j=0;j<3;j++)
                if(a[i][j]==a[i+1][j]||a[i][j]==a[i][j+1])
                gameover=0;
            for(i=0;i<3;i++)
                if(a[3][i]==a[3][i+1]||a[i][3]==a[i+1][3])
                gameover=0; 
        }
        if(gameover==1)
        {
            printf("Do you want to try again?Y/N");
            input=getch();
            if(input=='Y'||input=='y')
            {
                gameover=0;
                memset(a,0,sizeof(a));
                change=1;
                if(score>highest_score)
                highest_score=score;
                score=0;
                move=0; 
            }
            else if(input=='n'||input=='N')
            gameover=1;
        }
    }
    printf("\n                                      Game over!\n");
    getch();
    return 0;
} 

就是highest_score这个全局变量，为什么每次重新开始游戏的时候只在按过Y之后才会显示为score的值，之后一旦进行了一次操作，就右边为了0？而且我发现只要把它改为一个非零的数，就一切正常，这是为什么？

Answer 1

是因为你程序里面highest_score=score;是在按了Y之后，其实在gameover=1游戏结束的时候就可以判断和保存最高分了。