Python vs Java Loop

Question

I am working through some problems on HackerRank, and I thought I would try implementing the same solution in Python that I had already solved correctly in Java. Although my code almost exactly mirrors my previous Python solution, I am getting an out of bounds exception in the if input_str[i-1] == input_str[i] line. Is there different behavior in a Python loop that might be causing this discrepancy? The test cases are the same across both.

public class Solution {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String input = sc.nextLine();
        solve(input);
    }
    public static void solve(String str) {
        String s = new String(str);
        for (int i=1; i < s.length(); i++) {
           if (s.charAt(i-1) == s.charAt(i)) {
               s = s.substring(0, i-1) + s.substring(i+1, s.length());
               i = 0;
           }
           if (s.length() == 0) {
               System.out.println("Empty String");
               return;
           }
        }
        System.out.println(s);
    }
}

And this is the code for the same problem, but using Python 2.7.

input_str = raw_input()
for i in xrange(1, len(input_str)):
    if input_str[i-1] == input_str[i]:
        input_str = input_str[0:i-1] + input_str[i+1:len(input_str)]
        i = 0
    if len(input_str) == 0:
        print "Empty String"
        break

print input_str

Answer 1

In the Java loop, s.length() is recalculated every iteration. In Python, len(input_str) is only calculated once and does not reflect the correct length after you've modified input_str in the if block.

Similarly, assigning i = 0 does not work the way you want it to. i will take on the next value in the xrange , ignoring your attempted reset of its value.

Answer 2

I believe you are wasting a lot of time and system resources by doing it that way. A better solution would be to change the algorithm into one that only iterates input_str only once. In which case, you get O(n) execution time. My edited code looks thus:

input_str = input()
input_strR = input_str[0]
for i in range(1, len(input_str)):
    if input_str[i-1] != input_str[i]:
        input_strR = input_strR + input_str[i]
    if len(input_str) == 0:
        print ("Empty String")
        break

print (input_strR)

In this case, a new variable was introduced and all first characters were added while duplicates were dropped.

Answer 3

I would build on funaquarius24 and not even use indexes:

input_str = raw_input()
result = input_str[0]
for c in input_str[1:]:
    if c != result[-1]:
        result += c
print(result)