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How to count instances of numbers in an array using recursion?

I try to write a function (called: tally) using recursion (part of the exercise) to scan through an array of numbers and return an object with the numbers as key and the number of instances as value.
Example:
tally([2,3,4,5,5,5,5,5,5,5,6,7,,6,7,6,7,5,4,3,4,5,5,6])
//{2: 1, 3: 2, 4: 3, 5: 10, 6: 4, 7: 3}
I created the framework but i am not sure about the syntax to make it work:
function tally(arr) {
var obj = {}
if (/*check if object ('obj') has a key corresponding to the array element*/) {
//increase key's value by onee
} else {
//add key with value of 1
}
return obj
};
Any hints to complete the recursion function above? Please try to stick to my structure in your answers as much as possible since this is part of an exercise.

Here you are:
function tally(arr) {
if (arr.length == 0) {
return {}
}
var value = arr.pop()
var obj = tally(arr)
if (value in obj) {
obj[value] += 1
} else {
obj[value] = 1
}
return obj
};
EDIT:
It can also be done using slice() instead of pop():
function tally(arr) {
if (arr.length == 0) {
return {}
}
var value = arr[0]
var obj = tally(arr.slice(1))
if (value in obj) {
obj[value] += 1
} else {
obj[value] = 1
}
return obj
};

Using extra parameter for an index, i, the result, r -
const plus1 = (k = "", r = {}) =>
( k in r
? r[k] += 1
: r[k] = 1
, r
)
const tally = (a = [], i = 0, r = {}) =>
i >= a.length
? r
: tally
( a
, i + 1
, plus1(a[i], r)
)
console.log(tally([2,3,4,5,5,5,5,5,5,5,6,7,,6,7,6,7,5,4,3,4,5,5,6]))
Output
{
"2": 1,
"3": 2,
"4": 3,
"5": 10,
"6": 4,
"7": 3,
"undefined": 1
}

ok, so you are asked to do a recursion just for the sake of it.
This could be done (albeit is hacky) passing an extra parameter to tally. When you declare a function in vanilla js you can actually feed it extra stuff. So, in each recursion, pass obj as a second parameter:
EDIT
Thanks #Bergi, you're right. I'll edit the code
function tally(arr) {
let obj = arguments.length>1? arguments[1] : {};
if(arr.length===0) {
return obj;
}
let next_number=arr.pop();
obj[next_number]=obj[next_number]||0;
obj[next_number]++;
return tally(arr,obj);
};
let inputArr = [2,3,4,5,5,5,5,5,5,5,6,7,6,7,6,7,5,4,3,4,5,5,6],
outputObj=tally(inputArr);
console.log(outputObj);
console.log({outputEmpty:tally([])});

I am not sure how to guide you to an answer without giving it away entirely, but this is what I would recommend. (There are some problems such as you destroy arr in the process that you may want to consider)
function tally(arr, obj) {
// if the length is zero we've gone through every value
if(arr.length === 0)
return obj
// create obj if we didn't provide it
if(obj === undefined)
obj = {}
// pull the last value from arr
let val = arr.pop()
if (/*check if object ('obj') has a key corresponding to the array element*/) {
//increase key's value by onee
} else {
//add key with value of 1
}
// move onto the next value
return tally(arr,obj)
}
EDIT: took #Bergi's input

How to "splice" an object?

Here is what I want to achieve:
A function called Splice which can take 3 arguments:
The first is an object.
The second is numeric.
The third is numeric.
It takes properties from an object and uses them for a newly created object.
Which properties are defined by the two numbers. The second argument indicates the position of the key/value pair that I want to start taking properties from and the third indicates how many I want to remove.
So for instance splice(0,2) refers to the positions 0 and 1 of the object having the following object: var obj = {a: 1, b: 2, c: 2} and calling the function splice with the arguments obj, 0, 2 should return {a: 1, b: 2}
If the third argument is not passed it should default to 1.
This is what I have so far (not yet respecting the last part when only one number is passed in):
function splice(object, number, number2) {
var newOjb = {};
var count = 0;
object.forEach(function(element, index) {
count++;
if(count <= number2 && count > number) {
newObj[key]=item
} else if (number == count) {
newObj[key]=item
}
})
return newObj;
}
However, I seem to use some wrong syntax as my console tells me my forEach loop is not a function and the code, in general, doesn't seem to work.
Any ideas for basic solutions (no fancy techniques)?

Use Object.entries(object).forEach(function([key, item], index) { (Note: This will not guarantee order). You have also typo in var newOjb = {};
P.S. You don't need that count, you can use index in your conditions
number - index, number2 - count
function splice(object, number, number2) {
var newObj = {};
if (!number2) {
number2 = 1;
}
Object.entries(object).forEach(function([key, item], index) {
if (index >= number && index - number < number2) {
newObj[key] = item
}
})
return newObj;
}
let obj = {
a: 1,
b: 2,
c: 2
};
console.log(splice(obj, 0, 2))
console.log(splice(obj, 2))
console.log(splice(obj, 1, 2))

FWIW, here's a function that behaves exactly like Array.splice for objects (don't know if this qualifies as "fancy" though):
function spliceObject(obj, start, deleteCount, ...items) {
let e = Object.entries(obj);
let r = e.splice(start, deleteCount, ...items.flatMap(Object.entries));
Object.keys(obj).forEach(k => delete obj[k]);
e.forEach(([k, v]) => obj[k] = v);
return Object.fromEntries(r);
}
//
obj = {a:1, b:2, c:3, d:4, e:5, f:6 }
ret = spliceObject(obj, 1, 3, {xyz:88}, {blah:99})
console.log(obj)
console.log(ret)
Do note however that you generally shouldn't rely on object properties being ordered in any particular way. The ordering is defined and mandated by the standard, but not all engines and tools get it right.

For your default value question, this is a pattern that works well
function myfunction1(val) {
this.val = val ? val : 1;
console.log(this.val);
}

how to print a unique number in a array

The problem is to find the unique number in a array such as [2,2,2,5].
The output should be 5 as it is the 1 unique element in the array.
I have attempted this:
function findUniq(arr) {
var b= arr[0];
var c;
for(var i=0; i<arr.length; i++)
{
if(arr[i]===b )
{
b=arr[i]
}
else
{
c=arr[i];
}
}
return c
console.log(findUniq([3, 5, 3, 3, 3]))
This works fine unless the unique number is the first element in the array. How do I fix this?

You can use indexOf and lastIndexOf to see if a value occurs more than once in the array (if it does, they will be different), and if so, it is not the unique value. Use filter to process the array:
let array = [2,2,2,5];
console.log(array.filter(v => array.indexOf(v) === array.lastIndexOf(v)));
array = [5,3,3,3,3];
console.log(array.filter(v => array.indexOf(v) === array.lastIndexOf(v)));
array = [4,4,5,4];
console.log(array.filter(v => array.indexOf(v) === array.lastIndexOf(v)));

You can create a recursive function that will take the first element of the array and see if it exists in the rest of it, if it does, it will take the next element and do the same, return the element if it doesn't exist in the rest of the array :
const arr = [3, 3, 3, 5, 3];
const find = arr => {
const [f, ...rest] = arr;
if(rest.includes(f))
return find(rest);
else
return f;
}
const result = find(arr);
console.log(result);
Note that this will return the last element if all of them are the same [3,3,3] will return 3

Try something like this using a set, which only stores unique elements:
var set = new Set(arr);
// count instances of each element in set
result = {};
for(var i = 0; i < a.length; ++i) {
if(!result[arr[i]])
result[arr[i]] = 0;
++result[arr[i]];
}
for (var value in result) {
if (value == 1) {
return value;
}
}
// if there isn't any
return false;
This should work, please tell me if it doesn't.

This is another implementation that is surely less efficient than that of #Nick's, but it is a valid algorithm anyway:
function findUniq(arr) {
var elemCount = new Map();
var uniq = [];
// Initialize elements conts
for (var k of arr.values()) {
elemCount.set(k, 0);
}
// Count elements
for (var k of arr.values()) {
elemCount.set(k, elemCount.get(k) + 1);
}
// Add uniq elements to array
for (var [k, v] of elemCount.entries()) {
if (v === 1) uniq.push(k);
}
return uniq;
}
console.log(findUniq([3, 5, 3, 3, 3]))

if you prefer .reduce over .map for your use case (for performance/etc. reasons):
function existance(data) {
return data.reduce((a, c) => (data.indexOf(c) === data.lastIndexOf(c)) ? a.concat(c) : a, []);
}
console.log(existance([1,1,1,2]));
console.log(existance([1,1,2,3,4,5,5,6,6,6]));

How to sum the values of a JavaScript object?

I'd like to sum the values of an object.
I'm used to python where it would just be:
sample = { 'a': 1 , 'b': 2 , 'c':3 };
summed = sum(sample.itervalues())
The following code works, but it's a lot of code:
function obj_values(object) {
var results = [];
for (var property in object)
results.push(object[property]);
return results;
}
function list_sum( list ){
return list.reduce(function(previousValue, currentValue, index, array){
return previousValue + currentValue;
});
}
function object_values_sum( obj ){
return list_sum(obj_values(obj));
}
var sample = { a: 1 , b: 2 , c:3 };
var summed = list_sum(obj_values(a));
var summed = object_values_sum(a)
Am i missing anything obvious, or is this just the way it is?

It can be as simple as that:
const sumValues = obj => Object.values(obj).reduce((a, b) => a + b, 0);
Quoting MDN:
The Object.values() method returns an array of a given object's own enumerable property values, in the same order as that provided by a for...in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).
from Object.values() on MDN
The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.
from Array.prototype.reduce() on MDN
You can use this function like that:
sumValues({a: 4, b: 6, c: -5, d: 0}); // gives 5
Note that this code uses some ECMAScript features which are not supported by some older browsers (like IE). You might need to use Babel to compile your code.

You could put it all in one function:
function sum( obj ) {
var sum = 0;
for( var el in obj ) {
if( obj.hasOwnProperty( el ) ) {
sum += parseFloat( obj[el] );
}
}
return sum;
}
var sample = { a: 1 , b: 2 , c:3 };
var summed = sum( sample );
console.log( "sum: "+summed );
For fun's sake here is another implementation using Object.keys() and Array.reduce() (browser support should not be a big issue anymore):
function sum(obj) {
return Object.keys(obj).reduce((sum,key)=>sum+parseFloat(obj[key]||0),0);
}
let sample = { a: 1 , b: 2 , c:3 };
console.log(`sum:${sum(sample)}`);
But this seems to be way slower: jsperf.com

If you're using lodash you can do something like
_.sum(_.values({ 'a': 1 , 'b': 2 , 'c':3 }))

Now you can make use of reduce function and get the sum.
const object1 = { 'a': 1 , 'b': 2 , 'c':3 }
console.log(Object.values(object1).reduce((a, b) => a + b, 0));

A regular for loop is pretty concise:
var total = 0;
for (var property in object) {
total += object[property];
}
You might have to add in object.hasOwnProperty if you modified the prototype.

Honestly, given our "modern times" I'd go with a functional programming approach whenever possible, like so:
const sumValues = (obj) => Object.keys(obj).reduce((acc, value) => acc + obj[value], 0);
Our accumulator acc, starting with a value of 0, is accumulating all looped values of our object. This has the added benefit of not depending on any internal or external variables; it's a constant function so it won't be accidentally overwritten... win for ES2015!

Any reason you're not just using a simple for...in loop?
var sample = { a: 1 , b: 2 , c:3 };
var summed = 0;
for (var key in sample) {
summed += sample[key];
};
http://jsfiddle.net/vZhXs/

let prices = {
"apple": 100,
"banana": 300,
"orange": 250
};
let sum = 0;
for (let price of Object.values(prices)) {
sum += price;
}
alert(sum)

I am a bit tardy to the party, however, if you require a more robust and flexible solution then here is my contribution. If you want to sum only a specific property in a nested object/array combo, as well as perform other aggregate methods, then here is a little function I have been using on a React project:
var aggregateProperty = function(obj, property, aggregate, shallow, depth) {
//return aggregated value of a specific property within an object (or array of objects..)
if ((typeof obj !== 'object' && typeof obj !== 'array') || !property) {
return;
}
obj = JSON.parse(JSON.stringify(obj)); //an ugly way of copying the data object instead of pointing to its reference (so the original data remains unaffected)
const validAggregates = [ 'sum', 'min', 'max', 'count' ];
aggregate = (validAggregates.indexOf(aggregate.toLowerCase()) !== -1 ? aggregate.toLowerCase() : 'sum'); //default to sum
//default to false (if true, only searches (n) levels deep ignoring deeply nested data)
if (shallow === true) {
shallow = 2;
} else if (isNaN(shallow) || shallow < 2) {
shallow = false;
}
if (isNaN(depth)) {
depth = 1; //how far down the rabbit hole have we travelled?
}
var value = ((aggregate == 'min' || aggregate == 'max') ? null : 0);
for (var prop in obj) {
if (!obj.hasOwnProperty(prop)) {
continue;
}
var propValue = obj[prop];
var nested = (typeof propValue === 'object' || typeof propValue === 'array');
if (nested) {
//the property is an object or an array
if (prop == property && aggregate == 'count') {
value++;
}
if (shallow === false || depth < shallow) {
propValue = aggregateProperty(propValue, property, aggregate, shallow, depth+1); //recursively aggregate nested objects and arrays
} else {
continue; //skip this property
}
}
//aggregate the properties value based on the selected aggregation method
if ((prop == property || nested) && propValue) {
switch(aggregate) {
case 'sum':
if (!isNaN(propValue)) {
value += propValue;
}
break;
case 'min':
if ((propValue < value) || !value) {
value = propValue;
}
break;
case 'max':
if ((propValue > value) || !value) {
value = propValue;
}
break;
case 'count':
if (propValue) {
if (nested) {
value += propValue;
} else {
value++;
}
}
break;
}
}
}
return value;
}
It is recursive, non ES6, and it should work in most semi-modern browsers. You use it like this:
const onlineCount = aggregateProperty(this.props.contacts, 'online', 'count');
Parameter breakdown:
obj = either an object or an array
property = the property within the nested objects/arrays you wish to perform the aggregate method on
aggregate = the aggregate method (sum, min, max, or count)
shallow = can either be set to true/false or a numeric value
depth = should be left null or undefined (it is used to track the subsequent recursive callbacks)
Shallow can be used to enhance performance if you know that you will not need to search deeply nested data. For instance if you had the following array:
[
{
id: 1,
otherData: { ... },
valueToBeTotaled: ?
},
{
id: 2,
otherData: { ... },
valueToBeTotaled: ?
},
{
id: 3,
otherData: { ... },
valueToBeTotaled: ?
},
...
]
If you wanted to avoid looping through the otherData property since the value you are going to be aggregating is not nested that deeply, you could set shallow to true.

Use Lodash
import _ from 'Lodash';
var object_array = [{a: 1, b: 2, c: 3}, {a: 4, b: 5, c: 6}];
return _.sumBy(object_array, 'c')
// return => 9

I came across this solution from #jbabey while trying to solve a similar problem. With a little modification, I got it right. In my case, the object keys are numbers (489) and strings ("489"). Hence to solve this, each key is parse. The following code works:
var array = {"nR": 22, "nH": 7, "totB": "2761", "nSR": 16, "htRb": "91981"}
var parskey = 0;
for (var key in array) {
parskey = parseInt(array[key]);
sum += parskey;
};
return(sum);

A ramda one liner:
import {
compose,
sum,
values,
} from 'ramda'
export const sumValues = compose(sum, values);
Use:
const summed = sumValues({ 'a': 1 , 'b': 2 , 'c':3 });

We can iterate object using in keyword and can perform any arithmetic operation.
// input
const sample = {
'a': 1,
'b': 2,
'c': 3
};
// var
let sum = 0;
// object iteration
for (key in sample) {
//sum
sum += (+sample[key]);
}
// result
console.log("sum:=>", sum);

A simple solution would be to use the for..in loop to find the sum.
function findSum(obj){
let sum = 0;
for(property in obj){
sum += obj[property];
}
return sum;
}
var sample = { a: 1 , b: 2 , c:3 };
console.log(findSum(sample));

function myFunction(a) { return Object.values(a).reduce((sum, cur) => sum + cur, 0); }

Sum the object key value by parse Integer. Converting string format to integer and summing the values
var obj = {
pay: 22
};
obj.pay;
console.log(obj.pay);
var x = parseInt(obj.pay);
console.log(x + 20);

function totalAmountAdjectives(obj) {
let sum = 0;
for(let el in obj) {
sum += el.length;
}
return sum;
}
console.log(totalAmountAdjectives({ a: "apple" }))

A simple and clean solution for typescrip:
const sample = { a: 1, b: 2, c: 3 };
const totalSample = Object.values(sample).reduce(
(total: number, currentElement: number) => total + currentElement
);
console.log(totalSample);
Good luck!

Get next key-value pair in an object

Given a key, I want to find the next property in an object. I can not rely on the keys to be ordered or sequential (they're uuids). Please see below for trivial example of what I want:
var db = {
a: 1,
b: 2,
c: 3
}
var next = function(db, key) {
// ???
}
next(db, 'a'); // I want 2
next(db, 'b'); // I want 3
I also want a prev() function, but I'm sure it will be the same solution.
This seems like such a trivial problem but I can't for the life of me figure out how to do it.
Happy for the solution to use underscore.js or be written in coffeescript :)

ts / es6 version. I simply get the keys from the storeObject, look for the next Index.
let keys = Object.keys(storeObject);
let nextIndex = keys.indexOf(theCurrentItem) +1;
let nextItem = keys[nextIndex];

The correct answer is: you can't do that, as objects are unordered as per ECMAScript's spec.
I'd recommend that you use an ordered structure, like an array, for the purpose of the problem:
var db = [
{key: 'a', value: 1},
{key: 'b', value: 2},
{key: 'c', value: 3}
];
Then the next function can be something like:
var next = function(db, key) {
for (var i = 0; i < db.length; i++) {
if (db[i].key === key) {
return db[i + 1] && db[i + 1].value;
}
}
};
In case key does not exist on db or it was the last one, next returns undefined. if you're never going to ask for the next of the last item, you can simplify that function by removing the ternary && operator and returning db[i + 1].value directly.
You can also use some of Underscore.js utility methods to make next simpler:
var next = function(db, key) {
var i = _.pluck(db, 'key').indexOf(key);
return i !== -1 && db[i + 1] && db[i + 1].value;
};
(in this case next could return false sometimes... but it's still a falsy value :))
Now, a more pragmatic answer could be that, as most browsers will respect the order in which an object was initialized when iterating it, you can just iterate it with a for in loop as the other answers suggest. I'd recommend using Object.keys to simplify the job of iterating over the array:
// Assuming that db is an object as defined in the question.
var next = function(db, key) {
var keys = Object.keys(db)
, i = keys.indexOf(key);
return i !== -1 && keys[i + 1] && db[keys[i + 1]];
};

function next(db, key){
var found = 0;
for(var k in db){
if(found){ return db[k]; }
if(k == key){ found = 1; }
}
}

An immediate solution to this would be to store data in an array and use the object to simply store the index in the array at which an object exists.
var db = {
data: [1, 2, 3],
index: {
a: 0,
b: 1,
c: 2
}
};
function next(db, key) {
var next = db.index[key] + 1;
if (next >= db.data.length) {
return null;
}
return db.data[next];
}
function prev(db, key) {
var next = db.index[key] - 1;
if (next < 0) {
return null;
}
return db.data[next];
}
function add(db, key, value) {
db.index[key] = db.data.push(value) - 1;
}
function remove(db, key) {
var index = db.index[key], x, temp;
if (index !== undefined) {
delete db.index[key];
db.data.splice(index, 1);
// Update indices of any elements after the removed element
for (x in db.index) {
temp = db.index[x];
if (temp > index) {
db.index[x] = temp - 1;
}
}
}
}
The basic idea is to use an ordered structure, in this case the array, to hold the data in a sequential manner. In this case, next and prev are both constant time, add is amortized constant time, and delete is O(N).
The ordering of keys isn't guaranteed by the ECMA standard, so for/in doesn't need to be in the order keys were added (though in practice, that does tend to be the common implementation). In this solution, I use an array to explicitly keep track of insert order.
Edit: I overlooked a deletion issue earlier with splice. The index would become incorrect for all values after the spliced value for a remove. The fix doesn't impact the running time complexity of the operation. A faster version with fewer removes could let the array become sparse and instead of splicing, simply set the index to null to free any reference stored there. This would lower the remove operation to O(1).
function remove(db, key) {
var index = db.index[key];
if (index !== undefined) {
delete db.index[key];
db.data[index] = null;
}
}

Using undercore.js, you can take the keys of an object and do the trick. But I'm not sure if the key-value pairs are ordered in any way to begin with:
var next = function(db, key) {
var keys = _.keys(db);
var index = _.indexOf(keys, key);
if(index+1<keys.length){
return db[keys[index+1]];
}else{
return null;
}
}
jsFiddle: http://jsfiddle.net/QWhN2/

I landed here in 2021 so i'll post an Es6 solution.
A simple solution that let you navigate the object given a starting key:
const navObj = (obj, currentKey, direction) => {
return Object.values(obj)[Object.keys(obj).indexOf(currentKey) + direction];
};
const db = {
a: 1,
b: 2,
c: 3
};
console.log(navObj(db, 'a', 1));
console.log(navObj(db, 'a', 2));
console.log(navObj(db, 'b', -1));