Convert list comprehension python to javascript

Question

I am trying to convert some python code into JavaScript code.

I have a python list comprehension that take the paramente a as input. a is a string such as "bac".

asubstring = [a[i:i + j] for j in range(1, len(a) + 1) for i in range(len(a) - j + 1)]

The output is: ['b', 'a', 'c', 'ba', 'ac', 'bac']

I converted it into JavaScript by doing:

let j = 1
let i = 0
while(j < aTrimmed.length+1) {
    while(i < aTrimmed.length - j + 1) {
       aSubstring.push(aTrimmed.slice(i, i+j))
       i++
    }
    j++
}

However, my output is: [ 'b', 'a', 'c' ]

I am not sure what I am missing in the two while loops.

You didn't reset i = 0 in the inner loop. I would suggest writing this using nested for loops, then it's harder to make that mistake. — kaya3
– kaya3, Commented Feb 7, 2020 at 11:44

Aurea · Accepted Answer · 2020-02-07 11:54:28Z

1

The issue is that you are not resetting the i variable in the inner loop. In the Python version, for i in range(...) always makes i start from 0. Additionally, declaring an iteration variable, then using a while loop which ends in incrementing that variable is a bit weird, why not just use a for loop?

let aTrimmed = "abc";
let aSubstring = [];

for (let j = 1; j <= aTrimmed.length; j++) {
  for (let i = 0; i <= aTrimmed.length - j; i++) {
    aSubstring.push(aTrimmed.slice(i, i + j));
  }
}

console.log(aSubstring);

Also note the <= instead of < ... + 1.

answered Feb 7, 2020 at 11:54

Aurea

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Comments

Sapikelio · Accepted Answer · 2020-02-07 11:53:45Z

1

Use for loop instead of while because you are forgetting reset you i index to 0 at the end of the second loop.

let aTrimmed = "bac";
let aSubstring = [];


for(let j = 1; j < aTrimmed.length+1; j++) {
    for(let i = 0; i < aTrimmed.length - j + 1; i++) {
       aSubstring.push(aTrimmed.slice(i, i + j));
    }
}
alert(aSubstring);

answered Feb 7, 2020 at 11:53

Sapikelio

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Ben Aston · Accepted Answer · 2020-02-07 16:01:48Z

0

This version works more favourably with characters outside of the Unicode Basic Multilingual Plane. Although there remain issues with grapheme clusters.

let s1 = "abc"
let s2 = "🧽🍿🛥"

function substrings(s) {
    let us = [...s]
    let result = []

    for (let j = 1; j <= us.length; j++) {
      for (let i = 0; i <= us.length - j; i++) {
        result.push(us.slice(i, i + j).join(''))
      }
    }

    return result
}

console.log(substrings(s1))
console.log(substrings(s2))

answered Feb 7, 2020 at 16:01

Ben Aston

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