Please help me figure out what is it doing and why is it doing that.

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I was practicing python , and i dabbled into this problem "

Write a Python program to find three integers which gives the sum of zero in a given array of integers using Binary Search (bisect)."

<pre lang="Python">
<pre>
max_num = nums[-1]
        for i, v in enumerate(nums):
            if num_freq[v] >= 2:
                complement =  -2 * v
What is "complement = -2 * v " doing ? 
I can't figure out this part of the code.I can't understand what is "complement" doing here.

What I have tried:

<pre lang="Python">
from bisect import bisect, bisect_left
from collections import Counter
class Solution:
    def three_Sum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        triplets = []
        if len(nums) < 3:
            return triplets
        num_freq = Counter(nums)
        nums = sorted(num_freq)  # Sorted unique numbers
        max_num = nums[-1]
        for i, v in enumerate(nums):
            if num_freq[v] >= 2:
                complement =  -2 * v
                if complement in num_freq:
                    if complement != v or num_freq[v] >= 3:
                        triplets.append([v] * 2 + [complement])

            # When all 3 numbers are different.
            if v < 0:  # Only when v is the smallest
                two_sum = -v

                # Lower/upper bound of the smaller of remainingtwo.
                lb = bisect_left(nums, two_sum - max_num, i + 1)
                ub = bisect(nums, two_sum // 2, lb)                       
                for u in nums[lb : ub]:
                    complement = two_sum - u
                    if complement in num_freq and u != complement:
                        triplets.append([v, u, complement])
        return triplets
nums = [-20, 0, 20, 40, -20, -40, 80]
s = Solution()
result = s.three_Sum(nums)
print(result)
nums = [1, 2, 3, 4, 5, -6]
result = s.three_Sum(nums)
print(result)

Posted 30-Jan-21 3:51am

Ryan MaCanzie

Updated 30-Jan-21 6:32am

v3

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Comments

Patrice T 30-Jan-21 9:58am

" can't figure out what it is doing and why is it doing that."
This is not a description of a problem.

Ryan MaCanzie 30-Jan-21 10:06am

It doesn't have any errors, but i want to know is " max_num = nums[-1]
for i, v in enumerate(nums):
if num_freq[v] >= 2:
complement = -2 * v" what is this part of the code doing ?

Patrice T 30-Jan-21 10:12am

Use Improve question to update your question.
So that everyone can pay attention to this information.

2 solutions

Solution 2

Quote:
i can't figure out what it is doing and why is it doing that.

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

27.3. pdb — The Python Debugger — Python 3.6.1 documentation[^]
Debugging in Python | Python Conquers The Universe[^]
pdb – Interactive Debugger - Python Module of the Week[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

Posted 30-Jan-21 4:18am

Patrice T

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OriginalGriff · Accepted Answer · 2021-01-30T04:02:00

That's the problem when you search for code on the internet: if it doesn't do exactly what you need to hand in, it can be very difficult to work out what you have to change to get it to do that.

So if you are going to use that code, you need to work out exactly what it is doing, and how it does that - and the best tool for that is the debugger: pdb — The Python Debugger — Python 3.9.1 documentation[^]

Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!