higher order function variable undefined javascript

Question

I'm trying to get the maximum value of an element from an array of objects:

var graph = {};

graph.data = [
{x: 2008, y: 7.2},
{x: 2009, y: 8.1},
{x: 2010, y: 7.7},
{x: 2011, y: 6.8},
{x: 2012, y: 6.7}
];

to get the maximum, I use the following:

var highest = graph.data[0].y;
for (var i = graph.data.length - 1; i >=1; i--) {
    highest = Math.max(highest, graph.data[i].y);
}

which gives me the correct result. However when I try to make a higher order function, so I can reuse it:

var getMax = function (arr, para) {
    var highest = arr[0].para;
    for (var i = arr.length; i >= 1; i--) {
        highest = Math.max(highest, arr[i].para);
    }
}

and I try to use it like this:

getMax(graph.data, y);

I get an error, saying Uncaught: "Reference Error: y is not defined", any ideas? I did try to use the bracket notation instead of the dot one for para, it didn't work

This is not a higher-order function...

elclanrs
– elclanrs

2014-12-22 00:12:20 +00:00
Commented Dec 22, 2014 at 0:12 — elclanrs
– elclanrs, Commented Dec 22, 2014 at 0:12

artm · Accepted Answer · 2014-12-22 00:13:50Z

1

Try this:

var getMax = function (arr, para) {
    var highest = arr[0][para];

    for (var i = arr.length - 1; i >= 1; i--) {
        highest = Math.max(highest, arr[i][para]);
    }
    return highest;
}

getMax(graph.data, 'y');

You can't just pass y because y is not defined, so pass 'y' as a character. Also you need to return highest from you function and you need start your loop from length - 1

answered Dec 22, 2014 at 0:13

artm

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1 Comment

Bondifrench Over a year ago

Well spotted on the -1, thanks for the complete answer

elclanrs · Accepted Answer · 2014-12-22 00:21:43Z

0

A higher-order function is a function that takes functions as arguments, and/or returns a function. You can do this with higher-order functions, like pluck:

var pluck = function(x) {
  return function(xs) {
    return xs[x]
  }
}

var max = function(x, y) {
  return Math.max(x, y)
}

So you get:

var getMax = function(xs, x) {
  return xs.map(pluck(x)).reduce(max)
}

var result = getMax(graph.data, 'y')

console.log(result) //=> 8.1

answered Dec 22, 2014 at 0:21

elclanrs

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1 Comment

Bondifrench Over a year ago

My bad, I didn't use the correct terminology, but I appreciate your answer that gives me food for thought on how to organize my code.

RobG · Accepted Answer · 2014-12-22 00:30:57Z

0

You can use reduce for this also:

graph.data.reduce(function(pre, cur){return pre.y > cur.y? pre : cur}).y;

Which can be turned into a function as:

function getBiggest(arr, p) {
  return arr.reduce(function (pre, cur){return pre[p] > cur[p]? pre:cur})[p];
}

getBiggest(graph.data, 'y') // 8.1

Writing it as a loop might be a bit more code, but generally runs faster and may be easier to maintain:

function getBiggest(arr, p) {
  for (var i=0, x=-Infinity, iLen=arr.length; i<iLen; i++) {
    if (x < arr[i][p]) x = arr[i][p];
  }
  return x;
}

I think the use of Math.max to get the larger of two values is overkill, but it's good for getting the maximum of many values, e.g.:

function getBiggest(arr, p) {
  return Math.max.apply(Math, arr.map(function(v){return v[p]}));
}

edited Dec 22, 2014 at 0:30

answered Dec 22, 2014 at 0:25

RobG

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