Doomsday Manual

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<- Operator expressions ^ ^ Exceptions ->

====== Statements and compounds ======

===== Structure =====

Statements normally end at a newline character, unless there is a backslash at the end of a line.

  print 'Statement'
  
  print \
    'Statement'

Semicolons can be used on one line to separate  statements.

  print 'Statement 1.'; print 'Statement 2.'

Compounds are blocks of statements that end with the ''end'' keyword.

  if True
      print 'Compound inside "if"'
  end

As a special case, single-statement compounds can be created with a colon '':''.

  if True: print 'No "end" needed here'

The keyword ''pass'' can be used in place of an empty compound.

  if True: print "'Tis true."; else: pass

Each compound must be closed with its own ''end'' keyword.

  if True
      if True
          if True
              if True
              end
          end
      end
  end

===== Assignment =====

The assign statement is used to create new variables in the local namespace.

  i = 10

Variables are untyped, so any other kind of value can later be assigned to the same variable.  
  
  i = "abc"

Arrays are modifiable, so one can change the value of an individual element.

  $ array = [1, 2, 3, 4]  
    
  $ array[1] = 100
  
  $ print array
  [ 1, 100, 3, 4 ]

Nested arrays can also be assigned into.

  $ array[2] = ['a', 'b', 'c']
  
  $ array[2][1] = 'Bee'
  
  $ print array
  [ 1, 100, [ a, Bee, c ], 4 ]

One can assign to dictionaries to add or modify the values of keys.
  
  $ d = {1:2, 'Three':4}
  
  $ d[5] = 'Five'
  
  $ print d
  { 1: 2, 5: Five, Three: 4 }
  
When assigning, the right-hand value is copied to the new variable.

  array = [1, 2, 3]
  origArray = array
  array[1] = 10
  # origArray remains unmodified

==== Weak assignment ====

Weak assignment ''?='' only creates new variables. It never modifies existing ones.

  $ print "z exists?", 'z' in locals()
  z exists? False
  
  $ z ?= 3
  
  $ print "z exists?", 'z' in locals()
  z exists? True
  
  $ print 'z =', z
  z = 3
  
  $ z ?= 10    # this one does nothing
  
  $ print 'z =', z
  z = 3

==== const ====

You can use the ''const'' keyword when assigning to create a read-only variable.

  $ const z = 10
  
  $ print 'z has been consted to', z
  z has been consted to 10

Attempting to assign a new value to a const variable causes an exception.
  
  try
      z = 10
      print 'Allowed to reassign the same value...'
      z = 5
  catch ReadOnlyError
      print 'Have to delete z before its value can change.'
  end

Output:

  Allowed to reassign the same value...
  Have to delete z before its value can change.

==== del ====

Variables and functions can be deleted with the ''del'' keyword.

  del z

===== Flow control =====

==== if, elsif, else ====

''if'', ''elsif'', and ''else'' are used for conditional branching.

  if False
      print 'a)'
  elsif False
      print 'b)'
  elsif True > False
      print 'c)'
  else
      print 'd)'
  end

==== while ====

  i = 5
  while i > 0
      print 'Still looping because %i > 0' % i
      i -= 1
  end

Output:

  Still looping because 5 > 0
  Still looping because 4 > 0
  Still looping because 3 > 0
  Still looping because 2 > 0
  Still looping because 1 > 0

==== break ====

The ''break'' statement is used to break out of loops.

  while True
      print "I won't stay here."
      break
      print "Jumped over this."
  end

Optionally, a number argument can be given to ''break'' to break out of multiple nested loops. Below, the break escapes from two loops.

  while True
      while True
          print "Uh-oh, now I'm in trouble!"
          break 2
      end
  end
  print "Whew"

==== for ====

''for'' loops iterate over a sequence of array elements, dictionary key-value pairs, or string characters. The loop variable is not deleted afterwards, so it can be checked if the loop was terminated early with ''break''.

  for i in ['planes', 'trains', 'automobiles']
      print i
  end
  print 'Value of i after the "for" loop is', i

Output:

  planes
  trains
  automobiles
  Value of i after the "for" loop is automobiles

When iterating a dictionary, the loop variable is an array containing the key and the value.

  a = {'a': 1, 'b': 2}
  for i in a
      print i
  end

Output:

  [ a, 1 ]
  [ b, 2 ]

==== continue ====

The ''continue'' statement interrupts the loop and returns flow back to the beginning, with the next element.

  for i in ['planes', 'trains', 'automobiles']
      if i == 'trains': continue
      print i, 'are cool!'
  end

Output:

  planes are cool!
  automobiles are cool!
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