issues with array.concat()

Question

I'm trying to use a recursive call to concat a return array

Directions to this problem are: A stream of data is received and needs to be reversed.

Each segment is 8 bits long, meaning the order of these segments needs to be reversed, for example:

11111111 00000000 00001111 10101010 (byte1) (byte2) (byte3) (byte4) should become:

10101010 00001111 00000000 11111111 (byte4) (byte3) (byte2) (byte1) The total number of bits will always be a multiple of 8.

trying different combinations of things really...

 function dataReverse(data) { //split incoming array into array values consisting of 8 numbers each. //var octGroups = data.length / 8; var result = []; //recursive call function shuffler(array){ let input = array; //base case if(input.length === 0){ return result; } else { //concat result with 8 values at a time let cache = input.splice(-8,8); result.concat(cache); return shuffler(input); } return result; } var reversed = shuffler(data); //base case is if data.length === 0 return result else //reverse iterate through array, concating to new return array //return result return reversed; } console.log(dataReverse([1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,1,0,1,0,1,0,1,0])); 

it is expected to reverse through the input array, concating a result array with 8 values at a time starting at the end, but not reversing the order of the numbers.

My attempt above returns a zero length array. What have I done wrong?

Answer 1

Use join instead of concat

 function dataReverse(data) { //split incoming array into array values consisting of 8 numbers each. //var octGroups = data.length / 8; var result = []; //recursive call function shuffler(array) { let input = array; //base case if (input.length === 0) { return result; } else { //concat result with 8 values at a time let cache = input.splice(-8, 8); result.push(cache.join('')); return shuffler(input); } return result; } var reversed = shuffler(data); //base case is if data.length === 0 return result else //reverse iterate through array, concating to new return array //return result return reversed; } console.log(dataReverse([1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 0, 1, 0])); 

Answer 2

concat returns a new array you need to assign it back to result

result = result.concat(cache);

And if you want each byte as string of 8 characters you can use join

result = result.concat(cache.join(''));

 function dataReverse(data) { //split incoming array into array values consisting of 8 numbers each. //var octGroups = data.length / 8; var result = []; //recursive call function shuffler(array) { let input = array; //base case if (input.length === 0) { return result; } else { //concat result with 8 values at a time let cache = input.splice(-8, 8); result = result.concat(cache); return shuffler(input); } return result; } var reversed = shuffler(data); //base case is if data.length === 0 return result else //reverse iterate through array, concating to new return array //return result return reversed; } console.log(dataReverse([1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 0, 1, 0])); 

You can just loop over array create group of 8 byte each and then reverse and then reduce back to single array

 let dataReverse = (data) => { let count = 0 let temp = [] let group = data.reduce((op, inp) => { temp.push(inp) if (count === 8) { op.push(temp) temp = [] } return op }, []) if (temp.length) group.push(temp) return group.reverse().reduce((op,inp)=>op.concat(inp)) } console.log(dataReverse([1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 0, 1, 0])); 

Answer 3

concat returns an array - assign the result:

 function dataReverse(data) { //split incoming array into array values consisting of 8 numbers each. //var octGroups = data.length / 8; var result = []; //recursive call function shuffler(array) { let input = array; //base case if (input.length === 0) { return result; } else { //concat result with 8 values at a time let cache = input.splice(-8, 8); result = result.concat(cache); return shuffler(input); } return result; } var reversed = shuffler(data); //base case is if data.length === 0 return result else //reverse iterate through array, concating to new return array //return result return reversed; } let reversed = dataReverse([1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 0, 1, 0]); //Code for pretty-printing in groups of 8 reversed = reversed.reduce((acc, curr, i) => { const c = Math.floor(i / 8); acc[c] = [].concat((acc[c] || []), curr); return acc; }, []); console.log(reversed.map(e => e.join(""))); 

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(Array chunk from this answer ).

Answer 4

You can chunk the array by 8 pieces each and then just use Array.reverse and Array.flat . The benefit of this is that you get a nice functional, reusable, chain-able and very readable code.

Consider this:

 let data = [1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,1,0,1,0,1,0,1,0] // utility function to chunk array by number const chunkBy = (arr, by=2) => arr.reduce((r,c,i) => (i%by==0 ? r.push([c]) : r[r.length-1] = [...r[r.length-1], c], r), []) let result = chunkBy(data, 8).reverse().flat() console.log('in: ', data.join('')) console.log('out: ', result.join('')) 

Here is the chunkBy function in more readable form:

 let data = [1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,1,0,1,0,1,0,1,0] const chunkBy = (arr, by=2) => // default to chunks of 2 arr.reduce((acc, cur, index) => { if(index % by == 0) // use modulo to check for the remainder acc.push([cur]) // if exact then we start a new chunk else // if not we keep adding to the previous chunk acc[acc.length-1] = [...acc[acc.length-1], cur] return acc }, []) console.log(chunkBy(data, 8)) 

If you are using lodash _.chunk is already there:

 let data = [1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,1,0,1,0,1,0,1,0] let result = _(data) .chunk(8) .reverse() .flatten() .value() console.log(result.join('')) 

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