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Valued Contributor III
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Shortest Path Between Two Points

Hi folks, 

 

Please consider the following code: 

# include <limits.h># include <stdio.h># include <stdlib.h> # define INFINITY INT_MAX# define MAXNODES 12# define MEMBER 1# define NONMEMBER 0 void shortpath(w, s, t, pd, precede) int w; int s, t, *pd, precede; { int distance, perm; int current, i, k, dc; int smalldist, newdist; /* initialization */ for (i = 0; i < MAXNODES; ++i){ perm = NONMEMBER; distance = INFINITY; } perm = MEMBER; distance = 0; current = s; while (current != t){ smalldist = INFINITY; dc = distance; for (i = 0; i < MAXNODES; i++) if (perm == NONMEMBER){ newdist = dc + w; if(newdist < distance){ /* distance from a to i through current is less than distance */ distance = newdist; precede = current; } /* short distance assignment */ if (distance < smalldist) { smalldist = distance; k = i; } } current = k; perm = MEMBER; } *pd = distance; } int main() { int w, i, j; int pd, s, t; int precede; for(i=0; i<MAXNODES; i++){ for(j=0; j<MAXNODES; j++){ w = INFINITY; } } for(i = 0; i < MAXNODES; i++){ for(j=0; j<MAXNODES; j++){ if(i == j) w = 0; } } w = 7; w = 9; w = 14; w = 10; w = 15; w = 11; w = 2; w = 6; w = 9; for(i = 0; i < MAXNODES; i++){ for(j=0; j<MAXNODES; j++){ if(i > j) w = w; } } /* checking if adjacency matrix is ok */ for(i = 0; i < 6; i++){ for(j=0; j<6; j++){ if(w > 1000) printf("M\t"); else printf("%d\t", w); } printf("\n"); } printf("Enter s-->"); scanf("%d", &s); while(s != -1){ printf("Enter t-->"); scanf("%d", &t); shortpath(w, s, t, &pd, precede); printf("Shortest distance between %d and %d --> %d\n", s, t, pd); fflush(stdin); printf("Enter s-->"); scanf("%d", &s); } system("PAUSE"); return(EXIT_SUCCESS); }  

 

It is expected for this code process the shortest distance between node "s" and node "t", given an adjaceny matrix "w" (containing weights between the nodes). Non-adjacency nodes are linked by an INFINITY weight. The function used return the distance in the "pd" variable. 

 

In a first test, only considering imediate adjacent nodes, this code is only working for the expected distances between (1,0), (3,4) and (5,2). The remaining adjacent distances, e.g., (0,2), (1,2), (3,1), etc, are giving an infinite distance. 

 

Two questions: 

1) Is it ok posting generalized C questions here? 

2) If 'yes' to the previous answer, what is wrong and what is the fix? 

 

Regards 

Jaraqui
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2 Replies
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Valued Contributor III
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1) Ask anything you like, no guarantee it will get answered. 

2) If you haven't fixed this already, I suggest you replace: 

this: 

newdist = dc + w[current]

 

with: 

if(w[current] != INFINITY) 

newdist = dc + w[current][i]; 

else 

newdist = INFINITY; 

 

I think you end up adding a positive number to INFINITY which gives you a large negative that incorrectly satisfies your check for "less than".
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Valued Contributor III
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Hi ghogerheiden, 

 

 

I fixed the code, but I don´t remember exactly what was the fix. 

But thank you very much for your cooperation! 

 

best regards 

Jaraqui
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