help me implement Dijkstra's Algorithm

The pseudo code of dijkstra's algorithm is

Code:

     function Dijkstra(Graph, source):
 2      for each vertex v in Graph:           // Initializations
 3          dist[v] := infinity               // Unknown distance function from source to v
 4          previous[v] := undefined          // Previous node in optimal path from source
 5      dist[source] := 0                     // Distance from source to source
 6      Q := the set of all nodes in Graph
        // All nodes in the graph are unoptimized - thus are in Q
 7      while Q is not empty:                 // The main loop
 8          u := vertex in Q with smallest dist[]
 9          if dist[u] = infinity:
10              break                         // all remaining vertices are inaccessible from source
11          remove u from Q
12          for each neighbor v of u:         // where v has not yet been removed from Q.
13              alt := dist[u] + dist_between(u, v)
14              if alt < dist[v]:             // Relax (u,v,a)
15                  dist[v] := alt
16                  previous[v] := u
17      return dist[]

It's from wikipedia.

I have several clueless area here.

Firstly i like to know how to implement line numbe 4

Code:

 previous[v] := undefined          // Previous node in optimal path from source

what does it mean?

Please help me.

Pick a number that you'll be able to recognise as "undefined" later on.

Originally Posted by Salem

Pick a number that you'll be able to recognise as "undefined" later on.

I wanted to know what type of data-structure previous is?

Originally Posted by iamnew

I wanted to know what type of data-structure previous is?

Hint: What is the most common sentinal value, and why?

okay i solved it.