Discussion 2

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# CS 61A – Discussion 2
## Allen Guo
## http://aguo.us/cs61a
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# Quiz
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# Announcements

- Hog due today

- Lab 2 due tomorrow

- Homework 3 due Tuesday

- Midterm 1 next Thursday (8-10pm)
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# Survey

## http://x.aguo.us
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# Worksheet
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# Environment Diagrams

- How do we perform lookup?

- How do we evaluate a primitive expression?

- How do we evaluate a call expression?

- What is the parent frame of a function?

.footnote[Source: [Environment Diagram Party Slides](https://docs.google.com/presentation/d/1pKado1dzDzJaAAoTAkovQeMB3YwK5uo6BNenCQgH8xI/edit?usp=sharing)]
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# Closure Example

```python
def f():
    x = 1
    def g(y):
        return x + y
    x = 2
    return g

g = f

x = 3

h = f()
print(h(0))
```

- Environment diagram demo: [Python Tutor](http://www.pythontutor.com/visualize.html#code=def%20f%28%29%3A%0A%20%20%20%20x%20%3D%201%0A%20%20%20%20def%20g%28y%29%3A%0A%20%20%20%20%20%20%20%20return%20x%20%2B%20y%0A%20%20%20%20x%20%3D%202%0A%20%20%20%20return%20g%0Ag%20%3D%20f%0Ax%20%3D%203%0Ah%20%3D%20f%28%29%0Aprint%28h%280%29%29&cumulative=true&curInstr=0&heapPrimitives=false&mode=display&origin=opt-frontend.js&py=3&rawInputLstJSON=%5B%5D&textReferences=false)

- The function with intrinsic name `g` is a closure

- What variable is "captured" or "closed over" by `g`?
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# Recursion Done Wrong

```python
def f(x):
    return f(x)
```

- What's wrong with it?

- No base case

- The recursive case doesn't make the problem smaller

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# Recursion

- Three things you need:

- One or more base cases.

- One or more ways to make the problem smaller.

- One or more ways to solve the "big" problem using the "small" problem.

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# Recursion Done Right

```python
def sum_up(n):
    if n == 0:
        return 0
    return n + sum_up(n - 1)
```

- What does this function do?

- Where's the base case?

- Where do we make the problem smaller?

- Where do we use the smaller problem to solve the bigger problem?