Ruby Loops - while, for, until

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 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 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:

Syntax:

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 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 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 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'sexpression 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 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 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 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 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 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 ............................