# Rao Discussion 2: Environment Diagrams, Higher-Order Functions disc02.pdf

This discussion worksheet is for the Rao offering of CS 61A. Your work is not graded and you do not need to submit anything.

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Last week, you learned about the environment diagram, which is a visual model for keeping track of the state of a computer program. We have already gone over rules for variable assignment and `def` statements. This week, we will go over several more rules for constructing environment diagrams.

# Scope

Scope refers to the idea that names have meaning only within a specific context. For example, the following program produces an error because the scope of the variable `b` is local—limited to the call to `f()`. Outside of that function call, `b` has no meaning, so the line `print(b)` errors.

``````def f():
b = 2
f()
print(b)``````

On the other hand, some variables are defined with a broader scope. The following code prints `2`. `b` is accessible within the call to `f()` because `b` is defined in the global scope, outside of any function.

``````b = 2
def f():
print(b)
f()``````

Scope can also be relevant in disambiguating variables. In the following code, there are two variables named `b`: one is defined in the global scope and the other is defined in the local scope of the call to `f(3)`. The line `print(b)` prints `3` instead of `2` because the local variable takes precedence over the global variable.

``````b = 2
def f(b):
print(b)
f(3)``````

# Call Expressions

In Python, local variables are created whenever a function is called. To formalize this idea of scope, we have rules for how call expressions, such as `square(2)`, should be treated in environment diagrams. Specifically, we create a new frame in our diagram to keep track of local variables:

1. Evaluate the operator, which should evaluate to a function.
2. Evaluate the operands from left to right.
3. Draw a new frame, labelling it with the following:

• A unique index (`f1`, `f2`, `f3`, ...).
• The intrinsic name of the function, which is the name of the function object itself as it was named in its `def` statement. For example, if the function object is `func square(x) [parent=Global]`, the intrinsic name is `square`.
• The parent frame (e.g. [`parent=Global`]).
4. In the new frame, bind the formal parameters to the argument values obtained in step 2 (e.g. bind `x` to 2).
5. Evaluate the body of the function in this new frame until a return value is obtained. Write down the return value in the frame.

If you never hit a `return` statement in your function, it implicitly returns `None`. In that case, the “Return value” box should contain `None`.

Note: Since we do not know how built-in functions like `min(...)` or imported functions like `add(...)` are implemented, we do not draw a new frame when we call them because we would not be able to fill it out accurately.

When we want to look up the value of a variable, we follow these rules:

1. Search for the variable in the current frame.
2. If the variable is not bound in the current frame, look up to the parent frame. If the variable is not bound in that frame, look up to its parent frame, etc.
3. If you look up all the way to the global frame and still cannot find the variable, throw a `NameError`.

### Q1: Call Diagram

Let’s put it all together! Draw an environment diagram for the following code. You may not need to use all of the frames and blanks provided to you.

``````def double(x):
return x * 2

hmmm = double
wow = double(3)
hmmm(wow)``````
Objects
Global frame
f1: [parent=]
 Return value
f2: [parent=]
 Return value

### Q2: Nested Calls Diagrams

Draw the environment diagram that results from executing the code below. You may not need to use all of the frames and blanks provided to you.

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

def g(x, y):
if x(y):
return not y
return y

x = 3
x = g(f, x)
f = g(f, 0)``````
Objects
Global frame
f1: [parent=]
 Return value
f2: [parent=]
 Return value
f3: [parent=]
 Return value
f4: [parent=]
 Return value

# Lambda Expressions

A lambda expression evaluates to a function, called a lambda function. For example, `lambda y: x + y` is a lambda expression, and can be read as "a function that takes in one parameter `y` and returns `x + y`."

A lambda expression by itself evaluates to a function but does not bind it to a name. Also note that the return expression of this function is not evaluated until the lambda function is called. This is similar to how defining a new function using a def statement does not execute the function’s body until it is later called.

``````>>> what = lambda x : x + 5
>>> what
<function <lambda> at 0xf3f490>``````

Unlike `def` statements, lambda expressions can be used as an operator or an operand to a call expression. This is because they are simply one-line expressions that evaluate to functions. In the example below, `(lambda y: y + 5)` is the operator and `4` is the operand.

``````>>> (lambda y: y + 5)(4)
9
>>> (lambda f, x: f(x))(lambda y: y + 1, 10)
11``````

Lambda functions do not have an intrinsic name, so we use the Greek letter lambda as a placeholder wherever we would usually use a function's intrinsic name.

### Q3: Lambda the Environment Diagram

Draw the environment diagram for the following code and predict what Python will output.

``````a = lambda x: x * 2 + 1
def b(b, x):
return b(x + a(x))
x = 3
x = b(a, x)``````
Objects
Global frame
f1: [parent=]
 Return value
f2: [parent=]
 Return value
f3: [parent=]
 Return value

# Higher Order Functions

A higher order function (HOF) is a function that manipulates other functions by taking in functions as arguments, returning a function, or both. For example, the function `compose` below takes in two functions as arguments and returns a function that is the composition of the two arguments.

``````def composer(func1, func2):
"""Returns a function f, such that f(x) = func1(func2(x))."""
def f(x):
return func1(func2(x))
return f``````

HOFs are powerful abstraction tools because they allow us to express certain general patterns (functions) as named concepts in our programs.

Environment diagrams can model more complex programs that utilize higher order functions.
``````def delete_num(x):
"""Returns a lambda function that takes in y and deletes x digits from y."""
return lambda y: y // (10 ** x) # Note that ** means exponent (^) in Python

delete_two = delete_num(2)
delete_two(614)``````

The parent of any function (including lambdas) is always the frame in which the function is defined. It is useful to include the parent in environment diagrams in order to find variables that are not defined in the current frame. In the previous example, when we call `delete_two` (which is really the lambda function), we need to know what `x` is in order to compute `y // (10 ** x)`. Since `x` is not in the frame `f2`, we look at the frame’s parent, which is `f1`. There, we find `x` is bound to 2.

As illustrated above, higher order functions that return a function have their return value represented with a pointer to the function object.

Draw the environment diagram for the following code:

``````n = 9
return lambda k: k + n
Objects
Global frame
f1: [parent=]
 Return value
f2: [parent=]
 Return value

There are 3 frames total (including the Global frame). In addition, consider the following questions:

1. In the Global frame, the name `add_ten` points to a function object. What is the intrinsic name of that function object, and what frame is its parent?
2. What name is frame `f2` labeled with (`add_ten` or lambda)? Which frame is the parent of `f2`?
3. What value is the variable `result` bound to in the Global frame?

### Q5: Make Keeper

Implement `make_keeper`, which takes a positive integer `n` and returns a function `f` that takes as its argument another one-argument function `cond`. When `f` is called on `cond`, it prints out the integers from 1 to `n` (including `n`) for which `cond` returns a true value when called on each of those integers. Each integer is printed on a separate line.

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# Currying

One important application of HOFs is converting a function that takes multiple arguments into a chain of functions that each take a single argument. This is known as currying. For example, here is a curried version of the `pow` function:

``````>>> def curried_pow(x):
...     def h(y):
...         return pow(x, y)
...     return h

>>> curried_pow(2)(3)
8
>>> pow(2, 3) == curried_pow(2)(3)
True``````

This is useful if, say, you need to calculate a lot of powers of 2. Using the normal `pow` function, you would have to put in `2` as the first argument for every function call:

``````>>> pow(2, 3)
8
>>> pow(2, 4)
16
>>> pow(2, 10)
1024``````

With `curried_pow`, however, you can create a one-argument function specialized for taking powers of 2 one time, and then keep using that function for taking powers of 2:

``````>>> pow_2 = curried_pow(2)
>>> pow_2(3)
8
>>> pow_2(4)
16
>>> pow_2(10)
1024``````

This way, you don't have to put `2` in as an argument for every call! If instead you wanted to take powers of 3, you could quickly make a similar function specialized in taking powers of 3 using `curried_pow(3)`.

### Q6: Currying

Write a function `curry` that will curry any two argument function.

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First, try implementing `curry` with `def` statements. Then attempt to implement `curry` in a single line using lambda expressions.

# HOFs and Lambdas

### Q7: Make Your Own Lambdas

For the following problem, first read the doctests for functions `f1`, `f2`, `f3`, and `f4`. Then, implement the functions to conform to the doctests without causing any errors. Be sure to use lambdas in your function definition instead of nested `def` statements. Each function should have a one line solution.

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# Extra Practice

This question is particularly challenging, so it's recommended if you're feeling confident on the previous questions or are studying for the exam.

### Q8: Match Maker

Implement `match_k`, which takes in an integer `k` and returns a function that takes in a variable `x` and returns `True` if all the digits in `x` that are `k` apart are the same.

For example, `match_k(2)` returns a one argument function that takes in `x` and checks if digits that are 2 away in `x` are the same.

`match_k(2)(1010)` has the value of `x = 1010` and digits 1, 0, 1, 0 going from left to right. `1 == 1` and `0 == 0`, so the `match_k(2)(1010)` results in `True`.

`match_k(2)(2010)` has the value of `x = 2010` and digits 2, 0, 1, 0 going from left to right. `2 != 1` and `0 == 0`, so the `match_k(2)(2010)` results in `False`.

Important: You may not use strings or indexing for this problem. You do not have to use all the lines; one staff solution does not use the line directly above the while loop.

Hint: Floor dividing by powers of 10 gets rid of the rightmost digits.

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