Digital map in A* search algorithm in Arduino
first off, you need 2D array to store map segments.
then you have to update them according to the real map.
you have to implement a few measures:


a series of sensors to qualify adjacent segments. e.g. walls/road. map edges are considered walls.
an array (list) of current adjacent segments. (just a 4 member array, containing the up/down/left/right location. this array stores the index of the mentioned segments in the main 2D array.)
a beacon or something like that at the goal position. you can evaluate the segments based on the distance from beacon. (the simplest method, cartesian distance). 
run a iteration of A* alg to find out which adjacent segment is suitable for next move.