# Discussion 1: Control, Environment Diagrams

## Vitamin

The vitamin for Discussion 1 is worth 1 point and located here.

# Control structures

**Control structures** direct the flow of a program using logical statements. For example, conditionals (`if-elif-else`

) allow a program to skip sections of code, and iteration (`while`

), allows a program to repeat a section.

## Conditional statements

**Conditional statements** let programs execute different lines of code depending on certain conditions. Let’s review the `if-elif-else`

syntax.

Recall the following points:

- The
`elif`

and`else`

clauses are optional, and you can have any number of`elif`

clauses. - A
**conditional expression**is an expression that evaluates to either a truthy value (`True`

, a non-zero integer, etc.) or a falsy value (`False`

,`0`

,`None`

,`""`

,`[]`

, etc.). - Only the
**suite**that is indented under the first`if`

/`elif`

whose conditional expression evaluates to a true value will be executed. If none of the conditional expressions evaluate to a true value, then the

`else`

suite is executed. There can only be one`else`

clause in a conditional statement!`if <conditional expression>: <suite of statements> elif <conditional expression>: <suite of statements> else: <suite of statements>`

## Boolean Operators

Python also includes the **boolean operators** `and`

, `or`

, and `not`

. These operators are used to combine and manipulate boolean values.

`not`

returns the opposite truth value of the following expression (so not will always return either`True`

or`False`

).`and`

evaluates expressions in order and stops evaluating (short-circuits) once it reaches the first false value, and then returns it. If all values evaluate to a true value, the last value is returned.`or`

short-circuits at the first true value and returns it. If all values evaluate to a false value, the last value is returned.`>>> not None True >>> not True False >>> -1 and 0 and 1 0 >>> False or 9999 or 1/0 9999`

### Q1: Jacket Weather?

Alfonso will only wear a jacket outside if it is below 60 degrees or it is raining.

Write a function that takes in the current temperature and a boolean value telling if it is raining. This function should return `True`

if Alfonso will wear a jacket and `False`

otherwise.

First, try solving this problem using an `if`

statement.

Note that we’ll either return `True`

or `False`

based on a single condition, whose truthiness value will also be either `True`

or `False`

. Knowing this, try to write this function using a single line.

### Q2: Case Conundrum

In this question, we will explore the difference between the `if`

and `elif`

keywords.

What is the result of evaluating the following code?

```
def special_case():
x = 10
if x > 0:
x += 2
elif x < 13:
x += 3
elif x % 2 == 1:
x += 4
return x
special_case()
```

What is the result of evaluating this piece of code?

```
def just_in_case():
x = 10
if x > 0:
x += 2
if x < 13:
x += 3
if x % 2 == 1:
x += 4
return x
just_in_case()
```

How about this piece of code?

```
def case_in_point():
x = 10
if x > 0:
return x + 2
if x < 13:
return x + 3
if x % 2 == 1:
return x + 4
return x
case_in_point()
```

Which of these code snippets result in the same output, and why? Based on your findings, when do you think using a series of `if`

statements has the same effect as using both `if`

and `elif`

cases?

## While loops

To repeat the same statements multiple times in a program, we can use iteration. In Python, one way we can do this is with a **while loop**.

```
while <conditional clause>:
<body of statements>
```

As long as `<conditional clause>`

evaluates to a true value, `<body of statements>`

will continue to be executed. The conditional clause gets evaluated each time the body finishes executing.

### Q3: Square So Slow

What is the result of evaluating the following code?

```
def square(x):
print("here!")
return x * x
def so_slow(num):
x = num
while x > 0:
x=x+1
return x / 0
square(so_slow(5))
```

### Q4: Is Prime?

Write a function that returns `True`

if a positive integer `n`

is a prime number and `False`

otherwise.

A prime number n is a number that is not divisible by any numbers other than 1 and n itself. For example, 13 is prime, since it is only divisible by 1 and 13, but 14 is not, since it is divisible by 1, 2, 7, and 14.

Run in 61A CodeHint: Use the

`%`

operator:`x % y`

returns the remainder of`x`

when divided by`y`

.

### Q5: Fizzbuzz

Implement `fizzbuzz(n)`

, which prints numbers from 1 to `n`

. However, for numbers divisible by 3, print "fizz". For numbers divisible by 5, print "buzz". For numbers divisible by both 3 and 5, print "fizzbuzz".

This is a standard software engineering interview question, but we're confident in your ability to solve it!

Run in 61A Code# Environment Diagrams

An **environment diagram** is a model we use to keep track of all the variables that have been defined and the values they are bound to. We will be using this tool throughout the course to understand complex programs involving several different assignments and function calls.

Here's a simple program and its corresponding diagram:

Remember that programs are simply a set of statements, or instructions—so drawing diagrams that represent these programs also involves following sets of instructions! Let’s dive in...

## Assignment Statements

Assignment statements, such as `x = 3`

, define variables in programs. To execute one in an environment diagram, record the variable name and the value:

- Evaluate the expression on the right side of the
`=`

sign - Write the variable name and the expression’s value in the current frame.

### Q6: Assignment Diagram

Use these rules to draw a simple diagram for the assignment statements below:

```
x = 11 % 4
y = x
x **= 2
```

`def`

Statements

A `def`

statement creates a function object and binds it to a name. To diagram `def`

statements, record the function name and bind the function object to the name. It’s also important to write the **parent frame** of the function, which is where the function is defined. **Very important note:** Assignments for def statements use pointers to functions, which can have different behavior than primitive assignments.

- Draw the function object to the right-hand-side of the frames, denoting the intrinsic name of the function, its parameters, and the parent frame (e.g.
`func square(x) [parent = Global]`

- Write the function name in the current frame and draw an arrow from the name to the function object.

### Q7: def Diagram

Use these rules and the rules for assignment statements to draw a diagram for the code below.

```
def double(x):
return x * 2
def triple(x):
return x * 3
hat = double
double = triple
```

## Call Expressions

**Call expressions**, such as `square(2)`

, apply functions to arguments. When executing call expressions, we create a new frame in our diagram to keep track of local variables:

- Evaluate the operator, which should evaluate to a function.
- Evaluate the operands from left to right.
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. For example, if the function object is`func square(x) [parent=Global]`

, the intrinsic name is`square`

. - The parent frame ([
`parent=Global`

])

- A unique index (
- Bind the formal parameters to the argument values obtained in step 2 (e.g.
bind
`x`

to 3). - 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 a function does not have a return value, 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, since we would not be able to fill it out accurately.

### Q8: Call Diagram

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

```
def double(x):
return x * 2
hmmm = double
wow = double(3)
hmmm(wow)
```

Return value |

Return value |

### Q9: 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)
```

Return value |

Return value |

Return value |

Return value |

# Exam prep

We recommending reading sections 1.1-1.5 from the textbook for these problems.

### Q10: Unique Digits

Write a function that returns the number of unique digits in a positive integer.Run in 61A Code

Hint:You may find it helpful to first define a function`has_digit(n, k)`

, which determines whether a number`n`

has digit`k`

.

We have provided two solutions:

- In one solution, we look at the current digit, and check if the rest of the number contains that digit or not. We only say it's unique if the digit doesn't exist in the rest. We do this for every digit.
- In the other, we loop through the numbers 0-9 and just call
`has_digit`

on each one. If it returns true then we know the entire number contains that digit and we can one to our unique count.

Video walkthrough:

### Q11: Run, K, Run

An *increasing run* of an integer is a sequence of consecutive digits in which each digit is larger than the
last. For example, the number 123444345 has four increasing runs: 1234, 4, 4 and 345. Each run can be
indexed **from the end** of the number, starting with index 0. In the previous example, the 0th run is 345,
the first run is 4, the second run is 4 and the third run is 1234.

Implement `get_k_run_starter`

, which takes in integers n and k and returns the 0th digit of the kth
increasing run within n. The 0th digit is the leftmost number in the run. You may assume that there are
at least k+1 increasing runs in n.

### Q12: Environment Diagrams - Challenge

These questions were originally developed by Albert Wu and are included here for extra practice. We recommend checking your work in PythonTutor after filling in the diagrams for the code below.

#### Challenge 1

Draw the environment diagram that results from executing the code below.

Guiding Notes:

- Pay special attention to the names of the frames!

```
def funny(joke):
hoax = joke + 1
return funny(hoax)
def sad(joke):
hoax = joke - 1
return hoax + hoax
funny, sad = sad, funny
result = funny(sad(1))
```

Return value |

Return value |

Return value |

#### Challenge 2

Draw the environment diagram that results from executing the code below.

```
def double(x):
return double(x + x)
first = double
def double(y):
return y + y
result = first(10)
```

Return value |

Return value |

Return value |