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I am trying to implement the A* algorithm in C++ (http://en.wikipedia.org/wiki/A*_search_algorithm).

The pseudocode i am trying to implement is:

```
function A*(start,goal)
closedset := the empty set // The set of nodes already evaluated.
openset := set containing the initial node // The set of tentative nodes to be evaluated.
g_score[start] := 0 // Distance from start along optimal path.
h_score[start] := heuristic_estimate_of_distance(start, goal)
f_score[start] := h_score[start] // Estimated total distance from start to goal through y.
while openset is not empty
x := the node in openset having the lowest f_score[] value
if x = goal
return reconstruct_path(came_from[goal])
remove x from openset
add x to closedset
foreach y in neighbor_nodes(x)
if y in closedset
continue
tentative_g_score := g_score[x] + dist_between(x,y)
[U] if y not in openset[/U]
add y to openset
tentative_is_better := true
elseif tentative_g_score < g_score[y]
tentative_is_better := true
else
tentative_is_better := false
if tentative_is_better = true
[U]came_from[y] := x[/U]
[U]g_score[y] := tentative_g_score[/U]
[U]h_score[y] := heuristic_estimate_of_distance(y, goal)[/U]
[U] f_score[y] := g_score[y] + h_score[y][/U]
return failure
function reconstruct_path(current_node)
if came_from[current_node] is set
p = reconstruct_path(came_from[current_node])
return (p + current_node)
else
return current_node
```

I am using for the openset a priority queue (of a custom data type, which contains x,y coordinates as well as g,h,f scores) in order to pop the element with the lower f score. The problem i am facing is in the underlined points, firstly how can I check if an element is included into the openset (in priority queue you can access only first element) and how can I modify that element when it is necessary ? Do I have to use a different structure for the openset ?

Thanks in advance !