Update example and add CMakeLists.txt in constant folder.

Author: MilanSkocic

example/CMakeLists.txt

@@ -9,6 +9,7 @@ endmacro(ADD_EXAMPLE)
 add_subdirectory(array)
 add_subdirectory(ascii)
 add_subdirectory(bitsets)
+add_subdirectory(constants)
 add_subdirectory(error)
 add_subdirectory(hashmaps)
 add_subdirectory(hash_procedures)

example/constants/CMakeLists.txt

@@ -0,0 +1 @@
+ADD_EXAMPLE(constants)

example/constants/example_constants.f90

@@ -1,13 +1,25 @@
 program example_constants
     use stdlib_constants, only: c, PI=>PI_dp
     use stdlib_codata, only: alpha=>ALPHA_PARTICLE_ELECTRON_MASS_RATIO
-    use stdlib_codata, only: ATOMIC_UNIT_OF_1ST_HYPERPOLARIZABILITY
+    use stdlib_codata_type, only : to_real
+    use stdlib_kinds, only: dp, sp
 
-    call alpha%print()
+    ! Use most common physical constants defined as deouble precision reals
+    print *, "speed of light in vacuum= ", c
+
+    ! Use of mathematical constants such as PI
+    print *, "PI as double precision real= ", PI
+    
+    ! Use codata_constant type for evaluating the value to the desired precision
+    print *, "Value of alpha... evaluated to double precision=", alpha%to_real(1.0_dp)
+    print *, "Uncertainty of alpha... evaluated to double precision=", alpha%to_real(1.0_sp, .true.)
+    print *, "Value of alpha... evaluated to single precision=", alpha%to_real(1.0_sp)
 
-    print *, ATOMIC_UNIT_OF_1ST_HYPERPOLARIZABILITY%eval_value(1.0)
+    ! Convert a codata constant to a real
+    print *, "Value of the alpha... evaluated to double precision=", to_real(alpha, 1.0_dp)
 
-    print *, c
-    print *, PI
+
+    ! Print out codata constant attributes: name, value, uncertainty and unit
+    call alpha%print()
 
 end program example_constants