为什么除main以外的函数中的数组初始化是临时的？

Question

This is the code in which i initialize array "turn[20]" in main as well as in function "checkCollisionOrFood()",the four values turn[0],turn[1],turn[2],turn[3] are initialized to zero in main function,rest are being intialized in "checkCollisionOrFood()".This is where fault starts.when i initialize turn[4]=0 in "checkCollisionOrFood()" and then access it anywhere,it remains 0 in any function,but! when i initialize next turn[] ie turn[5],the value of turn[4] gets depleted .ie turn[4] have garbage value.turn[20] is global variable,its index"head" is also global.I'm stuck with this problem plz help me get out of it.This is my complete code

#include <mega8.h>
#include <delay.h>
#include <stdlib.h>

unsigned short address[]=    {0b00001111,0b00000000,0b00000000,0b00000000,
  0b00000000,0b00000000,0b00000000,0b00000000};
unsigned short turn[20];//it is declared as global variable.
unsigned short head=3;
unsigned short colHead=0;
unsigned short rowHead =3;
unsigned short rowTail=0;
unsigned short colTail=0;
unsigned short size=4;
unsigned short foodx;
unsigned short foody;

...

unsigned short checkCollisionOrFood()
{
head=(head+1)%20;
if(turn[head-1]==0){
    turn[head]=0;

/ this is where turn[] is iniliazized and if i access turn[head] here ie just after iniliazition then it gives correct value but if i access its previous value means turn[head-1]then it gives garbage value /

    PORTB=255;PORTB&=~(1<<turn[4]);PORTD.7=0;delay_ms(200);PORTD.7=1;

/ by this statement i checked the value of turn[4] just after initilization of turn[4] which gives right answer and then after initialization of turn[5] and accessing turn[4] gives garbage. /

    rowHead=(rowHead+1)%8;
    if(!(address[colHead]&(1<<rowHead)))return 1;
    else if((address[colHead]&(1<<rowHead))&&
    (!((colHead==foody)&&(rowHead==foodx))))gameOver();
    else return 0;
    }
if(turn[head-1]==1){
    turn[head]=1;
    colHead=(colHead+1)%8;
    if(!(address[colHead]&(1<<rowHead)))return 1;
    else if((address[colHead]&(1<<rowHead))&&
    (!((colHead==foody)&&(rowHead==foodx))))gameOver();
    else return 0;
    }
}
void main(void)
{
turn[0]=0;turn[1]=0;turn[2]=0;turn[3]=0;

/ these values of turn[] are not changed irrespective of where they are accessed. /

while (1)
  {
  if(checkCollisionOrFood())
    {
    PORTB=(address[colHead] |=1<<rowHead);
    turnOffTail();
    blink();
    }
  else
    {
    PORTB=address[colHead];
    createFood();
    blink();
    }
  }

}

Plz suggest me to get out of it.

Answer 1

If you observer garbage in head this could easily due to accessing address out of bounds, as this is located in memrory directly before turn

Check the indexing of address to make sure you do not write beyond it's boundaries and with this overwrite data in the next variable, that is turn .

Answer 2

unsigned short checkCollisionOrFood()
{
    head=(head+1)%20;
    if(turn[head-1]==0){
        turn[head]=0;

You might have a problem when "head" is reaching "20". The value of "head" is then rolling back to 0, which leads you to test the value of "turn[-1]".