Issue with simple while loop in C terminating early - c

Question

I have the following code:
//get distances
int a = -1;
int b = -1;
while ((a != 0) && (b != 0)) {
scanf("%d %d", &a, &b);
printf("The shortest path from %d to %d is %.1lf meters.\n", a, b, D[a][b]);
}
The loop seems to terminate after one input, despite the fact that a and b are not inputed as 0.
ie:
0 2
The shortest path from 0 to 2 is 237.7 meters.
Not really sure why it's doing that so any help would be appreciated.
Then it terminates
Full code in case needed
#include <stdio.h>
#include <stdlib.h>
#define INF 41e6
double** new2Ddouble(int n, int m);
void free2D(double **a);
int main() {
//get # of nodes
int size = 0;
scanf("%d", &size);
//create matrix
double **D = new2Ddouble(size, size);
//fill with inf for D[i][j]
int i;
int j;
for(i=0; i<size; i++) {
for(j=0; j<size;j++){
D[i][j] = INF;
}
}
//fill D[i][i] with INF
for(i=0;i<size;i++) D[i][i] = INF;
int exit = 0;
int I;
int J;
double d;
while(exit != 1) {
//populate values in matrix
scanf("%d %d %lf", &I, &J, &d);
if(I == 0 && J == 0 && d == 0){
//we can exit
exit = 1;
} else {
D[I][J] = d;
}
}
//calculate distances
/* Floyd-Warshall Algorithm */
int k;
for (k=0; k<size; ++k)
for (i=0; i<size; ++i)
for (j=0; j<size; ++j)
if (D[i][k]+D[k][j] < D[i][j])
D[i][j] = D[i][k]+D[k][j];
exit = 0;
//get distances
int a = -1;
int b = -1;
while ((a != 0) && (b != 0)) {
scanf("%d %d", &a, &b);
printf("The shortest path from %d to %d is %.1lf meters.\n", a, b, D[a][b]);
}
return 0;
}
double** new2Ddouble(int n, int m) {
int i;
double **ret = (double**) malloc(n*sizeof(double*));
double *a = (double*) malloc(n*m*sizeof(double));
for (i=0; i<n; ++i) ret[i] = &a[i*m];
return ret;
}
void free2D(double **a) { free(a[0]); free(a); }

Answer 1

scanf("%d %d", &a, &b);
In this line you are scanning values for a and b 0 and 2 respectively.
So once a new value is scanned to these variables then your while condition fails because as you have showed a is 0 and the second condition is never checked because 0 && (0|1) = 0
Once a=0 the condition fails and exits the loop.

Answer 2

while ((a != 0) && (b != 0))
This loop will terminate when a or b is zero and will continue when a and b are not zero. You enter a as 0 and as the condition a!=0 fails,your loop terminates.

Answer 3

there are a couple of problems with this line:
scanf("%d %d", &a, &b);
Note: scanf does not automatically consume white space,
so on the second iteration
the next input char is a newline.
so scanf fails
Note: all I/O statements
should have the returned value checked
to assure success of the operation
to fix those items, write it like so:
if( 2 != scanf(" %d %d", &a, &b) )
{
perror( "scanf failed" );
exit( EXIT_FAILURE );
}
the leading ' ' in the format string causes leading
white space, like the newline, to be consumed
the 'if' checks the returned value from
scanf to assure that all the input conversions were successful