Needless return in recursive method

Question

I am writing a recursive code that counts the minimum number of operations we need to do in order that s2 is equals to s1 , the valid operations are insert 1 character (di) , delete 1 character (dc) and dn stays for do nothing

private static int editDistance(String s1, String s2) {

    if((s2.isEmpty() && (dn == 0 && dc == 0 && di == 0)) || (s1.isEmpty() && (dn == 0 && dc == 0 && di == 0)))
        return Integer.max(s1.length(), s2.length());

    if(s2.isEmpty()) {
        return 0;
    } else if(s1.isEmpty()) {
        dc++;
        return 1 + editDistance(s1, rest(s2));
    } else if(s1.charAt(0) == s2.charAt(0)) {
        dn++;
        return editDistance(rest(s1), rest(s2));
    } else if(s1.charAt(0) != s2.charAt(0) && dc <= di) {
        dc++;
        return 1 + editDistance(s1, rest(s2));
    } else if(s1.charAt(0) != s2.charAt(0) && dc > di) {
        di++;
        return 1 + editDistance(rest(s1), s2);
    }

    return 0;

}

For instance if we have s1 = "home" and s2 = "hote" there will be 1 delete operation (for 't'), 1 insert operation ('m') and 3 do nothing operations.

The problem is that my statements are annidated in those if/else if branches so for my program to compile i had to put a return 0 statement at the bottom which is pointless, how could i correct this?

Answer 1

Remove last if. Of course you should do it only if your cases cover all possible variants. Sometimes the proper way will be to throw exception after all if else to ensure that if you don't count something, you program doesn't continue with incorrect value.

private static int editDistance(String s1, String s2) {

    if((s2.isEmpty() && (dn == 0 && dc == 0 && di == 0)) || (s1.isEmpty() && (dn == 0 && dc == 0 && di == 0)))
        return Integer.max(s1.length(), s2.length());

    if(s2.isEmpty()) {
        return 0;
    } else if(s1.isEmpty()) {
        dc++;
        return 1 + editDistance(s1, rest(s2));
    } else if(s1.charAt(0) == s2.charAt(0)) {
        dn++;
        return editDistance(rest(s1), rest(s2));
    } else if(s1.charAt(0) != s2.charAt(0) && dc <= di) {
        dc++;
        return 1 + editDistance(s1, rest(s2));
    } else {
        di++;
        return 1 + editDistance(rest(s1), s2);
    }
}

Answer 2

If these conditions exhaust all possibilities, I would advise you not to remove any conditions (because that's documentation of why the program does what it does), but rather throw an exception - for instance AssertionError with message "this can't happen".

That's because sometimes due to refactoring or magic, things which should not happen DO happen, and it's best not to ignore them but to crash the application instead (since it's in an inconsistent state).

Java compiler simply can't always detect impossible scenarios (it would be computationally too expensive, and in some cases - impossible - see "halting problem").

Answer 3

You can solve this in two ways :

1. If there could be another possibilities that could happen if you removed the last if statement

    private static int editDistance(String s1, String s2) { if((s2.isEmpty() && (dn == 0 && dc == 0 && di == 0)) || (s1.isEmpty() && (dn == 0 && dc == 0 && di == 0))) return Integer.max(s1.length(), s2.length()); if(s2.isEmpty()) { return 0; } else if(s1.isEmpty()) { dc++; return 1 + editDistance(s1, rest(s2)); } else if(s1.charAt(0) == s2.charAt(0)) { dn++; return editDistance(rest(s1), rest(s2)); } else if(s1.charAt(0) != s2.charAt(0) && dc <= di) { dc++; return 1 + editDistance(s1, rest(s2)); } else if(s1.charAt(0) != s2.charAt(0) && dc > di) { di++; return 1 + editDistance(rest(s1), s2); }else return 0; } 

2. If the last if statement covers all the remained possibilities just replace the last if statement with else keyword .