For loop Difference Between Python and Java

Jie Zhang :

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 :

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.