Why minindex Does NOT Reset After the Inner Loop in Selection Sort

[Rajeev Bagra]

A beginner learning Selection Sort in Python may naturally think:

• minindex changes only temporarily inside the inner loop
• once the inner loop ends, minindex automatically becomes i again
• the inner loop has its own separate “temporary” version of minindex

This often creates another layer of confusion:

• Does a loop create a separate variable scope in Python?
• Why does minindex still contain the last updated value?
• How does Selection Sort remember the smallest element found?

The following Q&A explains the deeper conceptual idea behind variable updates inside loops in Python.

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Q1. Why do beginners think minindex resets automatically?

Consider:

for i in range(len(L)):

minindex = i

for j in range(i, len(L)):

    if L[j] < L[minindex]:
        minindex = j

 

Since:

• minindex = i appears before the inner loop
• minindex = j happens inside the inner loop

many learners imagine:

The inner loop temporarily changes minindex, but once the loop ends, it becomes i again.

But Python does not work that way.

---

Q2. Does a loop create a separate variable scope in Python?

No.

In Python:

• if blocks do not create separate variable scopes
• for loops do not create separate variable scopes
• variables updated inside loops still exist outside the loop

So:

x = 5

for i in range(3):
    x = 10

print(x)

Output:

10

The value survives after the loop ends.

---

Q3. What actually happens to minindex during Selection Sort?

Consider:

L = [7, 3, 9, 1]

Outer loop starts:

i = 0
minindex = 0

Now the inner loop begins searching for a smaller value.

---

Q4. What happens during the inner loop?

First iteration:

j = 0

Comparison:

7 < 7

False.

So:

minindex = 0

still remains.

---

Second iteration:

j = 1

Comparison:

3 < 7

True.

So now:

minindex = 1

---

Third iteration:

j = 2

Comparison:

9 < 3

False.

So:

minindex = 1

still remains.

---

Fourth iteration:

j = 3

Comparison:

1 < 3

True.

Now:

minindex = 3

---

Q5. What happens after the inner loop ends?

A beginner may expect:

minindex = 0

again automatically.

But Python does NOT reset it.

The variable still contains:

minindex = 3

because that was the last assigned value.

---

Q6. Why is this behavior important for Selection Sort?

Selection Sort needs to remember:

Which position contained the smallest value found during scanning.

That remembered position is stored in:

minindex

After the loop ends, the algorithm uses that saved index for swapping.

---

Q7. What swap happens next?

The algorithm performs:

L[i], L[minindex] = L[minindex], L[i]

which becomes:

L[0], L[3] = L[3], L[0]

Result:

[1, 3, 9, 7]

The algorithm successfully moves the smallest value to the correct position.

---

Q8. When does minindex become i again?

Only when the next outer-loop iteration starts.

Python executes again:

minindex = i

For example:

i = 1
minindex = 1

So:

• minindex is reinitialized once per outer-loop cycle
• it is NOT automatically reset after the inner loop ends

---

Q9. What is a simple mental model for this?

Think of:

minindex

as a sticky note.

Initially:

minindex = i

means:

“Currently I believe this position contains the minimum value.”

During scanning, whenever a smaller value is found, the sticky note is updated:

minindex = j

The sticky note keeps its latest value until something changes it again.

---

Q10. What is the biggest conceptual takeaway?

The major insight is:

Loops in Python do not create temporary versions of variables.

So:

• changes made inside the inner loop survive after the loop ends
• minindex keeps the last smallest index found
• Selection Sort depends on preserving that value for swapping

Once this mental model becomes clear, the logic of Selection Sort becomes much easier to understand.

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