How do I create a nested for loop where I have control of the initial loop index value

Question

There doesn't seem to be a way to write this type of for loop in Python because I'm trying to translate/rewrite this javascript code into Python. How do I set the inital loop index like j in the nested loop?

Here's my JS code:

// Write a function called findGreaterNumbers which accepts an array and returns the number of times a number is followed by a larger number.
// Examples:
// findGreaterNumbers([1,2,3]) // 3 (2 > 1, 3 > 2, and 3 > 1)
// findGreaterNumbers([6,1,2,7]) // 4
// findGreaterNumbers([5,4,3,2,1]) // 0
// findGreaterNumbers([]) // 0

function findGreaterNumbers(arr) {
  let count = 0
  for (let i= 0; i < arr.length; i++){
    for (let j= i + 1; j < arr.length; j++){
      if(arr[j] > arr[i]){
        count++;
    }
  }
}
  return count;
}

w3schools.com/python/ref_func_range.asp

JacobIRR
– JacobIRR

2020-02-05 00:10:23 +00:00
Commented Feb 5, 2020 at 0:10 — JacobIRR
– JacobIRR, Commented Feb 5, 2020 at 0:10

dcg · Accepted Answer · 2020-02-05 00:12:06Z

2

You can use the range built-in like the following:

for i in range(len(arr)):
    for j in range(i+1, len(arr)):
        # work with i and j as required

answered Feb 5, 2020 at 0:12

dcg

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Comments

blhsing · Accepted Answer · 2020-02-05 00:25:13Z

2

You can use itertools.combinations for the purpose:

from itertools import combinations
def findGreaterNumbers(arr):
    return sum(b > a for a, b in combinations(arr, 2))

edited Feb 5, 2020 at 0:25

answered Feb 5, 2020 at 0:19

blhsing

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Sơn Lê Trường · Accepted Answer · 2020-02-05 07:13:28Z

1

control of the initial loop index value:

In javascript: for (let i= 0; i < arr.length; i++){

In Python: for i in range(0, len(arr), 1) :

or: for i in range( len(arr) ) :

Loop in Loop:

In javascript:

  for (let i= 0; i < arr.length; i++){
     for (let j= i + 1; j < arr.length; j++){

In Python:

for i in range(len(arr)):
    for j in range(i+1, len(arr), 1):

Rewrite this javascript code into Python

def findGreaterNumbers(arr):
    count = 0
    for i in range(len(arr)):
        for j in range(i + 1, len(arr), 1):
            if arr[j] > arr[i]:
                count += 1
    return count

Currently, you may not see people using loop in loop in python for simple tasks, instead people often uselist comprehension. Examples :

arr = [6, 1, 2, 7]
result = [f'{x} < {i}' for c, x in enumerate(arr) for i in arr[c:] if x < i]
# ['6 < 7', '1 < 2', '1 < 7', '2 < 7']
len(result) # 4

answered Feb 5, 2020 at 7:13

Sơn Lê Trường

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1 Comment

tedico Over a year ago

Cool. Thanks so much for the detailed explanation! The list comprehension is sort of a mind-bender for me right now but I get the gist on how it works. Thanks again!

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