Update organising_code.md to cover optional arguments (#466)

* Update organising_code.md to cover optional arguments

* fixed typo in organising_code.md

* Update variables.md

Show declaration of multiple variables in one line, gives an example of reading more than one variable in one statement.

* Updated code in variables.md with a comment

* Updated program float in variables.md

Added a print statement so that the user sees that a double precision is stored to more digits than a single precision one.

* removed unused variable n in vector_norm

* declared norm in vector_norm

* Update source/learn/quickstart/variables.md

Co-authored-by: Federico Perini <federico.perini@gmail.com>

---------

Co-authored-by: Henil Panchal <henilp105@gmail.com>
Co-authored-by: Federico Perini <federico.perini@gmail.com>

source/learn/quickstart/organising_code.md

@@ -171,3 +171,37 @@ use my_mod, only: printMat=>print_matrix
 ```
 
 > Each module should be written in a separate `.f90` source file. Modules need to be compiled prior to any program units that `use` them.
+
+## Optional arguments
+
+An advantage of placing subroutines and functions in modules is that they can have ```optional``` arguments. In a procedure with an argument declared optional, the ```present``` function is used to test if the argument was set in the caller. Optional arguments that are not present may not be accessed within the procedure. Here is a generalization of the ```vector_norm``` function that can use powers other than 2 to compute the Lp norm.
+
+```
+module norm_mod
+  implicit none
+  contains
+  function vector_norm(vec,p) result(norm)
+    real, intent(in) :: vec(:)
+    integer, intent(in), optional :: p ! power
+    real :: norm
+    if (present(p)) then ! compute Lp norm
+      norm = sum(abs(vec)**p) ** (1.0/p)
+    else ! compute L2 norm
+      norm = sqrt(sum(vec**2))
+    end if
+  end function vector_norm
+end module norm_mod
+
+program run_fcn
+  use norm_mod
+  implicit none
+
+  real :: v(9)
+
+  v(:) = 9
+
+  print *, 'Vector norm = ', vector_norm(v), vector_norm(v,2)
+  print *, 'L1 norm = ', vector_norm(v,1)
+
+end program run_fcn
+```

source/learn/quickstart/variables.md

@@ -23,7 +23,7 @@ the variable type and any other variable attributes.
 The syntax for declaring variables is:
 
 ```
-<variable_type> :: <variable_name>
+<variable_type> :: <variable_name>, <variable_name>, ...
 ```
 
 where `<variable_type>` is one of the built-in variable types listed above and
@@ -39,7 +39,7 @@ program variables
   implicit none
 
   integer :: amount
-  real :: pi
+  real :: pi, e ! two `real` variables declared
   complex :: frequency
   character :: initial
   logical :: isOkay
@@ -104,16 +104,16 @@ In a similar way, we can read values from the command window
 using the `read` statement:
 
 ```{play-code-block} fortran
-program read_value
+program read_values
   implicit none
-  integer :: age
+  real :: x, y
 
-  print *, 'Please enter your age: '
-  read(*,*) age
+  print *, 'Please enter two numbers. '
+  read(*,*) x, y
 
-  print *, 'In ten years, your age will be ', age + 10
+  print *, 'The sum and product of the numbers are ', x+y, x*y
 
-end program read_value
+end program read_values
 ```
 
 This input source is commonly referred to as `standard input` or `stdin`.
@@ -138,11 +138,7 @@ The usual set of arithmetic operators are available, listed in order of preceden
 program arithmetic
   implicit none
 
-  real :: pi
-  real :: radius
-  real :: height
-  real :: area
-  real :: volume
+  real :: pi, radius, height, area, volume
 
   pi = 3.1415927
 
@@ -180,7 +176,7 @@ program float
 
   float32 = 1.0_sp  ! Explicit suffix for literal constants
   float64 = 1.0_dp
-
+  print*, float32, float64
 end program float
 ```