For loop Difference Between Python and Java

Question

In Python we do for loop like:

for i in range(len(nums))

In java: we have:

for (int i = 0; i < nums.length; i++)

Are these two for loop identical? If we made some change within the for loop, let's say in the for loop i has been plus 3, for Java, next i in for loop will be 4? while Python still start i from 2

Leetcode 594. Longest Harmonious Subsequence.

We define a harmonious array is an array where the difference between its maximum value and its minimum value is exactly 1

The solution written in Java as follow:

nums=[1,3,2,2,5,2,3,7]
public class Solution {
    public int findLHS(int[] nums) {
        Arrays.sort(nums);
        int prev_count = 1, res = 0;
        for (int i = 0; i < nums.length; i++) {
            int count = 1;
            if (i > 0 && nums[i] - nums[i - 1] == 1) {
                while (i < nums.length - 1 && nums[i] == nums[i + 1]) {
                    count++;
                    i++;
                }
                res = Math.max(res, count + prev_count);
                prev_count = count;
            } else {
                while (i < nums.length - 1 && nums[i] == nums[i + 1]) {
                    count++;
                    i++;
                }
                prev_count = count;
            }
        }
        return res;
    }
}

I converted to Python:

nums=[1,3,2,2,5,2,3,7]

nums=sorted(nums)
prev_count=1
res=0
i=0
for i in range(len(nums)-1):
    count=1
    if i>0 and nums[i]-nums[i-1]==1:
        while i<len(nums)-1 and nums[i] == nums[i+1]:
            count+=1
            i+=1
        res=max(res,count+prev_count)
        prev_count=count
    else:
        while i<len(nums)-1 and nums[i] == nums[i+1]:
            count+=1
            i+=1

        prev_count=count

print (res)

In Java

for (int i = 0; i < nums.length; i++) {
            int count = 1;
            if (i > 0 && nums[i] - nums[i - 1] == 1) {
                while (i < nums.length - 1 && nums[i] == nums[i + 1]) {
                    count++;
                    i++;
                }

i++ inside for loop so the i started from whatever i has been added.

In Python:

for i in range(len(nums)-1):
    count=1
    if i>0 and nums[i]-nums[i-1]==1:
        while i<len(nums)-1 and nums[i] == nums[i+1]:
            count+=1
            i+=1

after i+=1, it only applied to While loop, for loop still start from i=2 not 4 instead.

Java returns answer as 5 while python is 4. I debug the code and looks like Java start i for whatever i has been added while Python didn't take added i and always start for last i.

rdas · Accepted Answer · 2019-04-19 17:18:07Z

In java the for loop semantics are borrowed from C.

for (<initilization>; <termination condition>; <what to do in after each iteration>)

Do something at the start(intialization), after that until some condition is reached (termination condition), do something to make progress (what to do after each iteration). The idiomatic for-loop with i works because the state of the iteration is maintained within i. So if you make changes to i in the body of the loop, the state of iteration also changes.

The python syntax is akin to the bash loops:

for i in some_iterable:

Here i takes on each of the values from the some_iterable and the loop runs once for each value of i. If you change i within the body of the loop, doesn't matter; i is assigned the next value from the iterable during the next iteration. The state of the loop is maintained in the iterable, not i. i is just what lets you access the current value of the iterable.

Matt · Accepted Answer · 2019-04-19 17:24:10Z

2

Python for-loops are essentially the same as Java's enhanced for-loops. For your example, since range(len(nums)) returns [0, 1, 2, ...], these two are more or less equivalent:

Python:

    array = [0, 1, 2, 3, 4, 5, 6]
    for i in array:
        // i represents each item in the array

Java:

    int[] array = {0, 1, 2, 3, 4, 5, 6};
    for (int i : array) {
        // i represents each item in the array
    }

answered Apr 19, 2019 at 17:24

Matt

1,20211 silver badges21 bronze badges

Comments

Patrick Artner · Accepted Answer · 2019-04-20 08:51:22Z

This does not work in python - the i is "reset" every time it gets back to the for i in .... :

for i in range(20) :
   print(i)    # prints i
   i += 99     # has no influence over the next iterations i
   print(i)    # prints (i + 99)

A way to solve it in python would be:

from collections import Counter

nums=[1,3,2,2,5,2,3,7]

c = Counter(nums)

# create possible keys from c that are 1 apart
one_apart_keys = [ (a, a+1) for a in c if a+1 in c]     

# get the key that has the max value of counts
# will pick first one if multiple equals possible
max_key = max(one_apart_keys, key = lambda x: c[x[0]]+c[x[1]]) 

# get all the numbers in order from list
collec = [x for x in nums if x in max_key]  

print(collec)

# c is                Counter({2: 3, 3: 2, 1: 1, 5: 1, 7: 1})
# one_apart_keys is   [(1, 2), (2, 3)]
# max_key is          (2, 3)

Output:

[3, 2, 2, 2, 3]

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