Placement of variable declaration in Ruby (beginner) - ruby

Question

I'm working through some online practice problems to learn Ruby, and while I was able to solve this one, I'm struggling to understand something about the placement of variable declaration.
Why does the following code work when I declare/define "pair" (sum of array indices) inside the idx2 while-loop, but not when I do so directly after declaring the indices themselves?
def two_sum(nums)
idx=0
idx2=0
while idx<nums.length-1
idx2=idx+1
while idx2<nums.length
pair=nums[idx]+nums[idx2]
if pair==0
return [idx, idx2]
else
idx2+=1
end
end
idx+=1
end
end
The version above works, but the structure below does not.
def two_sum(nums)
idx=0
idx2=0
pair=nums[idx]+nums[idx2]
while idx<nums.length-1
idx2=idx+1
while idx2<nums.length
if pair==0
return [idx, idx2]
else
idx2+=1
end
end
idx+=1
end
end
If anyone can provide an explanation or some entry-level resources about this, I'd greatly appreciate it. Thanks.
Edit:
Sorry for indentation and specificity issues regarding desired output. I'm entirely new to this and sometimes details slip by, I appreciate the feedback. Thanks for the step-by-step logical run through, that's exactly what I was looking for.

Answer 1

Let's look at the working code and the broken code one at a time.
Let's say nums = [1, 2, -1, 3]
Supposably Working Code
1 def two_sum(nums)
2 idx = 0
3 idx2 = 0
4
5 while idx < nums.length-1
6 idx2 = idx+1
7 while idx2 < nums.length
8 pair = nums[idx] + nums[idx2]
9 if pair == 0
10 return [idx, idx2]
11 else
12 idx2 += 1
13 end
14 end
15 idx += 1
16 end
17 end
When we start off
idx = 0, idx2 = 0
Now we are in the first while loop (line 6)
idx = 0, idx2 = 1
2nd while loop (line 8)
idx = 0, idx2 = 1, pair = nums[0] + nums[1] = 3
pair != 0, so we are now in the else statement. (line 12)
idx = 0, idx2 = 2, pair is still 3
We now loop back to the 2nd while loop (line 8 again)
idx = 0, idx2 = 2, pair = nums[0] + nums[2] = 0
pair == 0, so we return [0, 2] (line 10)
This is how the code is suppose to run.
Broken Code
1 def two_sum(nums)
2 idx = 0
3 idx2 = 0
4 pair = nums[idx] + nums[idx2]
5 while idx < nums.length-1
6 idx2 = idx+1
7 while idx2 < nums.length
8
9 if pair == 0
10 return [idx, idx2]
11 else
12 idx2 += 1
13 end
14 end
15 idx += 1
16 end
17 end
We will run through the same logic again.
Before we hit the first while loop (line 2 - 4)
idx = 0, idx2 = 0, pair = nums[0] + nums[0] = 2
Now we are in the first while loop, (line 6)
idx = 0, idx2 = 1, pair = 2
And now in second while loop, pair != 0, so we hit the else statement, (line 12)
idx = 0, idx2 = 2, pair = 2
Now we loop back to the start of the second while loop. pair != 2, so we hit the else statement.
idx = 0, idx2 = 3, pair = 2
pair != 0, so we hit the else statement.
idx = 0, idx2 = 4, pair = 2
So now, idx2 < nums.length is false, so we exit the second while loop, and nowidx = 1`, and we continue to cycle.
As you can see, because you declared pair outside of the while loop, it never recalculates to different values, so you will never hit the if statement, because pair will never equal zero, unless of course, you get 0 on the initial sum.
while loops are meant for cases where you are not sure how many loops you need. An example is checking a user's entry, where you may need to loop once, or ten times, depending on when the user gets the entry correct. In this case, you know exactly how many times you need to run. So, an iterator is better for the job here.
Here's an example of how that can be written. Keep in mind there's probably better ways to do this, I just want to show an example.
def two_sum(nums)
nums.each_with_index do |num1, idx1|
nums.each_with_index do |num2, idx2|
next if idx1 == idx2
return [idx1, idx2] if num1 + num2 == 0
end
end
end