In VS I am told this function “does not return a value in all control paths.” A bot told me specifically the issue is with this line: else if (letter + key <= 90). It said that if the outcome results in letter + key equally exactly 90 then a value is not returned, but I thought that was covered where ‘<=’ means ‘less than or equals.’
char rotate(char letter, int key)
{
if (isalpha(letter) == true)
{
if (letter + key > 90)
{
int overage = letter + key - 90;
letter = 64 + overage;
while (letter > 90)
{
overage = letter - 90;
letter += overage;
}
return letter;
}
else if (letter + key <= 90)
{
letter += key;
return letter;
}
}
else if (isalpha(letter) == false)
{
return letter;
}
This particular compiler isn’t smart enough to recognize that
isalpha(letter) == trueandisalpha(letter) == falseare mutually exclusive conditions. It thinks there’s a third scenario that you haven’t accounted for.That’s because they’re not necessarily mutually exclusive. The function is being called twice so there’s no way to guarantee the result will be the same both times without knowing what it does under the hood.
Consider a case where
isalphaperforms a coin flip, 50% chance each call to return true. The first call returns false so the first condition fails, then the second call returns true so the second condition fails; in 25% of cases neither code block executes.You could store the result of the first call in a local variable and reuse it if you really wanted to, but the smart solution is to either use if/else properly or switch to early returns instead.
Expect isalpha is part of the standard library not an arbitrary function, a compile should be able to optimize standard calls.
Compiler optimisations don’t apply when you’re breaking the rules of the language. It won’t compile.
Right, the compiler isn’t smart enough to recognize that
isalpha()is pure and deterministic.
you are missing a returning
elseblock.Ah I see. I had a bad habit of using else if statements instead of else statements because I thought else if could be better in seeing the condition it’s testing for so it was clearer. I get the logic is actually different now.
I’m a fan of early returns.
Soif(isalpha(letter) == false) return letter if(letter whatever) { Do thing Return letter } if(letter something else) { Do something else Return letter } throw error("unprocessable letter")I find it lets me mentally walk through all the paths more easily.
And if something gets too complex for this, then I need to break it down into further functions
