Way to multiply each element of one array to PARTS of another?

Question

I am newbie in programming and try to find out how to multiply

[1,1,0]

to

[4,9,7,2,1,6]

for the next result output

[4,9,7,2,0,0]

As you see I want to multiply each value of [1,1,0] array to each two of second array by shifting in them

[1..] * [4,9..] = [4,9]
[.1.] * [.7,2.] = [7,2]
[..0] * [..1,6] = [0,0]

As example in js i writed something like

var firstArray = [1,1,0];
var secondArray = [4,9,7,2,1,6];
var shift = secondArray / firstArray;
var startpos = 0;
var outArray = [];

for(i=0; i< firstArray.length; i++){
    for(z=i; z< shift+i; z++){
        outArray.push(firstArray[i] * secondArray[z]);
    }
}

console.log(outArray);

It may be in python

You are looking for a solution in python only or in javascript as well? — Pankaj Shukla, May 04 '17 at 09:17

DeepSpace · Answer 1 · 2017-05-04T09:43:05.430

1

You can abuse zip and list slicing:

a = [1, 1, 0]
b = [4, 9, 7, 2, 1, 6]

shift = len(b) // len(a)  # use / in Python 2
li = []
for num_a, num_b1, num_b2 in zip(a, b[::shift], b[1::shift]):
    li.extend([num_a * num_b1, num_a * num_b2])

print(li)
# [4, 9, 7, 2, 0, 0]

edited May 04 '17 at 09:43

answered May 04 '17 at 09:10

DeepSpace

78,697
11
109
154

For what is `shift` used in this example? – Richy May 04 '17 at 09:11
@Richy as the slicing "skip" argument, I edited the answer – DeepSpace May 04 '17 at 09:28
Thank you guys! You are awesome – Max May 04 '17 at 09:47
@DeepSpace now it makes sense :-) – Richy May 04 '17 at 09:55

Nina Scholz · Answer 2 · 2017-05-04T09:16:34.877

In Javascript, you could use a more functional approach by using

Array#reduce for iterating the factors and returning a new array,
Array#concat for adding a part result after multiplying to the result set,
Array#slice, for getting only two elements of the values array,
Array#map for multiplying the part array with the given factor,
at least use an array as start value for reduce.

var factors = [1, 1, 0],
    values = [4, 9, 7, 2, 1, 6],
    result = factors.reduce(
        (r, f, i) => r.concat(
            values
                .slice(i * 2, (i + 1) * 2)
                .map(v => f * v)
        ),
        []
    );

console.log(result);

score 0 · Answer 3 · edited May 23 '17 at 11:54

You can express this as a standard matrix multiplication by making your inputs 2D arrays.

I got the multiplication algorithm from here: https://stackoverflow.com/a/27205510/5710637

The calculation would now look like:

[1,0,0]   [4,9]   [4,9]
[0,1,0] * [7,2] = [7,2]
[0,0,0]   [1,6]   [0,0]

function multiplyMatrices(m1, m2) {
    var result = [];
    for (var i = 0; i < m1.length; i++) {
        result[i] = [];
        for (var j = 0; j < m2[0].length; j++) {
            var sum = 0;
            for (var k = 0; k < m1[0].length; k++) {
                sum += m1[i][k] * m2[k][j];
            }
            result[i][j] = sum;
        }
    }
    return result;
}

var in1 = [[1, 0, 0], [0, 1, 0], [0, 0, 0]];
var in2 = [[4, 9], [7, 2], [1, 6]]

console.log(multiplyMatrices(in1, in2))

Pankaj Shukla · Answer 4 · 2017-05-04T09:20:42.683

You can do this:

Iterate over first array so that you get a number to multiply.
Then extract two elements from the target array and multiply, store the result in res array.

var mul = [1,1,0];


    var target = [4,9,7,2,1,6];
    var start = 0;
    var res = [];
    mul.forEach(function(v,i) {
      var arr = target.slice(start, start+2);//from start index extract two 
                                             //elements
      arr.forEach(function(val,i) {
        res.push(v * val);
      });
      start += 2;
    });

    console.log(res);

score 0 · Answer 5 · answered May 04 '17 at 09:21

You can use map() on array2 and use one var for incrementing index of array1 for each two elements and then multiply current element of array1 with element from array2 that has index as that var.

var a1 = [1,1,0];
var a2 = [4,9,7,2,1,6];

var j = 0;
var result = a2.map(function(e, i) {
  if(i % 2 == 0 && i != 0) j++
  return e * a1[j];
})

console.log(result)

score 0 · Answer 6 · answered May 04 '17 at 09:21

In python you can use the following code. I'm assuming the length of the second array is larger and it is divisible by the length of the shorter array

def multiply(a, b):
    x, y = len(a), len(b)                        # Find lengths
    assert x % y == 0                            
    n = x / y
    result = [a[i] * b[i / n] for i in range(x)] # For each index in a, calculate the appropriate element in output by multiplying with relevant part
    print result

Way to multiply each element of one array to PARTS of another?

6 Answers6