================== Control Structures ================== In this chapter we are going to learn about the control structures provided by the Ring programming language. .. index:: pair: Control Structures; Branching Branching ========= * If Statement Syntax: .. code-block:: none if Expression Block of statements but Expression Block of statements else Block of statements ok Example: .. code-block:: none see " Main Menu --------- (1) Say Hello (2) About (3) Exit " give nOption if nOption = 1 see "Enter your name : " give name see "Hello " + name + nl but nOption = 2 see "Sample : using if statement" + nl but nOption = 3 bye else see "bad option..." + nl ok * Switch Statement Syntax: .. code-block:: none switch Expression on Expression Block of statements other Block of statements off Example: .. code-block:: none See " Main Menu --------- (1) Say Hello (2) About (3) Exit " Give nOption Switch nOption On 1 See "Enter your name : " Give name See "Hello " + name + nl On 2 See "Sample : using switch statement" + nl On 3 Bye Other See "bad option..." + nl Off .. index:: pair: Control Structures; Looping Looping ======= * While Loop Syntax: .. code-block:: none while Expression Block of statements end Example: .. code-block:: none While True See " Main Menu --------- (1) Say Hello (2) About (3) Exit " Give nOption Switch nOption On 1 See "Enter your name : " Give name See "Hello " + name + nl On 2 See "Sample : using while loop" + nl On 3 Bye Other See "bad option..." + nl Off End * For Loop Syntax: .. code-block:: none for identifier=expression to expression [step expression] Block of statements next Example: .. code-block:: none # print numbers from 1 to 10 for x = 1 to 10 see x + nl next Example: .. code-block:: none # Dynamic loop See "Start : " give nStart See "End : " give nEnd See "Step : " give nStep For x = nStart to nEnd Step nStep see x + nl Next Example: .. code-block:: none # print even numbers from 0 to 10 for x = 0 to 10 step 2 see x + nl next Example: .. code-block:: none # print even numbers from 10 to 0 for x = 10 to 0 step -2 see x + nl next * For in Loop Syntax: .. code-block:: none for identifier in List/String [step expression] Block of statements next Example: .. code-block:: none aList = 1:10 # create list contains numbers from 1 to 10 for x in aList see x + nl next # print numbers from 1 to 10 .. index:: pair: Control Structures; Step Option Using The Step option with For in ================================= We can use the Step option with For in to skip number of items in each iteration Example: .. code-block:: none aList = 1:10 # create list contains numbers from 1 to 10 # print odd items inside the list for x in aList step 2 see x + nl next .. index:: pair: Control Structures; for in to modify lists Using For in to modify lists ============================= When we use (For in) we get items by reference. This means that we can read/edit items inside the loop. Example: .. code-block:: none aList = 1:5 # create list contains numbers from 1 to 5 # replace list numbers with strings for x in aList switch x on 1 x = "one" on 2 x = "two" on 3 x = "three" on 4 x = "four" on 5 x = "five" off next see aList # print the list items .. index:: pair: Control Structures; Do Again Loop Do Again Loop ============= Syntax: .. code-block:: none do Block of statements again expression Example: .. code-block:: none x = 1 do see x + nl x++ again x <= 10 .. index:: pair: Control Structures; Exit Exit Command ============ Used to go outside one or more of loops. Syntax: .. code-block:: none exit [expression] # inside loop Example: .. code-block:: none for x = 1 to 10 see x + nl if x = 5 exit ok next Exit from two loops =================== The next example presents how to use the exit command to exit from two loops in one jump. Example: .. code-block:: none for x = 1 to 10 for y = 1 to 10 see "x=" + x + " y=" + y + nl if x = 3 and y = 5 exit 2 # exit from 2 loops ok next next .. index:: pair: Control Structures; Loop * Loop Command Used to jump to the next iteration in the loop. Syntax: .. code-block:: none loop [expression] # inside loop Example: .. code-block:: none for x = 1 to 10 if x = 3 see "Number Three" + nl loop ok see x + nl next Exit/Loop inside sub functions ============================== While we are inside a loop, we can call a function then use the exit and/or loop command inside that function and the command will work on the outer loop. Example: .. code-block:: none # print numbers from 1 to 10 except number 5. for x = 1 to 10 ignore(x,5) see x + nl next func ignore x,y if x = y loop ok .. index:: pair: Control Structures; Short-circuit evaluation Short-circuit evaluation ======================== The logical operators and/or follow the `short-circuit evaluation `_. If the first argument of the AND operator is zero, then there is no need to evaluate the second argument and the result will be zero. If the first argument of the OR operator is one, then there is no need to evaluate the second argument and the result will be one. Example: .. code-block:: none /* output ** nice ** nice ** great */ x = 0 y = 10 if (x = 0 and nice()) and (y = 10 and nice()) see "great" + nl ok func nice see "nice" + nl return 1 Example: .. code-block:: none # No output x = 0 y = 10 if (x = 1 and nice()) and (y = 10 and nice()) see "great" + nl ok func nice see "nice" + nl return 1 Example: .. code-block:: none /* output ** nice ** great */ x = 0 y = 10 if (x = 0 and nice()) or (y = 10 and nice()) see "great" + nl ok func nice see "nice" + nl return 1 Comments about evaluation ========================= * True, False, nl & NULL are variables defined by the language * True = 1 * False = 0 * nl = new line * NULL = empty string = "" * Everything evaluates to true except 0 (False). Example: .. code-block:: none # output = message from the if statement if 5 # 5 evaluates to true because it's not zero (0). see "message from the if statement" + nl ok