W9D1_examples.js

//Iterative versions of https://gist.github.com/Rosuav/5736378d02c6cd459470582ce301b4b4

//Exercise 1: Take an integer as input, and return a boolean indicating whether
//the value is even or odd.
//Editorial comment: This isn't even iterative, just fomulaic. Exercise 4 could
//be done similarly. Avoiding iteration AND recursion generally gives the best
//performance, when it's possible (but usually it won't be).
function is_even(int) {
    return (int % 2) == 0;
}

//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: Obviously arr.map() is the normal way to do this.
function double_all(arr) {
    var ret = Array(arr.length);
    for (var i = 0; i < arr.length; ++i)
        ret[i] = arr[i] * 2;
    return ret;
}

//Exercise 3: Take a string as input, reverse the string, and return the new
//string.
//Direct transformation of the tail-recursive form.
function reverse_tail(str) {
    var accumulator = "";
    while (str !== "") {
    	accumulator = str[0] + accumulator;
    	str = str.slice(1);
    }
    return accumulator;
}

//Exercise 4: Calculates the nth triangular number.
//Should always return n*(n+1)/2
function triangle(n) {
    var tot = 0;
    for (var i = 1; i <= n; ++i) tot += n;
    return tot;
}

//Exercise 5: Split a string based upon a separator (similar to
//String.prototype.split).
//Editorial comment: There are more efficient ways to do this, but this is a
//fairly direct translation of the recursive version.
function split(str, sep) {
    var ret = [];
    while (true) {
        var idx = str.indexOf(sep);
        if (idx == -1) break;
	ret.push(str.slice(0, idx))
	str = str.slice(idx + sep.length);
    }
    ret.push(str);
    return ret;
}