Control Structures

In this chapter we are going to learn about the control structures provided by the Ring programming language.

Branching

• If Statement

Syntax:

if Expression
        Block of statements
but Expression
        Block of statements
else
        Block of statements
ok

Example:

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:

switch Expression
on Expression
        Block of statements
other
        Block of statements
off

Example:

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

Looping

• While Loop

Syntax:

while Expression
        Block of statements
end

Example:

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:

for identifier=expression to expression [step expression]
        Block of statements
next

Example:

# print numbers from 1 to 10
for x = 1 to 10  see x + nl  next

Example:

# 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:

# print even numbers from 0 to 10
for x = 0 to 10 step 2
        see x + nl
next

Example:

# print even numbers from 10 to 0
for x = 10 to 0 step -2
        see x + nl
next

• For in Loop

Syntax:

for identifier in List/String  [step expression]
        Block of statements
next

Example:

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

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:

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

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:

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

Do Again Loop

Syntax:

do
        Block of statements
again expression

Example:

x = 1
do
        see x + nl
        x++
again x <= 10

Exit Command

Used to go outside one or more of loops.

Syntax:

exit [expression]       # inside loop

Example:

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:

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

• Loop Command

Used to jump to the next iteration in the loop.

Syntax:

loop [expression]       # inside loop

Example:

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:

# 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

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:

/* 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:

# 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:

/* 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:

# 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