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--- script:walkthrough:statements_and_compounds [2017-03-20 20:25] – skyjake
+++ script:walkthrough:statements_and_compounds [2019-11-20 17:30] (current) – [Assignment] skyjake
@@ Line 1: / Line 1: @@
+<- 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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