Creating an array of object with cumulative sum dynamically in javascript

Question

I have array of objects like this and require to calculate running total(accumulate sum) dynamically for N keys.

  let a = [
  {"subject":  "Civil", "a": 100, "b":50, "c":100 },
  {"subject":  "aero",  "a": 200, "b":130,"c":100 },
  {"subject":  "tribe", "a": 100, "b":100,"c":600 },
  {"subject":  "solen", "a": 400, "b":150,"c":100},
  {"subject":  "space", "a": 100, "b":100,"c":900 }
];

Required Output as below :

[
{"subject": "Civil", "a": 100, "b": 50,  "c": 100},
{"subject": "aero",  "a": 300, "b": 180, "c": 200},
{"subject": "tribe", "a": 400, "b": 280, "c": 800},
{"subject": "solen", "a": 800, "b": 430, "c": 900},
{"subject": "space"," a": 900, "b": 530, "c": 1800}]

I tried below using MAP as below :

let b = a.map( x => ({...x,"a":a1+=x.a,"b":b1+=x.b,"c":c1+=x.c}) )

I want to achieve it dynamically without writing a, b, c.

what is expected output?

kyun
– kyun

2021-11-30 00:29:49 +00:00
Commented Nov 30, 2021 at 0:29 — kyun
– kyun, Commented Nov 30, 2021 at 0:29

Phil · Accepted Answer · 2021-11-30 01:02:21Z

2

If you want to cumulatively sum every property other than subject, you can do so by mapping the array to new values, keeping an object reference for each running total as you go

const a = [{"subject":"Civil","a":100,"b":50,"c":100},{"subject":"aero","a":200,"b":130,"c":100},{"subject":"tribe","a":100,"b":100,"c":600},{"subject":"solen","a":400,"b":150,"c":100},{"subject":"space","a":100,"b":100,"c":900}]

const cf = {} // store running totals in here
const b = a.map(({ subject, ...props }) => ({
  subject,
  ...Object.fromEntries(Object.entries(props).map(([ key, val ]) => [
    key,
    cf[key] = (cf[key] ?? 0) + val // value is the result of the assignment
  ]))
}))

console.log(b)

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edited Nov 30, 2021 at 1:02

answered Nov 30, 2021 at 0:55

Phil

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2 Comments

Jeery sum code Over a year ago

can I add an Initial value as well?

Phil Over a year ago

@RajseGodia you can initialise cf to whatever you want if that's what you mean, eg const cf = { a: 500, b: -2 }

PeterKA · Accepted Answer · 2021-11-30 16:30:44Z

0

You can use a combination of .map(), .slice() and .reduce() array methods as follows:

let a = [
  {"subject":  "Civil", "a": 100, "b":50, "c":100 },
  {"subject":  "aero",  "a": 200, "b":130,"c":100 },
  {"subject":  "tribe", "a": 100, "b":100,"c":600 },
  {"subject":  "solen", "a": 400, "b":150,"c":100},
  {"subject":  "space", "a": 100, "b":100,"c":900 }
];

let b = a.map((x,i) => ({
    subject:x.subject,
    a:a.slice(0,i+1).map(({a}) => a).reduce((x,y) => x + y),
    b:a.slice(0,i+1).map(({b}) => b).reduce((x,y) => x + y),
    c:a.slice(0,i+1).map(({c}) => c).reduce((x,y) => x + y)
}));

console.log( b );

Alternatively, this can be done without reference to any particular properties as follows:

let a = [
  {"subject":  "Civil", "a": 100, "b":50, "c":100, "zz":0},
  {"subject":  "aero",  "a": 200, "b":130,"c":100, "zz":40},
  {"subject":  "tribe", "a": 100, "b":100,"c":600, "zz":0},
  {"subject":  "solen", "a": 400, "b":150,"c":100, "zz":60},
  {"subject":  "space", "a": 100, "b":100,"c":900, "zz":200}
];

let b = a.map((val, i) => ({
        ...val,
        //introduce a temp property 'index'
        index: i + 1 
    }))
    .map(({
        subject,
        index,
        ...rest
    }) => ({
        subject,
        ...Object.keys(rest).reduce((obj, cur) => ({
            ...obj,
            ...{
                //use temp propterty to compute running total for every
                //property but 'subject', 
                [cur]: a.slice(0, index).map(v => v[cur]).reduce((x, y) => x + y)
            }
        }), {})
    }));   

console.log( b );

edited Nov 30, 2021 at 16:30

answered Nov 30, 2021 at 4:18

PeterKA

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2 Comments

Jeery sum code Over a year ago

i want to achieve it dynamically i.e. don't want to hard code a,b,c

PeterKA Over a year ago

No problem. The second option should take care of that.

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