# How does one pick the col/row or x/y coordinates of N random elements of a 2D matrix without item/coordinate duplicates?

I have a 2D matrix, here’s an example `data = [["a", "b", "c", "d"], ["e", "g"], ["i", "j", "k"]]`

I need to get N random `(x, y)` indexes without duplicates.

I already asked a different question same context and this is the solution to pick 2 x, y combos

``````const data = [["a", "b", "c", "d"], ["e", "g"], ["i", "j", "k"]];

function combinations(data) {
const i11 = Math.floor(Math.random() * data.length);
const i12 = Math.floor(Math.random() * data[i11].length);

const dataLength = data[i11].length > 1 ? data.length : data.length - 1;

let i21 = Math.floor(Math.random() * dataLength);
if (i21 >= i11 && data[i11].length === 1) ++i21;

const innerDataLength = i21 === i11 ? data[i21].length - 1 : data[i21].length;
let i22 = Math.floor(Math.random() * innerDataLength);
if (i21 === i11 && i22 >= i12) ++i22;

return [[i11, i12], [i21, i22]];
}

console.log(combinations(data));

for (let i = 0; i < 10000; ++i) {
const [[i11, i12], [i21, i22]] = combinations(data);
if (i11 === i21 && i12 == i22) console.log('Test failed!');
}
``````

### >Solution :

``````function pickNOf(list, n) {
// - create a new shallow array copy.
// - does decouple the original reference, thus it
//   prevents its further mutation by e.g. `splice`.
list = Array.from(list);

// - creates and returns an array of the desired length
//   by a mapping task which constantly slices random
//   items from the shallow array copy.
return Array.from({ length: n }, () => list.splice(
// - `splice` does mutate the list by removing a
//   single item from its randomly chosen index.
Math.floor(Math.random() * list.length), 1
));
}

function pickNUniqueItemsFromNestedArray(arr, n) {
// - flatten nested array structure,
// - channel it through a `Set` in order
//   to keep just unique values,
// - return the result of the forwarded
//   `pickNOf` call.
return pickNOf([...new Set(
arr.flat(Infinity)
)], n);
}

const data = [
["a", "b", "c", "d"],
["e", "g"],
["i", "j", "k"]
];
console.log(
"pickNUniqueItemsFromNestedArray(data, 9) ...",
pickNUniqueItemsFromNestedArray(data, 9)
);
console.log(
"pickNUniqueItemsFromNestedArray(data, 5) ...",
pickNUniqueItemsFromNestedArray(data, 5)
);
console.log(
"pickNUniqueItemsFromNestedArray(data, 3) ...",
pickNUniqueItemsFromNestedArray(data, 3)
);``````
``.as-console-wrapper { min-height: 100%!important; top: 0; }``

Edit due to …

Thank you it works, but i need indexes –
Amine

But that’s not what the question’s topic doe state … "How to pick N random elements from a 2D matrix without duplicates in javascript?" Nevertheless I will provide a second solution which utilizes the first given approach in order to fully meet the OP’s requirements. –
Peter Seliger

In order to meet the OP’s requirements the above given approach changes slightly to …

``````function pickNOf(list, n) {
// - create a new shallow array copy.
// - does decouple the original reference, thus it
//   prevents its further mutation by e.g. `splice`.
list = Array.from(list);

// - creates and returns an array of the desired length
//   by a mapping task which constantly slices random
//   items from the shallow array copy.
return Array.from({ length: n }, () => list.splice(
// - `splice` does mutate the list by removing a
//   single item from its randomly chosen index.
Math.floor(Math.random() * list.length), 1
));
}

function pickNUniqueIndexTuplesFrom2dMatrix(matrix, n) {
// - create a flat array of any of a
//   matrix' index coordinates/tuples.
return pickNOf(
matrix.flatMap((arr, rowIdx) =>
arr.map((_, colIdx) => [rowIdx, colIdx])
), n
);
}

const data = [
["a", "b", "c", "d"],
["e", "g"],
["i", "j", "k"]
];
console.log(
"pickNUniqueIndexTuplesFrom2dMatrix(data, 9) ...",
pickNUniqueIndexTuplesFrom2dMatrix(data, 9)
);
console.log(
"pickNUniqueIndexTuplesFrom2dMatrix(data, 5) ...",
pickNUniqueIndexTuplesFrom2dMatrix(data, 5)
);
console.log(
"pickNUniqueIndexTuplesFrom2dMatrix(data, 3) ...",
pickNUniqueIndexTuplesFrom2dMatrix(data, 3)
);``````
``.as-console-wrapper { min-height: 100%!important; top: 0; }``

Note

Despite or maybe even because of the misunderstanding it might become more clear that the base approach itself does not change. Which is … pick `n` random items from an array without duplicates … and …

• Split the main task into more specialized sub tasks. (Like demonstrated with the implementation of `pickNOf` which solves a single problem perfectly, thus it does not need to be refactored.)

• The other task/s has/have to transform the input data in a way that it can be passed to the specialized task of picking `n` random items from an array without duplicates (which hopefully got proofed by the two different approaches each targeting a different requirement).