fix(example): eliminate real-equality warning

Replace example that relied on a check for equality of real values
with one that relies on an inequality.

Author: rouson

example/simple_assertions.f90

@@ -3,30 +3,34 @@ program assertion_examples
   !! of two kinds of constraints:
   !! 1. Preconditions: requirements for correct execution at the start of a procedure and
   !! 2. Postconditions: requirements for correct execution at the end of a procedure.
+  use assert_m, only : assert
+  use intrinsic_array_m, only : intrinsic_array_t
   implicit none
 
-  print *, reciprocal(2.)
+  print *, "roots: ", roots(a=1.,b=0.,c=-4.)
 
 contains
 
-  pure real function reciprocal(x) result(reciprocal_of_x)
-    !! Erroneous calculation of the reciprocal of the function's argument
-    use assert_m, only : assert
-    real, intent(in) :: x
+  pure function roots(a,b,c) result(zeros)
+    !! Calculate the roots of a quadratic polynomial
+    real, intent(in) :: a, b, c
+    real zeros(2)
 
-    call assert(assertion = x /= 0., description = "reciprocal: x /= 0", diagnostic_data = x) ! Precondition passes
+    associate(discriminant => b**2 - 4*a*c)
+      call assert(assertion = (discriminant >= 0.), description = "roots: nonnegative discriminant", diagnostic_data = discriminant)
 
-    reciprocal_of_x = 0. ! incorrect value for the reciprocal of x
+      associate(radical => sqrt(discriminant))
+        zeros = [-b + radical, -b - radical]/(2*a)
 
-    block
-      real, parameter :: tolerance = 1.E-06
-
-      associate(error => x*reciprocal_of_x - 1.) 
-
-        call assert(abs(error) < tolerance, "reciprocal: abs(error) < tolerance", error) ! Postcondition fails
+        block
+          real, parameter :: tolerance = 1.E-06
 
+          associate(errors => a*zeros**2 + b*zeros + c)
+            call assert(maxval(abs(errors)) < tolerance, "roots: |max(error)| > tolerance", intrinsic_array_t([errors]))
+          end associate
+        end block
       end associate
-    end block
+    end associate
 
   end function