add 4-xpr1.c

pset2/4-xpr1.c

@@ -0,0 +1,15 @@
+#include <math.h>
+#include <stdio.h>
+
+int main(void) {
+
+  // initialize variables
+  double a, b, c, d;
+  scanf("%lf %lf %lf %lf", &a, &b, &c, &d);
+
+  // trinary expression magic. same as above
+  (b * c - d < 1e-6 && b * c - d > -1e-6) ? printf("error")
+                                          : printf("%.1lf", a / (b * c - d));
+
+  return 0;
+}