Transcript Ruby - Electrical Engineering & Computer Science at CSM

and other languages…
 Selection
Statements
 Iterative Statements

Unconditional Branching
 Not
covered
• Guarded Commands
if expr
code
end
# newline after expr
# false or nil, don’t
execute, else do
# no () around expr
# end required even if only
one statement in code
if expr then code end
if expr
code
else
code
end
if expr1
code
elsif expr2
code2
…
else
code
end
 Return
value is last expression executed
OR nil
x=5
# topic: return values
name = if x==1 then "Cyndi"
else "Nancy"
end
puts name
if expr then code end
 equivalent to:
code if expr
 Best
practice: use when condition is
trivial or normally true
Perl also has this syntax
unless expr
code
end
code unless expr
tax = case income
when 0..7550
income * 0.1
when 7550..30650
income * 0.15
when 3065..50000
income * 0.25
else
income * 0.9
end
also other forms of case, not covered
Compare to switch. Consider language readabilty.
while expr do
code
end
until expr do
code
end
code while expr
code until expr
Pascal had repeat… until.
for var in collection do
code
end
# do is optional, can use newline
hash.each do |key, value|
puts “#{key} => #{value}”
end
 Integer.times
 Enumerable.each
 Enumerable.Map
 Integer.upto
 make
use of yield (next slide)
 Examples
2.times { puts “OK” }
array.each {|x| puts x }
[5,10,15].map { |x| x*x*x }
factorial = 1
2.upto(20) { |x| factorial *= x}
 yield
temporarily returns control from
iterator to calling method
 QUICK
EXERCISE:
 Trace the code on the next two slides.
Turn in for class participation.
 Format flexible… draw arrows etc, just
show you understand
 yield
temporarily returns control from
iterator to calling method
def test
puts "You are in the method"
yield
puts "You are again back to the method"
yield
You are in the method
end
You are in the block
You are again back to the method
You are in the block
test {puts "You are in the block"}

method must be invoked with a block (otherwise, what
would you yield to?)
from: http://www.tutorialspoint.com/ruby/ruby_blocks.htm
def test
yield 5
puts "You are in the method test"
yield 100
end
test {|i| puts "You are in the block #{i}"}
You are in the block 5
You are in the method test
You are in the block 100
Java = pull (caller controls iteration), Ruby = push (iterator controls iteration)


Silly code example. Write code that will display the modulo
15 of all numbers from 100 to 90. Your yield block should
return two values (look up the syntax).
Example output:
100 modulo 15 is 10
99 modulo 15 is 9
98 modulo 15 is 8
97 modulo 15 is 7
96 modulo 15 is 6
95 modulo 15 is 5
94 modulo 15 is 4
93 modulo 15 is 3
92 modulo 15 is 2
91 modulo 15 is 1

Nothing to submit.
 What’s
really going on here? Language
design: importance of blocks
 Read:
http://www.artima.com/intv/closures.html







Transfers execution control to a specified place in the
program
Represented one of the most heated debates in 1960’s
and 1970’s
Well-known mechanism: goto statement
Major concern: Readability
Some languages do not support goto statement (e.g.,
Module-2, Java, Python, Ruby)
C# offers goto statement (can be used in switch
statements)
Loop exit statements are restricted and somewhat
camouflaged goto’s
 Language
Concepts
 Ruby
• selection/conditionals
• if/elsif
• return value
• conditional return
• iteration
• unconditional
branching
•
•
•
•
•
•
value
expression modifier
unless
while/until
for
each
yield