Creating a random map into an array

programming flash
1

I’m trying to create a program that creates a random map for a side scrolling cave.
So I need to store the roof and the grounds y coordinate into a big array, like 10 000 pieces of wall.

The only way I’ve gotten it to accept some data into the array is like this but I want something cleaner and something that can go on forever.

How should I format the first 4 lines of code so that the array accepts commands like this and expands for the for loop as needed?
level*[j] = something;

var level = [ 
["0","0","0","0","0","0"],//this will be the roof 
["0","0","0","0","0","0"]//this will be the ground 
]; 
//need something better for the code above

for (i=0;i<10;i++){ 
    for (j=0;j<2;j++){ 

        level[j]* = 1; 
    } 
     
} 

for (i=0;i<10;i++){ 
    for (j=0;j<2;j++){ 

        trace(j + "j i" + i +"-" + level[j]*); 
    } 
     
}