javascript map and reduce array list

Question

I have a list of array in this format

 data = [[1,2,3,4,5,6,7,8,9,10],
 [11,12,13,14,15,16,17,18,19,20],
 [21,22,23,24,25,26,27,28,29,30]]

I want the output in this format

    [[11/1, 12/2,13/3,14/4,15/5,16/6,17/7,18/8,19/9,20/10],
     [21/11,22/12,23/13,24/14,25/15,26/16,27/17,28/18,29/19,30/20]]

I have used for loop and this is how it looks

const totalData = data.length;
for(var i =0 ; i < totalData ; i++){
  for(var j =0; j < data[i].length; j++){
    console.log(data[i+1][j]/data[i][j]);
  }
}

I want to convert this using javascript map and reduce? is there any possible ways? Thank you

Answer 1

for loops aren't bad practice, they just don't fit in a functional style of programming. The following solution presupposes that the arrays' lengths are equal:

 const data = [ [1,2,3,4,5,6,7,8,9,10], [11,12,13,14,15,16,17,18,19,20], [21,22,23,24,25,26,27,28,29,30] ]; const result = data.map((arr, index) => { const next = data[index + 1]; if (!Array.isArray(next)) { return; } return arr.map((item, i) => next[i] / item); }).filter(Boolean); console.log(result); 

I am sure you can figure out what to do if the arrays' lengths are not equal.

Answer 2

You could use a single reduce and map the items for a new array.

 var data = [[1, 2, 3, 4, 5, 6, 7, 8, 9, 10], [11, 12, 13, 14, 15, 16, 17, 18, 19, 20], [21, 22, 23, 24, 25, 26, 27, 28, 29, 30]], result = []; data.reduce((a, b) => (result.push(a.map((c, i) => b[i] / c)), b)); console.log(result); 

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Answer 3

You need to create separated functions for each task:

/**
 * Combine 2 arrays
 * @param Array a 
 * @param Array b 
 */
function combineArrays(a, b) {

    var combined = [];

    for (var i = 0; i < a.length; i++)
        combined.push(a[i] / b[i]);

    return combined;
}

/**
 * Combine an group of arrays
 * @param Array group 
 */
function combineGroup(group) {
    var newGroup = [];
    for (var i = 0; i < group.length - 1; i++)
        newGroup.push(combineArrays(group[i], group[i + 1]));

    return newGroup;

}

The first function combine 2 arrays only. The second function combine many arrays and return what you want.

Answer 4

if the lengths are not equal, the items in the tail of the longer one are ignored.

data = [
  [1,2,3,4,5,6,7,8,9,10],
  [11,12,13,14,15,16,17,18,19,20],
  [21,22,23,24,25,26,27,28,29,30]
];

result = data.reduce((acc,current,i)=>{
      var ret;
      if(data[i].length <= data[i-1].length)
        ret = data[i].map((item,j)=>(item / data[i-1][j]));
      else
        ret = data[i-1].map((item,j)=>(data[i][j] / item));
      if(i==1)
        return [ret]
      else
        return acc.concat([ret])
})
console.log(result)