Filter Array of Objects with a Object reside in Nested Array property

Question

I have the following use-case,

I have,

An array of objects that contains a list of courses
An array of objects that contains students with a nested array: studies

I need to find a courses which are not studied by any student.

How to achieve that?

follow is the code sinnpient.

let courses = [
    { id: 'A' },
    { id: 'B' },
    { id: 'C' },
    { id: 'D' }, <-- not studied by any one
    { id: 'E' },
    { id: 'F' }, <-- not studied by any one
];

let students = [
    {
        name: 'STD1',
        study: [
            { id: 'A' },
            { id: 'C' }
        ]
    },
    {
        name: 'STD2',
        study: [
            { id: 'B' },
            { id: 'E' }
        ]
    }

];

expected output

  const notUsedCourse = [{ id: 'D' }, { id: 'F' }];

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– T.J. Crowder, Commented Oct 22, 2019 at 9:30
Please first try something (use .filter(), .some(), things like that). Then, if you are really stuck, we can help you. But Stackoverflow is not a free code writing platform. We can help you but not develop the whole solution for you. — Jeremy Thille
– Jeremy Thille, Commented Oct 22, 2019 at 9:32

Harry Chang · Accepted Answer · 2019-10-22 14:53:45Z

1

You can save course ids which have been studied by students into a Set so that we can check if a course has been studied later. The advantage over the solution with filter and some combination is that this solution will be much faster when the size of courses and students gets bigger since the former has the time complexity of O(n^3) .

const courses = [
    { id: 'A' },
    { id: 'B' },
    { id: 'C' },
    { id: 'D' },
    { id: 'E' },
    { id: 'F' },
];

const students = [
    {
        name: 'STD1',
        study: [
            { id: 'A' },
            { id: 'C' }
        ]
    },
    {
        name: 'STD2',
        study: [
            { id: 'B' },
            { id: 'E' }
        ]
    }

];

const usedCourseIds = new Set(students.flatMap(student => student.study).map(course => course.id));
const notUsedCourses = courses.filter(course => !usedCourseIds.has(course.id));

console.log(notUsedCourses);

edited Oct 22, 2019 at 14:53

answered Oct 22, 2019 at 9:42

Harry Chang

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2 Comments

altocumulus Over a year ago

Note, that .flatMap() instead of .flat().map() will perform even better.

Harry Chang Over a year ago

Thanks for the advice! Though .flatMap() is equivalent to .map().flat() instead of .flat().map().

Blue · Accepted Answer · 2019-10-22 09:37:56Z

0

You can use .filter with .some to loop through and search if a student has the course:

let courses = [
    { id: 'A' },
    { id: 'B' },
    { id: 'C' },
    { id: 'D' },
    { id: 'E' },
    { id: 'F' },
];

let students = [
    {
        name: 'STD1',
        study: [
            { id: 'A' },
            { id: 'C' }
        ]
    },
    {
        name: 'STD2',
        study: [
            { id: 'B' },
            { id: 'E' }
        ]
    }

];

let notUsedCourse = courses.filter(
  course => !students.some(
    student => student.study.some(
      study => study.id === course.id
    )
  )
);

console.log(notUsedCourse);

edited Oct 22, 2019 at 9:37

answered Oct 22, 2019 at 9:35

Blue

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3 Comments

Blue Over a year ago

It's an easy enough problem for an experienced programmer, and didn't seem like a "write this code for me" question. This one can be a bit tricky with the nested .some()s, which is why I chose to answer it. @JeremyThille

Blue Over a year ago

He posted an expected output, a clear problem statement. By all means: This fits our site.

Jeremy Thille Over a year ago

You were silently downvoted, and my comment, in which I meant no harm and I was only wondering something, was silently censored... classic Stackoverflow.

Nina Scholz · Accepted Answer · 2019-10-22 10:07:59Z

0

You could get the visited courses first and then filter all courses.

var courses = [{ id: 'A' }, { id: 'B' }, { id: 'C' }, { id: 'D' }, { id: 'E' }, { id: 'F' }],
    students = [{ name: 'STD1', study: [{ id: 'A' }, { id: 'C' }] }, { name: 'STD2', study: [{ id: 'B' }, { id: 'E' }] }],
    seen = students.reduce(
        (seen, { study }) => study.reduce((s, { id }) => s.add(id), seen),
        new Set
    ),
    missing = courses.filter(({ id }) => !seen.has(id));

console.log(missing)

answered Oct 22, 2019 at 10:07

Nina Scholz

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