A Star algorithm in C implementation with arrays? - c

Question

I'm new here. I am trying to implement A-Star algorithm in C by myself. I don't know how to use Hashmaps or Lists (but i am open too learn as long as they are simple enough for me) so I use arrays.
The problem is simple: There is a NxN array. You can either go up/down or left/right, no diagonial. Horizontal is better (less cost =5) than vertical movement (high cost=10).
There are some obstacle-cells. Free cells are represented by number 0 in the NxN array, while obstacle cells with the number 9. The obstacle cells occur as a proportion of the area of the table (for example if the table is 10*10 and the independent possibility to have an obstacle in each cell is 0.1, there will be approximately 10 9's in the table.
With the number 1 the starting point is represented and with 2 and 3 the two final goals to go, G1 and G2.
I have tried this below:
#include<stdio.h>
#include <stdlib.h>
int main(void) {
//create a NxN array
int N, sX, sY, g1X,g1Y,g2X,g2Y,i,j,w;
double p;
float r;
printf("Give N\n");
scanf("%d",&N);
printf("Give p\n");
scanf("%lf",&p);
printf("Give S x k y\n");
scanf("%d",&sX);
scanf("%d",&sY);
printf("Give G1 x & y\n");
scanf("%d",&g1X);
scanf("%d",&g1Y);
printf("Give G2 x & y\n");
scanf("%d",&g2X);
scanf("%d",&g2Y);
int table[N][N];
for(i=0; i<N; i++){
for (j=0; j<N; j++){
r=(float)(rand() % 10)/10; // [0,1)
// printf("%f",&r);
if (sX==i && sY==j){
table[i][j]=1;
// printf("1");
}
else if(g1X==i && g1Y==j){
table[i][j]=2;
// printf("2");
}
else if( g2X==i && g2Y==j){
table[i][j]=3;
// printf("3");
}
else if (p>=0 && r<=p){
table[i][j]=9;
// printf("9");
}
else{
table[i][j]=0;
// printf("0");
}
printf("%d ",table[i][j]);
}
printf("\n");
}
// Create the open list
int cX=sX, cY=sY;
while (cX!=g1X && cY!=g1Y)
{
int openList[4][2];
//TOP
if(cX>0 && table[cX-1][cY]!=9){
openList[0][0]=(cX-1);
openList[0][1]=cY;
}
else{
openList[0][0]=-1;
openList[0][1]=-1;
}
//BOTTOM
if(cX+1<N && table[cX+1][cY]!=9 ){
openList[1][0]=(cX+1);
openList[1][1]=cY;
}
else{
openList[1][0]=-1;
openList[1][1]=-1;
}
//RIGHT
if(cY+1<N && table[cX][cY+1]!=9){
openList[2][0]=cX;
openList[2][1]=(cY+1);
}
else{
openList[2][0]=-1;
openList[2][1]=-1;
}
//LEFT
if(cY>0 && table[cX][cY-1]!=9){
openList[3][0]=cX;
openList[3][1]=(cY-1);
}
else{
openList[3][0]=-1;
openList[3][1]=-1;
}
printf("Open List of current cell:%d,%d\n",&cX, &cY);
for (i=0;i<4;i++){
printf("%d , %d\n",openList[i][0],openList[i][1]);
cX=g1X; cY=g2Y;
}
}
return 0;
}
Questions:
I know I didn't yet add the current cell in the open list. I should add it right?
Both the openlist and the closed list should be a Hashmap?
How do you think I should keep a connection with the parent of selected cell?

Answer 1

The open list needs to be a priority queue. That is a queue that allows new entrants to "push in" according to their priority or importance, but we always shrink by taking from the front. Now you can do this naively with an array and sorting on each insertion, but that will be slow. A linked list won't help much (linked lists have very bad cache performance). Really you need a specially-written priority queue which keeps as much as possible together in memory.