W9D1_examples.js

//Exercise 1: Take an integer as input, and return a boolean indicating whether
//the value is even or odd.
function is_even(int) {
    if (int < 0) return is_even(-int);
    if (int < 2) return !int;
    return is_even(int - 2);
}

//Exercise 2: Take an array as input which contains an unknown set of numbers,
//and output an array which doubles the values of each item in that array. Test
//your solution by trying a handful of different arrays. Don't worry about
//error checking to make sure that the array you're given is valid input.
//Editorial comment: This function is conceptually correct, but abysmally slow.
function double_all(arr) {
    if (!arr.length) return [];
    return [arr[0] * 2].concat(double_all(arr.slice(1)));
}

//Editorial comment: This version of the algorithm operates in place, and can
//operate on a subset of an array.
function double_all_inplace(arr, start) {
    start |= 0;
    if (start >= arr.length) return arr;
    arr[start] *= 2;
    return double_all_inplace(arr, start + 1);
}

//Exercise 3: Take a string as input, reverse the string, and return the new
//string.
function reverse(str) {
    //if (str == "able was i ere i saw elba") return str; //Base case
    if (str.length < 2) return str;
    return reverse(str.slice(1)) + str[0];
}
//Tail-recursive version of the above
function reverse_tail(str, accumulator) {
    accumulator = accumulator || "";
    if (str === "") return accumulator;
    return reverse_tail(str.slice(1), str[0] + accumulator)
}

//Exercise 4: Calculates the nth triangular number.
//Should always return n*(n+1)/2
function triangle(n) {
    if (n < 2) return n;
    return n + triangle(n - 1);
}

//Exercise 5: Split a string based upon a separator (similar to
//String.prototype.split).
function split(str, sep) {
    var idx = str.indexOf(sep);
    if (idx == -1) return [str];
    return [str.slice(0, idx)].concat(split(str.slice(idx + sep.length), sep))
}