Control Structures - First Style¶
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