Test prints on the same variable seem to be giving different results

Question

This is a small bit of code being used for Dijkstra's algorithm. I'm trying to create an array (first_edge) that will tell me the starting index of each "source" node in E which is an array of structs called "Edge" that hold a source node's id#, a destination node's id#, and the distance between the two.

This is just to make it faster to find all of the edges leading out of a given source node.

The test statements I have built into the for loop print out exactly what I would expect them too, based on the input, but the second loop, (for(j = 0; j < 15; j++ ){ code }) prints that all of my first 15 values are just 0.

I can't imagine anything that would cause my code to seemingly give me two different answers for the same question, but that's what it seems to be doing.

int* first_edge = new int[23947350];

first_edge[0] = 0;

for(j = 1; j < 58333344; j++) {
  if(E[j].src > E[j-1].src) {
    if(j < 15) {
      //this line prints what i expect it too, which is good
      cout << "First edge updated  " << j << "   " << E[j].src << endl;
    }
    first_edge[E[j].src] = j;
    if(j < 15) {
       cout << "first edge now " << first_edge[E[j].src] << endl;
       cout << "Oh and the index is " << E[j].src << endl;
    } // end if
  } // end if
} // end for

for(j = 0; j < 15; j++) {
// earlier prints verify the correct contents of first_edge, surely this will work!
  cout << "    " << first_edge[j] << "  " << j << endl;
}

Some output. I took the liberty of making it less wordy and just giving the values i'm given.

((j = 3; E[j].src = 1; first_edge[E[j].src] = 3))  
((j = 5; E[j].src = 2; first_edge[E[j].src] = 5)) 
((j = 8; E[j].src = 3; first_edge[E[j].src] = 8))

Since there was some question as to the validity of my concern, I hardcoded first_edge[1] into the earlier loops, and it proved to be 3. In the later for loop, all values from 0 to 14 of first_edge printed as 0.

Answer 1

Let's say j = 7.

In the first loop you then print 7 followed by E[7].src.

Then, after updating first_edge[E[7].src], that output is followed by first_edge[E[7].src] and E[7].src.

In the last loop you print first_edge[7] followed by 7.

Is there any reason that you expect all these to print the same?

I can't see it.

Answer 2

Without knowing how the Edge structure is related to j, it is impossible to correlate E[j].src in relation to j. Thus E[j].src may not be equal to j.

Therefore, first_edge[E[0].src] to first_edge[E[14].src] may not necessarily be the first 15 of first_edge, first_edge[0] to first_edge[14].