Hey everyone! I've been watching an intro to programming series on YouTube from freeCodeCamp, and I recently learned about recursion. They explained that a recursive function is one that calls itself, which seemed pretty simple at first. However, when I looked up examples, I came across this code from w3schools that I modified slightly to accept user input.
Here's the code I'm working with:
```python
print("Recursion ")
print()
k = int(input("Enter number: "))
def tri_recursion(k):
if (k > 0):
result = k + tri_recursion(k - 1)
print(result)
else:
result = 0
return result
print("nnRecursion Example Results")
tri_recursion(k)
```
When I run this code with k set to 10, I get a sequence of outputs:
```
Recursion Example Results
1
3
6
10
15
21
28
36
45
55
```
I understand that the recursion stops when k reaches 0, but I'm confused about how the function actually accumulates the totals. It looks to me like it just adds 1 to 0 and then adds that to 2, but I feel like that's oversimplifying it. What am I missing in terms of understanding how this all works?
5 Answers
Good question! Essentially, when you call the function tri_recursion, it keeps pushing calls to the stack until k reaches 0. Starting from k=10, it computes down to k=0, and only then does it start returning results back up. You’ll see each layer adding up to your final result, which becomes clear once you trace through the calls step by step. Hope that clears things up!
A clearer example would be to add more print statements to visualize the recursion process better. Here’s a modified version:
```python
def tri_recursion(k, depth=0):
print(f"{'-' * depth} f({k}) called")
if (k > 0):
result = k + tri_recursion(k - 1, depth + 1)
print(f"{'-' * depth} f({k}) = {result}")
else:
result = 0
return result
tri_recursion(5)
```
Run this and see how it helps to visualize what’s happening at each level of the recursion!
It might help to trace it out on paper! Try a smaller number like k=3 and see how it breaks down. Keep track of each call and what’s happening in each step. Just remember, with recursion, each call is its own separate instance that doesn't carry over values directly from previous calls.
Your understanding of the stopping condition is a bit off! The function actually keeps calling itself with reduced k values until k reaches 0. Imagine a stack of calls being built up as it goes deeper until it hits zero, then it begins to unwind back down, adding the results together as it goes. So it’s not stopping exactly, it’s accumulating results during the unwinding phase!
A call stack is just the series of function calls your program makes. Each time a function is called, it's added to the stack. Once the function reaches the base case (in this case, k=0) and returns, it pops off the stack, returning values back up as it goes. Does that help?
Think of it like counting down your numbers. Every time you call `tri_recursion`, you’re counting down by 1 until you hit 0. The `else` part stops the counting when k hits 0 and then adds the results as it unwinds back. So with k=10, it counts down all the way to 1 and adds everything up, resulting in 55!
Thanks for breaking that down! It really helps.
What's a call stack? I’m new and feel a bit lost.