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---
title: Ethiopian multiplication
id: 599d1566a02b571412643b84
challengeType: 5
---
## Description
< section id = 'description' >
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Ethiopian multiplication is a method of multiplying integers using only addition, doubling, and halving.
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< strong > Method:< / strong >
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< ol >
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< li > Take two numbers to be multiplied and write them down at the top of two columns< / li >
< li > In the left-hand column repeatedly halve the last number, discarding any remainders, and write the result below the last in the same column, until you write a value of < code > 1< / code > < / li >
< li > In the right-hand column repeatedly double the last number and write the result below. stop when you add a result in the same row as where the left hand column shows < code > 1< / code > < / li >
< li > Examine the table produced and discard any row where the value in the left column is even< / li >
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< li > Sum the values in the right-hand column that remain to produce the result of multiplying the original two numbers together< / li >
< / ol >
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< strong > For example:< / strong > < code > 17 × 34< / code >
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< pre >
17 34
< / pre >
Halving the first column:
< pre >
17 34
8
4
2
1
< / pre >
Doubling the second column:
< pre >
17 34
8 68
4 136
2 272
1 544
< / pre >
Strike-out rows whose first cell is even:
< pre >
17 34
8 < strike > 68< / strike >
4 < strike > 136< / strike >
2 < strike > 272< / strike >
1 544
< / pre >
Sum the remaining numbers in the right-hand column:
< pre >
17 34
8 --
4 ---
2 ---
1 544
====
578
< / pre >
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So < code > 17< / code > multiplied by < code > 34< / code > , by the Ethiopian method is < code > 578< / code > .
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< / section >
## Instructions
< section id = 'instructions' >
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The task is to define three named functions/methods/procedures/subroutines:
< ol >
< li > one to halve an integer,< / li >
< li > one to double an integer, and< / li >
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< li > one to state if an integer is even< / li >
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< / ol >
Use these functions to create a function that does Ethiopian multiplication.
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< / section >
## Tests
< section id = 'tests' >
```yml
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tests:
- text: < code > eth_mult</ code > is a function.
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testString: assert(typeof eth_mult === 'function', '< code > eth_mult< / code > is a function.');
- text: < code > eth_mult(17,34)</ code > should return < code > 578</ code > .
testString: assert.equal(eth_mult(17, 34), 578, '< code > eth_mult(17,34)< / code > should return < code > 578< / code > .');
- text: < code > eth_mult(23,46)</ code > should return < code > 1058</ code > .
testString: assert.equal(eth_mult(23, 46), 1058, '< code > eth_mult(23,46)< / code > should return < code > 1058< / code > .');
- text: < code > eth_mult(12,27)</ code > should return < code > 324</ code > .
testString: assert.equal(eth_mult(12, 27), 324, '< code > eth_mult(12,27)< / code > should return < code > 324< / code > .');
- text: < code > eth_mult(56,98)</ code > should return < code > 5488</ code > .
testString: assert.equal(eth_mult(56, 98), 5488, '< code > eth_mult(56,98)< / code > should return < code > 5488< / code > .');
- text: < code > eth_mult(63,74)</ code > should return < code > 4662</ code > .
testString: assert.equal(eth_mult(63, 74), 4662, '< code > eth_mult(63,74)< / code > should return < code > 4662< / code > .');
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```
< / section >
## Challenge Seed
< section id = 'challengeSeed' >
< div id = 'js-seed' >
```js
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function eth_mult(a, b) {
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// Good luck!
}
```
< / div >
< / section >
## Solution
< section id = 'solution' >
```js
function eth_mult(a, b) {
let sum = 0; a = [a]; b = [b];
let half = a => a / 2,
double = a => a * 2,
is_even = a => a % 2 == 0;
while (a[0] !== 1) {
a.unshift(Math.floor(half(a[0])));
b.unshift(double(b[0]));
}
for (let i = a.length - 1; i > 0; i -= 1) {
if (!is_even(a[i])) {
sum += b[i];
}
}
return sum + b[0];
}
```
< / section >