9. Condition and Loops in Ruby

Conditions:

Ruby offers conditional structures that are pretty common to modern languages. Here, we will explain all the conditional statements and modifiers available in Ruby.

Ruby if...else Statement:

Syntax:

if conditional [then]
   code...
[elsif conditional [then]
   code...]...
[else
   code...]
end
if expressions are used for conditional execution. The values false and nil are false, and everything else are true. Notice Ruby uses elsif, not else if nor elif.
Executes code if the conditional is true. If the conditional is not true, code specified in the else clause is executed.
An if expression's conditional is separated from code by the reserved word then, a newline, or a semicolon.

Example:

#!/usr/bin/ruby

x=1
if x > 2
   puts "x is greater than 2"
elsif x <= 2 and x!=0
   puts "x is 1"
else
   puts "I can't guess the number"
end
x is 1

Ruby if modifier:

Syntax:

code if condition
Executes code if the conditional is true.

Example:

#!/usr/bin/ruby

$debug=1
print "debug\n" if $debug
This will produce the following result:
debug

Ruby unless Statement:

Syntax:

unless conditional [then]
   code
[else
   code ]
end
Executes code if conditional is false. If the conditional is true, code specified in the else clause is executed.

Example:

#!/usr/bin/ruby

x=1
unless x>2
   puts "x is less than 2"
 else
  puts "x is greater than 2"
end
This will produce the following result:
x is less than 2

Ruby unless modifier:

Syntax:

code unless conditional
Executes code if conditional is false.

Example:

#!/usr/bin/ruby

$var =  1
print "1 -- Value is set\n" if $var
print "2 -- Value is set\n" unless $var

$var = false
print "3 -- Value is set\n" unless $var
This will produce the following result:
1 -- Value is set
3 -- Value is set

Ruby case Statement

Syntax:

case expression
[when expression [, expression ...] [then]
   code ]...
[else
   code ]
end
Compares the expression specified by case and that specified by when using the === operator and executes the code of the when clause that matches.
The expression specified by the when clause is evaluated as the left operand. If no when clauses match, case executes the code of the else clause.
A when statement's expression is separated from code by the reserved word then, a newline, or a semicolon.
Thus:
case expr0
when expr1, expr2
   stmt1
when expr3, expr4
   stmt2
else
   stmt3
end
is basically similar to the following:
_tmp = expr0
if expr1 === _tmp || expr2 === _tmp
   stmt1
elsif expr3 === _tmp || expr4 === _tmp
   stmt2
else
   stmt3
end

Example:

#!/usr/bin/ruby

$age =  5
case $age
when 0 .. 2
    puts "baby"
when 3 .. 6
    puts "little child"
when 7 .. 12
    puts "child"
when 13 .. 18
    puts "youth"
else
    puts "adult"
end
This will produce the following result:

little child


Loops:

Loops in Ruby are used to execute the same block of code a specified number of times. This chapter details all the loop statements supported by Ruby.

Ruby while Statement:

Syntax:

while conditional [do]
   code
end
Executes code while conditional is true. A while loop's conditional is separated from code by the reserved word do, a newline, backslash \, or a semicolon ;.

Example:

#!/usr/bin/ruby

$i = 0
$num = 5

while $i < $num  do
   puts("Inside the loop i = #$i" )
   $i +=1
end
This will produce the following result:
Inside the loop i = 0
Inside the loop i = 1
Inside the loop i = 2
Inside the loop i = 3
Inside the loop i = 4

Ruby while modifier:

Syntax:

code while condition

OR

begin 
  code 
end while conditional
Executes code while conditional is true.
If a while modifier follows a begin statement with no rescue or ensure clauses, code is executed once before conditional is evaluated.

Example:

#!/usr/bin/ruby

$i = 0
$num = 5
begin
   puts("Inside the loop i = #$i" )
   $i +=1
end while $i < $num
This will produce the following result:
Inside the loop i = 0
Inside the loop i = 1
Inside the loop i = 2
Inside the loop i = 3
Inside the loop i = 4

Ruby until Statement:

until conditional [do]
   code
end
Executes code while conditional is false. An until statement's conditional is separated from code by the reserved word do, a newline, or a semicolon.

Example:

#!/usr/bin/ruby

$i = 0
$num = 5

until $i > $num  do
   puts("Inside the loop i = #$i" )
   $i +=1;
end
This will produce the following result:
Inside the loop i = 0
Inside the loop i = 1
Inside the loop i = 2
Inside the loop i = 3
Inside the loop i = 4
Inside the loop i = 5

Ruby until modifier:

Syntax:

code until conditional

OR

begin
   code
end until conditional
Executes code while conditional is false.
If an until modifier follows a begin statement with no rescue or ensure clauses, code is executed once before conditional is evaluated.

Example:

#!/usr/bin/ruby

$i = 0
$num = 5
begin
   puts("Inside the loop i = #$i" )
   $i +=1;
end until $i > $num
This will produce the following result:
Inside the loop i = 0
Inside the loop i = 1
Inside the loop i = 2
Inside the loop i = 3
Inside the loop i = 4
Inside the loop i = 5

Ruby for Statement:

Syntax:

for variable [, variable ...] in expression [do]
   code
end
Executes code once for each element in expression.

Example:

#!/usr/bin/ruby

for i in 0..5
   puts "Value of local variable is #{i}"
end
Here, we have defined the range 0..5. The statement for i in 0..5 will allow i to take values in the range from 0 to 5 (including 5). This will produce the following result:
Value of local variable is 0
Value of local variable is 1
Value of local variable is 2
Value of local variable is 3
Value of local variable is 4
Value of local variable is 5
A for...in loop is almost exactly equivalent to:
(expression).each do |variable[, variable...]| code end
except that a for loop doesn't create a new scope for local variables. A for loop's expression is separated from code by the reserved word do, a newline, or a semicolon.

Example:

#!/usr/bin/ruby

(0..5).each do |i|
   puts "Value of local variable is #{i}"
end
This will produce the following result:
Value of local variable is 0
Value of local variable is 1
Value of local variable is 2
Value of local variable is 3
Value of local variable is 4
Value of local variable is 5

Ruby break Statement:

Syntax:

break
Terminates the most internal loop. Terminates a method with an associated block if called within the block (with the method returning nil).

Example:

#!/usr/bin/ruby

for i in 0..5
   if i > 2 then
      break
   end
   puts "Value of local variable is #{i}"
end
This will produce the following result:
Value of local variable is 0
Value of local variable is 1
Value of local variable is 2

Ruby next Statement:

Syntax:

next
Jumps to next iteration of the most internal loop. Terminates execution of a block if called within a block (with yield or call returning nil).

Example:

#!/usr/bin/ruby

for i in 0..5
   if i < 2 then
      next
   end
   puts "Value of local variable is #{i}"
end
This will produce the following result:
Value of local variable is 2
Value of local variable is 3
Value of local variable is 4
Value of local variable is 5

Ruby redo Statement:

Syntax:

redo
Restarts this iteration of the most internal loop, without checking loop condition. Restarts yield or call if called within a block.

Example:

#!/usr/bin/ruby

for i in 0..5
   if i < 2 then
      puts "Value of local variable is #{i}"
      redo
   end
end
This will produce the following result and will go in an infinite loop:
Value of local variable is 0
Value of local variable is 0
............................

Ruby retry Statement:

Syntax:

retry
If retry appears in rescue clause of begin expression, restart from the beginning of the 1begin body.
begin
   do_something # exception raised
rescue
   # handles error
   retry  # restart from beginning
end
If retry appears in the iterator, the block, or the body of the for expression, restarts the invocation of the iterator call. Arguments to the iterator is re-evaluated.
for i in 1..5
   retry if some_condition # restart from i == 1
end

Example:

#!/usr/bin/ruby

for i in 1..5
   retry if  i > 2
   puts "Value of local variable is #{i}"
end
This will produce the following result and will go in an infinite loop:
Value of local variable is 1
Value of local variable is 2
Value of local variable is 1
Value of local variable is 2
Value of local variable is 1
Value of local variable is 2
............................

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