How to find the middle letter among the specified letters?-CodePudding

I've created a function that accepts 2 parameters to find the middle value between those parameters in an Alphabet variable.

Example:

the middle part between Q and U is S,
the middle part between R and U is ST,
the middle part between T and Z is W,

What I'm confused about is how do I take the value one by one starting at index 1 in the Alphabet variable?

function letterMiddleValue(a, b) {
    let alpha1 = Alphabet.indexOf(a);
    let alpha2 = Alphabet.indexOf(b);
    let center = (alpha1   alpha2) / 2;
    let letterLength;

    if (center % 2 == 1) {
        letterLength = 1;
    } else {
        letterLength = 2;
    }
    return Alphabet.substring(center, center   letterLength);
}

var Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

console.log(letterMiddleValue("Q", "U"));
console.log(letterMiddleValue("R", "U"));
console.log(letterMiddleValue("T", "Z"));

CodePudding user response：

Center for r & u is 18.5. 18.5 % 2 is 0.5 so you need to check for 0.5 condition:

function letterMiddleValue(a, b) {
    let alpha1 = Alphabet.indexOf(a);
    let alpha2 = Alphabet.indexOf(b);
    let center = (alpha1   alpha2) / 2;
    let letterLength;

    if (center % 2 == 0.5) {
        letterLength = 2;
    } else {
        letterLength = 1;
    }

    return Alphabet.substring(center, center   letterLength);
}

var Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

console.log(letterMiddleValue("Q", "U"));
console.log(letterMiddleValue("R", "U"));
console.log(letterMiddleValue("T", "Z"));

That's because how you calculate center. (7 5) / 2 = 6.5 (6 10) / 2 = 8 and modulo gives back what's not dividable by 2

CodePudding user response：

I think there is an alternative approach, which is just to calculate the distance between and then decide whether to extract one or two characters.

This also copes with there being nothing between, or the b value coming lower down the alphabet (or not being in the defined Alphabet)

function letterMiddleValue(a, b) {
    const aPos = Alphabet.indexOf(a);
    const bPos = Alphabet.indexOf(b);
    const len = bPos - aPos

    if (len < 2) {
        return '[none]'
    }

    const start = aPos   (len / 2)
    const end = start   1   (len % 2)
    return Alphabet.slice(Math.floor(start), end)
}

const Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

console.log(letterMiddleValue("Q", "U"));   // S
console.log(letterMiddleValue("R", "U"));   // ST
console.log(letterMiddleValue("T", "Z"));   // W
console.log(letterMiddleValue("A", "B"));   // none
console.log(letterMiddleValue("A", "C"));   // B
console.log(letterMiddleValue("A", "D"));   // BC
console.log(letterMiddleValue("A", "A"));   // none
console.log(letterMiddleValue("Z", "A"));   // none
console.log(letterMiddleValue("z", "A"));   // none
console.log(letterMiddleValue("Z", "a"));   // none