Comprehensions in Python and Javascript are only very basic? - javascript

Looking at comprehensions in Python and Javascript, so far I can't see some of the main features that I consider most powerful in comprehensions in languages like Haskell.
Do they allow things like multiple generators? Or are they just a basic map-filter form?
If they don't allow multiple generators, I find them quite disappointing - why have such things been left out?

Python allows multiple generators:
>>> [(x,y,x*y) for x in range(1,5) for y in range(1,5)]
[(1, 1, 1), (1, 2, 2), (1, 3, 3), (1, 4, 4),
(2, 1, 2), (2, 2, 4), (2, 3, 6), (2, 4, 8),
(3, 1, 3), (3, 2, 6), (3, 3, 9), (3, 4, 12),
(4, 1, 4), (4, 2, 8), (4, 3, 12), (4, 4, 16)]
And also restrictions:
>>> [(x,y,x*y) for x in range(1,5) for y in range(1,5) if x*y > 8]
[(3, 3, 9), (3, 4, 12), (4, 3, 12), (4, 4, 16)]
Update: Javascript's syntax is similar (results from using the javascript shell on firefox):
var nums = [1, 2, 3, 21, 22, 30];
var s = eval('[[i,j] for each (i in nums) for each (j in [3,4]) if (i%2 == 0)]');
s.toSource();
[[2, 3], [2, 4], [22, 3], [22, 4], [30, 3], [30, 4]]
(For some reason, something about the context stuff is evaluated in in the javascript shell requires the eval indirection to have list comprehensions work. Javascript inside a <script> tag doesn't require that, of course)

Yes, you can have multiple iterables in a Python list comprehension:
>>> [(x,y) for x in range(2) for y in range(3)]
[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2)]

Add an if statement as well...
>>> [(x,y) for x in range(5) for y in range(6) if x % 3 == 0 and y % 2 == 0]
[(0, 0), (0, 2), (0, 4), (3, 0), (3, 2), (3, 4)]

Comprehensions is very powerful in Haskell to a large extent because Haskell is functional, so it makes extremely much sense for them to be. Python is not functional so it makes less sense.
You can make a lot of complex things with comprehensions in Python but it quickly becomes hard to read, thereby defeating the whole purpose (meaning you should do it some other way).
However, as pointed out here, python does allow multiple generators in comprehensions.

Related

extract 2D array from another 2D array represented as a 1D array

So if I have a 2D array such as
const array = [
[1, 2, 3, 4, 5],
[9, 8, 7, 6, 5],
[1, 8, 3, 6, 5],
[9, 8, 7, 6, 5],
[1, 8, 3, 6, 5],
[1, 2, 3, 4, 5]
];
which could for the point of the question be any size, square or not square.
and I want to extract 3x3 arrays out of it, for instance at 1,1 that would be const sub = [[8, 7, 6], [8, 3, 6], [8, 7, 6]]. So far so good - I can do this. However I am flattening the 2D array so that its represented as a 1D array (long story as to why), i.e
const array = [1, 2, 3, 4, 5, 9, 8, 7, 6, 5, 1, 8, 3, 6, 5, 9, 8, 7, 6, 5, 1, 8, 3, 6, 5, 1, 2, 3, 4, 5];
What I'm trying to do is extract the same (or any) 3x3 array out of this but while its represented as a 1D array, so I would then get back [8, 7, 6, 8, 3, 6, 8, 7, 6].
I almost got there, however I made an error of only working with arrays that were 9x9 and always extracting 3x3 subsets which mean that my solution only works for that specific case, and the more I stare at my solution, the more I cannot work out what the generic solution would look like.
My solution:
const extractSubsetFrom1D = (array, subHeight, subWidth, startRow, startCol) => {
const kWH = subWidth * subHeight
const subset = array.slice(((((kWH - 2) * startRow) + startCol) * kWH), ((((kWH - 2) * startRow) + startCol) * kWH) + kWH)
return subset
}
In my case subHeight and subWidth were always equalling 3 respectively, and as the array itself was always 9x9 I believe I accidentally stumbled on a solution for that specific case as they divide nicely into each other.
To be clear my solution will fail for the startRow = 1 startCol = 0 for the provided array (it works for the startRow = 0 scenario
It's not entirely clear to me how you came to your current implementation, but I can at least tell:
✅ You correctly determined the size of the sub grid array to return (kWH)
❌ You incorrectly assume you can slice out a sub grid as one continuous part of the original 1d array
🟠 The calculation of the first element seems kind-of-right but is actually wrong (probably because of the previous mistake)
From (y,x) to i
Let's start from scratch and work our way up to a nice one liner.
In a 2d-array, you can get a cell's value by doing:
cellValue = grid2d[y][x]
Once you flatten it, you'll need to do:
cellValue = grid1d[y * GRID_WIDTH + x]
y * GRID_WIDTH takes you to the start of the right row, and + x gets you to the right column.
As you can see, you need to know the original grid's size before you can even query a specific cell. That means your extract function would need an argument to pass the original width (or, if the grids are guaranteed to be square, you can do Math.sqrt(array.length).
A slice per row
Let's use this math to find the indices of a 2x2 sub grid at (1,1) extracted from a 3x3 source grid:
0 1 2
3 [4][5]
6 [7][8]
As you can see, the resulting indices are [4,5,7,8]. There is no way to slice these indices out of the source array directly because we don't want to include the 6.
Instead, we can use a nested loop to skip the gaps between our rows:
const to1d = (x, y, w) => y * w + x;
const extractSubGrid1D = (grid, gridWidth, x, y, w, h) => {
const yTop = y;
const yBottom = y + h
const xLeft = x;
const xRight = x + w;
const subgrid = [];
for (let y = yTop; y < yBottom; y += 1) {
for (let x = xLeft; x < xRight; x += 1) {
const index = to1d(x, y, gridWidth);
subgrid.push(grid[index]);
}
}
return subgrid;
}
const originalGrid = [
0, 1, 2,
3, 4, 5,
6, 7, 8
];
console.log(
extractSubGrid1D(originalGrid, 3, 1, 1, 2, 2)
)
Once you get a feel for the logic, feel free to refactor.
The other way around
To go from a 1d-index to a 2d coordinate, you can do:
x = i % w
y = Math.floor(i / w)
Applying this logic, you can also fill your sub grid like so:
Create a new array of the right size
For each of its indices, determine the original grid's (x, y) coordinate
Transform that coordinate back to an index to query the original grid with
const to1d = (x, y, w) => y * w + x;
const extractSubGrid1D = (grid, gridWidth, x, y, w, h) => Array.from(
{ length: w * h },
(_, i) => grid[to1d(x + i % w, y + Math.floor(i / w), gridWidth)]
)
const originalGrid = [
0, 1, 2,
3, 4, 5,
6, 7, 8
];
console.log(
extractSubGrid1D(originalGrid, 3, 1, 1, 2, 2)
)

Subset Sum Variation - subset has X number of items

I'm looking for an algorithm that will allow me to quickly access (not necessarily print) every X item subset of a 300 item set that, when the values of the items in the subset are added up, equals Y. Repetition is allowed and order is important. All values of the 300 item set are positive.
So for example,
300 item set: [1.5, 1.34, 3, .25, 2.333, 1.75, .125, .675, 2, 4, .75, ....]
X = 5
Y = 6
Algorithm generates:
[2, 2, 1.5, .25, .25]
[2, 2, .25, 1.5, .25]
[2, 1.5, 2, .25, .25]
[1.5, 2, 2, .25, .25]
[2, 1.75, .5, .5, 1.25]
[2, 1.75, .5, 1.25, .5]
[2, 1.75, 1.25, .5, .5]
[2, 1.25, .5, .5, 1.75]
[1.25, 2, .5, .5, 1.75]
[.5, 2, .5, 1.25, 1.75]
[3, 1, 1, .5, .5]
[1, 3, 1, .5, .5]
[1, 1, 3, .5, .5]
[1, 1, .5, 3, .5]
[1, 1, .5, .5, 3]
And so on....
[2, 2, 1.5, .25, .25] is allowed and
[2, 1.75, .5, .5, 1.25] is not the same thing as [1.25, .5, .5, 1.75, 2].
I realize that this is a variation of the Subset Sum problem but I can't seem to find any helpful examples of this variation anywhere online. Right now my current solution implements nested loops (the number of nested loops is determined by the value of X) That works fine when X is small but quickly get very slow when X starts going up. Any input would be appreciated!
An approach is to collect subsets and check if the length of the subset and sum fits.
If necessary, you could permutate the subsets.
function subsetSum(array, sum, items) {
function iter(taken, index, subsum) {
if (taken.length === items && subsum === sum) return result.push(taken);
if (taken.length === items || subsum === sum || index === array.length) return;
iter([...taken, array[index]], index, subsum + array[index]);
iter(taken, index + 1, subsum);
}
var result = [];
iter([], 0, 0);
return result;
}
var result = subsetSum([1.5, 1.34, 3, .25, 2.333, 1.75, .125, .675, 2, 4, .75], 6, 5);
console.log(result.map(a => a.join(' ')));
.as-console-wrapper { max-height: 100% !important; top: 0; }

Find array index on a 1d array from a 2d grid javascript

Let's suppose I have the following grid and each cell of the grid has an index mapped to a 1d array.
0, 1, 2
3, 4, 5,
6, 7, 8
I could represent this with a 1d array like: [0, 1, 2, 3, 4, 5, 6, 7, 8]
I would like to know a simple way to map a 2d coordinate like (3,1) to its index in the array, in this case, would be 2.
After researching a lot, I found a lot of people suggesting this equation: index = x + (y * width), but it doesn't seem to work in my tests.
For example for (1, 1), the result would be index = 1 + (1 * 3) = 4, and for (3, 1) would be index = 3 + (1 * 3) = 6, which does not make any sense to me.
Is it possible to achieve this in a simple way? Or I would need to use iterators like a for?
2D matrix notation is commonly (row, col), with indexes starting at 0.
Thus, (3, 1) is invalid: only 3 rows, from 0 to 2. (1, 1) means 2nd row, 2nd colum, which is 4 in your example. The formula is thus:
(row * width) + col
(2, 1) = 2*3+1 = index 7
once again using 0 for the first row/col.
If you really want to keep thinking with indexes starting at one, just change the formula to:
((row - 1) * width) + (col - 1) = 1D index
In your case it would be index = (x - 1) + ((y - 1) * width) as your coordinate system starts from 1 and arrays start from 0.
let arr = [0, 1, 2, 3, 4, 5, 6, 7, 8];
function getPosition(x, y, width) {
return x - 1 + (y - 1) * width;
}
console.log({
position: getPosition(3, 1, 3),
element: arr[getPosition(3, 1, 3)]
});
It is indeed index = x + y * width (the parens are unnecessary) or index = y + x * width, depending on whether you want your flat array to keep the rows together as in your question ([0, 1, 2, 3, 4, 5, 6, 7, 8, 9], index = x + y * width), or keep columns together ([0, 3, 6, 1, 4, 7, 2, 5, 8], index = y + x * width). But indexes usually start at 0, not 1. So your (1, 1) would be (0, 0) and your (3, 1) would be (2, 0).
Here's the first:
// 0, 1, 2
// 3, 4, 5
// 6, 7, 8
const a = [0, 1, 2, 3, 4, 5, 6, 7, 8];
let x = 0, y = 1;
let index = x + y * 3;
console.log(`(${x}, ${y}) is index ${index}, value ${a[index]}`);
x = 2;
y = 0;
index = x + y * 3;
console.log(`(${x}, ${y}) is index ${index}, value ${a[index]}`);
Here's the second:
// 0, 1, 2
// 3, 4, 5
// 6, 7, 8
const a = [0, 3, 6, 1, 4, 7, 2, 5, 8];
let x = 0, y = 1;
let index = y + x * 3;
console.log(`(${x}, ${y}) is index ${index}, value ${a[index]}`);
x = 2;
y = 0;
index = y + x * 3;
console.log(`(${x}, ${y}) is index ${index}, value ${a[index]}`);

Build a chain out of integer pairs

I have a set of integers pairs:
[4, 3], [0, 1], [0, 4], [2, 3], [2, 1]
The task is to chain it to get the output:
[0, 1], [1, 2], [2, 3], [3, 4], [4, 0]
Are there any existed optimised algorithms to do it?
Make a graph where numbers are vertices and pairs are edges.
Check whether Eulerian path exists (all vertex degrees are even (perhaps except for two vertices))
If yes, build this path

How to play a specific frequency with Javascript?

I want a function that works like this:
playSound(345, 1000)
Which would play a tone of 345 hz for 1000 milliseconds. What is the simplest way to achieve this in JavaScript? I don't mind if it uses a sample (maybe of a sin wave, or piano), or uses the computer's hardware to generate the sound.
As already pointed out in the comments, the way to do it is through the OscillatorNode.
// create web audio api context
var audioCtx = new(window.AudioContext || window.webkitAudioContext)();
function playNote(frequency, duration) {
// create Oscillator node
var oscillator = audioCtx.createOscillator();
oscillator.type = 'square';
oscillator.frequency.value = frequency; // value in hertz
oscillator.connect(audioCtx.destination);
oscillator.start();
setTimeout(
function() {
oscillator.stop();
playMelody();
}, duration);
}
function playMelody() {
if (notes.length > 0) {
note = notes.pop();
playNote(note[0], 1000 * 256 / (note[1] * tempo));
}
}
notes = [
[659, 4],
[659, 4],
[659, 4],
[523, 8],
[0, 16],
[783, 16],
[659, 4],
[523, 8],
[0, 16],
[783, 16],
[659, 4],
[0, 4],
[987, 4],
[987, 4],
[987, 4],
[1046, 8],
[0, 16],
[783, 16],
[622, 4],
[523, 8],
[0, 16],
[783, 16],
[659, 4]
];
notes.reverse();
tempo = 100;
playMelody();
There is a library called simpleTones.js that greatly simplifies the Web Audio API to do exactly what you are attempting.
Once the library is included in your project, playing a timed frequency is as easy as calling
playTone(345, sine, 1)
345 being the frequency in Hz, sine being the wave pattern(there are other wave pattern options as well) and "1" being one second, or 1000 milliseconds.
You can download the library and read the documentation here: https://github.com/escottalexander/simpleTones.js
Best of luck on your project.

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