How to get a new array containing some random elements from an old array in Javascript

Question

I am making a quiz app in react native, it has this array of objects in which each object contains a question statement,choices and the correct choice. There will be total 15 objects in this, but i require a new array that contains only 5 of the objects of this array. They should be selected at random everytime i start the quiz. How to get that array, help

const questions = [
    new quizQuestions(
        'What is capital of Pakistan?',
        ['Karachi', 'Islamabad', 'Lahore', 'Peshawar'],
        'Islamabad',
    ),
    new quizQuestions(
        'What is capital of India?',
        ['delhi', 'Islamabad', 'zcjzj', 'Peshawar'],
        'delhi',
    ),
    new quizQuestions(
        'What is capital of Pakistan?',
        ['Karachi', 'Islamabad', 'Lahore', 'Peshawar'],
        'Islamabad',
    ),
]

Azad · Accepted Answer · 2020-05-10 11:14:54Z

2

use JS random method to generate random numbers from 0 to 15

//your whole set of questions array
    const questions = [
        new quizQuestions(
            'What is capital of Pakistan?',
            ['Karachi', 'Islamabad', 'Lahore', 'Peshawar'],
            'Islamabad',
        ),
        new quizQuestions(
            'What is capital of India?',
            ['delhi', 'Islamabad', 'zcjzj', 'Peshawar'],
            'delhi',
        ),
        new quizQuestions(
            'What is capital of Pakistan?',
            ['Karachi', 'Islamabad', 'Lahore', 'Peshawar'],
            'Islamabad',
        ),
    ];

//random 5 question array

const selectedQuestionArr = [];
let addedIndex = [];
for (var i = 0; i < 5; i++) {
  let randomIdx;
  do {

    randomIdx = Math.floor(Math.random() * 15);
  }
  while(addedIndex.indexOf(randomIdx) > -1);

  //record added index to not include it in the next cycle
  addedIndex.push(randomIdx);

  //push the randomly selected question
  selectedQuestionArr.push(questions[randomIdx]);
}

console.log(selectedQuestionArr);

edited May 10, 2020 at 11:14

answered May 10, 2020 at 10:43

Azad

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5 Comments

Moeez Atlas Over a year ago

this might repeat some question if Math.random returns old index value. Wont it?

Azad Over a year ago

@MoeezAtlas I modified the answer

Moeez Atlas Over a year ago

yeah i think this works. i changed randomIdx.push(randomIdx) to addedindex.push(randomIdx) as previously got error

Azad Over a year ago

@MoeezAtlas isn't this solution helped you?

Azad Over a year ago

no way to accept 2 answers, you should decide which answer would give you the best and reliable solution for your current problem

xdeepakv · Accepted Answer · 2020-05-10 11:21:17Z

1

Very much random what you need.

Using generator function:

const questions = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15];
function* randomise(arr, numOfElem) {
  // Create a copy of question so that wont effect actual array
  let copyArray = [...arr];

  // Use copyArray to generate random as closure, so that it never go out of index
  const getRandom = () => Math.floor(Math.random() * copyArray.length);
  // This is generator function run always
  while (true) {
    // If numOfElem is greater, then clone again

    // suppose you have 15 question but you take 6. SO after 2 iteration, copyArray will have only 3 element so clone again
    if (numOfElem > copyArray.length) {
      // Clone again
      copyArray = [...arr];
    }
    // pick 5 random data.
    let result = [];
    for (let i = 0; i < numOfElem; i++) {
      result.push(copyArray.splice(getRandom(), 1)[0]);
    }
    yield result;
  }
}

const questionGenerator = randomise(questions, 5);
console.log(questionGenerator.next().value); // random [ 11, 3, 7, 2, 8 ]

console.log(questionGenerator.next().value); // random [ 12, 13, 14, 9, 5 ]

console.log(questionGenerator.next().value); // random [ 6, 1, 4, 15, 10 ]

console.log(questionGenerator.next().value); // random [ 14, 12, 8, 2, 13 ]

console.log(questionGenerator.next().value); // random [ 9, 3, 10, 6, 11 ]

Same but without generator function.

const questions = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15];
const randomise = (arr, numOfElem) => {
  let copyArray = [...arr];
  const getRandom = () => Math.floor(Math.random() * copyArray.length);
  return () => {
    // If numOfElem is greater, then clone again

    // suppose you have 15 question but you take 6. SO after 2 iteration, copyArray will have only 3 element so clone again
    if (numOfElem > copyArray.length) {
      // Clone again
      copyArray = [...arr];
    }
    // pick 5 random data.
    let result = [];
    for (let i = 0; i < numOfElem; i++) {
      result.push(copyArray.splice(getRandom(), 1)[0]);
    }
    return result;
  };
};
const questionGenerator = randomise(questions, 5);
console.log(questionGenerator()); // random [ 11, 3, 7, 2, 8 ]

console.log(questionGenerator()); // random [ 12, 13, 14, 9, 5 ]

console.log(questionGenerator()); // random [ 6, 1, 4, 15, 10 ]

console.log(questionGenerator()); // random [ 14, 12, 8, 2, 13 ]

console.log(questionGenerator()); // random [ 9, 3, 10, 6, 11 ]

edited May 10, 2020 at 11:21

answered May 10, 2020 at 10:57

xdeepakv

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6 Comments

xdeepakv Over a year ago

Lol! :-D let me know if you need help to understand! Please accept as answer :-) if works for you

Moeez Atlas Over a year ago

Yeah i was just checking it inside my code it works.

xdeepakv Over a year ago

i added a real generator function example. You can check that too. I add details later! as comment. You can check later.

xdeepakv Over a year ago

Please check i added few comments. to help you.! cheers! keep coding!

Moeez Atlas Over a year ago

Thanks bro comments help a lot :)

|

RishabhRathod · Accepted Answer · 2020-05-10 11:11:21Z

1

For your particular case, i couldn't find any lodash function for this i have implemented my own javascript logic for this you can have a look.

https://codesandbox.io/s/shuffled-array-ltzud

answered May 10, 2020 at 11:11

RishabhRathod

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Comments

s-s · Accepted Answer · 2020-05-28 20:41:32Z

0

Almost a repeat of Azad's answer :D but I found our method to be very simple and straight forward. pass in selectedQuestions as an empty array

function randomizeQuestions(allQuestions,selectedQuestions,numberToSelect)
{
  selectedIndex = [];
  var randIndex;
  for(var i = 0; i < numberToSelect; i++)
  {
  
	  do{
	     randIndex = Math.floor(Math.random() * allQuestions.length);
	  }while(selectedIndex.includes(randIndex));

	  selectedIndex.push(randIndex);
	  selectedQuestions.push(allQuestions[randIndex]); 
  }
}

answered May 28, 2020 at 20:41

s-s

313 bronze badges

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