C/math.h/isnan

int isnan(fpval): Returns nonzero if fpval is not a number

<source lang="cpp">

1. include <stdio.h>
2. include <math.h>

int main(void) {

 printf("isnan(-1.0) %d\n", isnan(-1.0));
 printf("isnan(1.0) %d\n", isnan(1.0));
 printf("isnan(0.0) %d\n", isnan(0.0));
 return 0;

}

/* isnan(-1.0) 0 isnan(1.0) 0 isnan(0.0) 0

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      </source>