Why is this an issue?

When the same code is duplicated in two or more separate branches of a conditional, it can make the code harder to understand, maintain, and can potentially introduce bugs if one instance of the code is changed but others are not.

Having two when clauses in a case statement or two branches in an if chain with the same implementation is at best duplicate code, and at worst a coding error.

if a >= 0 && a < 10
  doFirstThing()
  doTheThing()
elsif a >= 10 && a < 20
  doTheOtherThing()
elsif a >= 20 && a < 50
  doFirstThing()
  doTheThing()   # Noncompliant; duplicates first condition
else
  doTheRest()
end

case i
  when 1
    doFirstThing()
    doSomething()
  when 2
    doSomethingDifferent()
  when 3 # Noncompliant; duplicates case 1's implementation
    doFirstThing()
    doSomething()
  else
    doTheRest()
end

If the same logic is needed for both instances, then:

• in an if structure they should be combined

if (a >= 0 && a < 10) || (a >= 20 && a < 50)
  doFirstThing()
  doTheThing()
elsif a >= 10 && a < 20
  doTheOtherThing()
else
  doTheRest()
end

• for a case, the values should be put in the when expression list.

case i
  when 1, 3
    doFirstThing()
    doSomething()
  when 2
    doSomethingDifferent()
  else
    doTheRest()
end

Exceptions

Blocks in an if chain that contain a single line of code are ignored, as are blocks in a case statement that contain a single line of code.

if a ==
  doSomething()  # no issue, usually this is done on purpose to increase the readability
elsif a == 2
  doSomethingElse()
else
  doSomething()
end

But this exception does not apply to if chains without else-s, or to case-es without else clauses when all branches have the same single line of code. In the case of if chains with else-s, or of case-es with else clauses, rule {rule:ruby:S3923} raises a bug.

if a == 1
  doSomething()  # Noncompliant, this might have been done on purpose but probably not
elsif a == 2
  doSomething()
end

Resources

Related rules

• {rule:ruby:S3923} - All branches in a conditional structure should not have exactly the same implementation