7th class: if, else

Reference site

Chart type ruby-III (if, case, Array.each by leap year)

if-else statement

Conditional branching used in various languages ​​as well as Ruby Basically with Python

if expression[then] 
formula...
[elsif expression[then]
formula... ]
...
[else
formula... ]
end

Works with or without movements in []

Just note that else if is written as elsif

Writing example:

if a=20
 b true
elsif c=30
 b true
else
 b false
end

Theme: Is it a leap year?

A program that displays whether the number entered is a leap year

・ Years that are divisible by 4 are leap years ・ However, a year divisible by 100 is not a leap year ・ In addition, a year divisible by 400 is a leap year.

First of all, whether it is divisible by 4

[2004, 1999].each do |year|
  p year
  if year % 4 == 0
    p true #True if divisible by 4(leap year)
  else
    p false #False if not divisible by 4(Not a leap year)
  end
end

The first line puts 2004 and 1999 in each year

Next, put in the process of whether it is divisible by 100

[1900, 2004, 1999].each do |year|
  p year
  if year % 100 == 0
    p false #False if divisible by 100
    next
  end
  if year % 4 == 0
    p true
  else
    p false
  end
end

When next is executed, the process after next is not executed and it shifts to the next iteration. It is a little different from break which interrupts the iteration itself.

Finally, put in the process of whether it is divisible by 400

def leap?(year)
  if year % 400 == 0
    p true #True if divisible by 400
  elsif year % 100 == 0
    p false
  elsif year % 4 == 0
    p true
  else
    p false
  end
end
[2000, 1900, 2004, 1999].each do |year|
  p year
  leap?(year)
end

By the way, I implemented it as a function leap?

EX

If you write neatly using case instead of if, it will be like this

def leap?(year)
  case
  when year % 400 ==0 ; p true
  when year % 100 ==0 ; p false
  when year % 4 ==0 ;   p true
  else ;                p false
  end
end

[2000, 1900, 2004, 1999].each do |year|
  p year
  leap?(year)
end

note

It is difficult to think about conditional branching in advance and implement it, but may it be better to implement it step by step?

• source ~/grad_members_20f/members/batamon-427/task7.org