Assign Value to Node in Multidimensional array javascript

Question

I am trying to assign an element within a json object with some new value (text/object/array). I have a swap function which takes in the json object, an array with the indexes to retreive the element and the value to replace it with. Currently I am using eval, which accoridng to some is "evil". Is there a better way to do this without eval or is eval ok in this case? Keep in mind it must be dynamic because the array may change. Also it may be important to note that I am programatically creating the array parameter.

//array looks like: ["cluster", "2", "segment", "0", "node", "3"]    
JsonManager.prototype.swap = function(json, array, val){
        var statement = "json";
        for (var i = 0; i < array.length; i++) {
            if(!isNumeric(array[i]))
            {
            statement += "[\"" + array[i] + "\"]";          
            }else{
                statement += "[" + array[i] + "]"   
            }
        }
        statement += " = val";
        eval(statement);
    };

Example JSON Object:

var customers = {
    "cluster": [{
        "clusterid": "cluster1.1",
        "color": "blue",
        "flights": "784",
        "profit": "524125",
        "clv": "2364",
        "segment": [{
            "segmentid": "segment1.1",
            "color": "green",
            "flights": "82",
            "profit": "22150",
            "clv": "1564",
            "node": [{
                "nodeid": "node1.1",
                "color": "orange",
                "xpos": "1",
                "ypos": "1"
            }, {
                "nodeid": "node1.2",
                "color": "blue",
                "xpos": "1",
                "ypos": "2"
            }, {
                "nodeid": "node1.3",
                "color": "orange",
                "xpos": "1",
                "ypos": "3"
            }, {
                "nodeid": "node1.4",
                "color": "orange",
                "xpos": "1",
                "ypos": "4"
            }]
        }, {
            "segmentid": "segment1.2",
            "color": "red",
            "flights": "2",
            "profit": "2150",
            "clv": "1564",
            "node": [{
                "nodeid": "node2.1",
                "color": "tan",
                "xpos": "2",
                "ypos": "1"
            }, {
                "nodeid": "node2.2",
                "color": "tan",
                "xpos": "2",
                "ypos": "2"
            }, {
                "nodeid": "node2.3",
                "color": "tan",
                "xpos": "2",
                "ypos": "3"
            }, {
                "nodeid": "node2.4",
                "color": "tan",
                "xpos": "2",
                "ypos": "4"
            }]
        }]
    }, {
        "clusterid": "cluster1.2",
        "flights": "4",
        "profit": "5245",
        "clv": "2364",
        "segment": [{
            "segmentid": "segment1.2",
            "flights": "2",
            "profit": "2150",
            "clv": "1564",
            "node": [{
                "nodeid": "node3.1",
                "xpos": "3",
                "ypos": "1"
            }, {
                "nodeid": "node3.2",
                "xpos": "3",
                "ypos": "2"
            }, {
                "nodeid": "node3.3",
                "xpos": "3",
                "ypos": "3"
            }, {
                "nodeid": "node3.4",
                "xpos": "3",
                "ypos": "4"
            }]
        }]
    }, {
        "clusterid": "cluster1.3",
        "flights": "10",
        "profit": "456978",
        "clv": "548",
        "segment": [{
            "segmentid": "segment1.3",
            "flights": "2",
            "profit": "2150",
            "clv": "1564",
            "node": [{
                "nodeid": "node4.1",
                "xpos": "4",
                "ypos": "1"
            }, {
                "nodeid": "node4.2",
                "xpos": "4",
                "ypos": "2"
            }, {
                "nodeid": "node4.3",
                "xpos": "4",
                "ypos": "3"
            }, {
                "nodeid": "node4.4",
                "xpos": "4",
                "ypos": "7"
            }]
        }]
    }]
};

Here is my test method:

JsonManager.prototype.init = function(){
    var clause = new Clause("nodeid", "node4.4");
    var indexes = this.search(customers, clause);
    this.swap(customers, indexes.reverse(), {"name": "kevin"});
    var test = customers["cluster"][2]["segment"][0]["node"][3];  //hard coded pointer to node4.4
    var breakPoint = "breakpoint";  //Just used as a point to stop the debugger to see test
};

For future reference here is the solution further commented:

JsonManager.prototype.swap = function(obj, path, value) {

   //This is the inner function we are recursing into 
   function descend(obj, path) {
    /*This if statement is used to stop the recrusion,
    when we have iterated through all the paths, it returns
    the object above our desired object */
        if (path.length == 0) {
            return obj;
        }
    /*Recurse into the function passing in the top level object and remove
    the top level object from our path*/ 
        return descend(obj[path[0]], path.slice(1));
    }
//Pass in the object and the (path - the last element)
    var node = descend(obj, path.slice(0, -1));
//Get the last node in path, pull it from node and assign the value
    node[path[path.length - 1]] = value;
};

Samir Talwar · Accepted Answer · 2011-05-14 12:12:22Z

1

Your "JSON" object is just a JavaScript object. More importantly, it's a tree, and trees are easiest to traverse using recursion.

JsonManager.prototype.swap = function(obj, path, value) {
    function descend(obj, path) {
        if (path.length == 0) {
            return obj;
        }
        return descend(obj[path[0]], path.slice(1));
    }

    var node = descend(obj, path.slice(0, -1));
    node[path[path.length - 1]] = value;
};

slice will take a chunk out of an array. So path.slice(1) returns path without the first element, and path.slice(0, -1) returns it without the last one. This means that we descend to the second-to-last node of the object, then set the last node using normal array notation. The easiest way to get your head around it is to work through it manually on paper with an example such as the one you have above.

answered May 14, 2011 at 12:12

Samir Talwar

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7 Comments

Guffa Over a year ago

Processing an entire tree is easier with recursion, but accessing a single element is actually easier with a simple loop.

Kevin Bowersox Over a year ago

@Samir Talwar, thanks for your answer the method works, I am going to sit down and understand what is going on. It looks like your using a closure within recursion. Is that correct?

Kevin Bowersox Over a year ago

@Samir Talwar How does your object retain a reference to the object, while Guffa's does not? I worked through your code and commented it so I understand it, its included in my question.

Samir Talwar Over a year ago

@kmb385: Guffa's code should also work well. I prefer my approach, but it's simple preference.

Kevin Bowersox Over a year ago

@Samir Talwar I looked further into Guffa's code, essentially his function boils down to j = val; While at the point of assignment j is equal to the node, however we are simply assigning j with the value. We are not assigning the object element with the value. In your code you capture the array node and assign the array node with the value. The new value is placed in the object via reference. I thank you for your time and also thanks Guffa. I learned a lot from this

|

Guffa · Accepted Answer · 2011-05-14 23:33:10Z

1

There is no reason to use eval for that, just assign the object to a variable, and use that variable to step into each level:

JsonManager.prototype.swap = function(json, array, val){
  var j = json;
  for (var i = 0; i < array.length - 1; i++) {
    j = j[array[i]];
  }
  j[array[array.length - 1]] = val;
};

edited May 14, 2011 at 23:33

answered May 14, 2011 at 12:14

Guffa

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3 Comments

Kevin Bowersox Over a year ago

I tried using this method before posting the question. I believe there is a problem with it because although the json object is passed in by reference, when you assign a segment of it to j it is done by value. When I swap your function in place of mine the value does not change. Am I missing something? I have added my tester method to the initial post.

Guffa Over a year ago

@kmb385: Yes, you are right, but it's only the last step that is the problem as that is the only one that is a value, not an object. I changed the code to to the last step directly instead of assigning it to the variable.

Kevin Bowersox Over a year ago

thanks for your solution, I'm going to place a stopwatch on both functions and see which is most efficient.

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