Lecture 4

class: title-slide, left, bottom

# Lecture 4
----
## **DANL 100: Programming for Data Analytics**
### Byeong-Hak Choe
### September 8, 2022

---
# Announcement

### <p style="color:#00449E">Office Hours</p>

- Office: South Hall 117B.

- Office Hours: 
  - Mondays 3:30 PM-5:30 PM 
  - Wednesdays 1:30 PM-3:30 PM.

---
# Announcement

### <p style="color:#00449E">TA and Tutors</p>

- Teaching Assistant (TA): Emine Morris 
  - Major in Accounting
  - Minor in Data Analytics and Mathematics

- TA Hours: 
  - Mondays and Wednesdays: 5:00 PM-6:30 PM 
  - Tuesdays and Thursdays: 3:00 PM-4:45 PM

- Where: Data Analytics Lab (South Hall, 3rd Floor)

- Tutoring schedules are coming soon.

---
# Tips on using Presentation Slides

- To go to a previous/next page (or tab), use keyboard arrows, <svg aria-hidden="true" role="img" viewBox="0 0 448 512" style="height:1em;width:0.88em;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:currentColor;overflow:visible;position:relative;"><path d="M447.1 256C447.1 273.7 433.7 288 416 288H109.3l105.4 105.4c12.5 12.5 12.5 32.75 0 45.25C208.4 444.9 200.2 448 192 448s-16.38-3.125-22.62-9.375l-160-160c-12.5-12.5-12.5-32.75 0-45.25l160-160c12.5-12.5 32.75-12.5 45.25 0s12.5 32.75 0 45.25L109.3 224H416C433.7 224 447.1 238.3 447.1 256z"/></svg> and <svg aria-hidden="true" role="img" viewBox="0 0 448 512" style="height:1em;width:0.88em;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:currentColor;overflow:visible;position:relative;"><path d="M438.6 278.6l-160 160C272.4 444.9 264.2 448 256 448s-16.38-3.125-22.62-9.375c-12.5-12.5-12.5-32.75 0-45.25L338.8 288H32C14.33 288 .0016 273.7 .0016 256S14.33 224 32 224h306.8l-105.4-105.4c-12.5-12.5-12.5-32.75 0-45.25s32.75-12.5 45.25 0l160 160C451.1 245.9 451.1 266.1 438.6 278.6z"/></svg>.

- To see a tile view of the lecture slides, use the alphabet key, `o`.

- If we hover a mouse cursor on the code block, we can see and click the "Copy Code".

---
# Installing the Tools
### <p style="color:#00449E"> Spyder Environment </p>
.pull-left[
<img src="../lec_figs/spyder_env.png" width="100%" style="display: block; margin: auto;" />
]

.pull-right[
- Blue box is **Console Pane** that allows us to interact directly with the Python interpreter and type commands where Python will immediately execute them.

]
---
# Installing the Tools
### <p style="color:#00449E"> Spyder Environment </p>
.pull-left[
<img src="../lec_figs/spyder_env.png" width="100%" style="display: block; margin: auto;" />
]

.pull-right[
- Yellow box is **Variable Explorer** where we can see the values of variables, data frames, and other objects that are currently stored in memory.
  - Double-clicking a variable will open a specialized viewer, allowing us to inspect its contents.

]

---
class: inverse, center, middle

# Workflow
<html><div style='float:left'></div><hr color='#EB811B' size=1px width=796px></html>

---
# Workflow
### <p style="color:#00449E"> Shortcuts </p>
- **F9** runs a current line (where the blinking cursor bar is) or selected lines.

- **Home/End** moves the blinking cursor bar to the beginning/end of the line.
  - **Fn + ** <svg aria-hidden="true" role="img" viewBox="0 0 448 512" style="height:1em;width:0.88em;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:currentColor;overflow:visible;position:relative;"><path d="M447.1 256C447.1 273.7 433.7 288 416 288H109.3l105.4 105.4c12.5 12.5 12.5 32.75 0 45.25C208.4 444.9 200.2 448 192 448s-16.38-3.125-22.62-9.375l-160-160c-12.5-12.5-12.5-32.75 0-45.25l160-160c12.5-12.5 32.75-12.5 45.25 0s12.5 32.75 0 45.25L109.3 224H416C433.7 224 447.1 238.3 447.1 256z"/></svg> / <svg aria-hidden="true" role="img" viewBox="0 0 448 512" style="height:1em;width:0.88em;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:currentColor;overflow:visible;position:relative;"><path d="M438.6 278.6l-160 160C272.4 444.9 264.2 448 256 448s-16.38-3.125-22.62-9.375c-12.5-12.5-12.5-32.75 0-45.25L338.8 288H32C14.33 288 .0016 273.7 .0016 256S14.33 224 32 224h306.8l-105.4-105.4c-12.5-12.5-12.5-32.75 0-45.25s32.75-12.5 45.25 0l160 160C451.1 245.9 451.1 266.1 438.6 278.6z"/></svg> works too.

.pull-left[
### <p style="color:#00449E"> Mac </p>
- **command + N** opens a new script.
- **command + 1** is the shortcut for #.
- **command + 4** is the shortcut for block comment.
]

.pull-right[
### <p style="color:#00449E"> Windows </p>
- **Ctrl + N** opens a new script.
- **Ctrl + 1** is the shortcut for #.
- **Ctrl + 4** is the shortcut for block comment.
]

---
# Workflow
### <p style="color:#00449E"> More Shortcuts </p>

- **Ctrl** (**command** for Mac Users) **+ Z** undoes the previous action.
- **Ctrl** (**command** for Mac Users) **+ Shift + Z** redoes when undo is executed.

- **Ctrl** (**command** for Mac Users) **+ F** is useful when finding a phrase in the script.
- **Ctrl** (**command** for Mac Users) **+ R** is useful when replacing a specific phrase with something in the script.

- **Ctrl** (**command** for Mac Users) **+ D** deletes a current line.

---
# Workflow
### <p style="color:#00449E"> Code and comment style </p>
- The two main principles for coding and managing data are:
  - Make things easier for your future self.
  - Don't trust your future self.

- The `#` mark is Spyder's comment character.
  - The `#` character has many names: `hash`, `sharp`, `pound`, or `octothorpe`.
  - `#` indicates that the rest of the line is to be ignored.
  - Write comments before the line that you want the comment to apply to.

- Consider using block commenting for separating code sections.
  - `#%%` defines a coding block in Spyder.

---
class: inverse, center, middle

# Data: Types, Values, Variables, and Names
<html><div style='float:left'></div><hr color='#EB811B' size=1px width=796px></html>

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Python Data Are Objects </p>

- Python keeps track of where data are, and what (data type) they are.

- Python wraps each data value---booleans, integers, floats, strings, even large data structures, functions, and programs---in memory as an **object**.

- We can think of **objects** in Python programming as variable-sized boxes occupying spaces on memory shelves:

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Python Data Are Objects </p>

- In Python, an object is a chunk of data that contains at least the following:

- A **type** that defines what it can do, like a factory stamp on the box saying what it can do.
  
  - A **value** that is consistent with its type.
  
  - A *unique id* to distinguish it from other objects, like its location on the memory shelf.
  
  - A *reference count* that tracks how often this object is used.

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Python’s basic data types </p>

- **Type** contains the Python name of that type. 
- **Mutable?** indicates whether the value can be changed after creation.

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Mutability </p>

- The type determines whether the data value contained by the box can be changed (mutable) or is constant (immutable).

- A mutable object is like a box with a lid: not only can we see the value inside, we can also change it; 
  - However, we can’t change the type of an object.

.pull-left[

```python
a = (1, 2, 3)     # tuple
a[0]
a[0] = 5
a
```
]
.pull-right[

```python
b = [1, 2, 3]     # list
b[0]
b[0] = 5
b
```
]

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Variables </p>

- Python lets you define **variables**---names for *values* in
your computer’s memory that you want to use in a program.
  
- Variable names can contain only these characters:
  - Lowercase letters (`a` through `z`)
  - Uppercase letters (`A` through `Z`)
  - Digits (`0` through `9`)
  - Underscore (`_`)
  - They are case-sensitive: `thing`, `Thing`, and `THING` are different names.
  - They must begin with a letter or an underscore, not a digit.
  - Names that begin with an underscore are treated specially.

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Variables </p>

- Variable names cannot be one of Python’s reserved words (also known as *keywords*):

- Within a Python program, we can find the reserved words with:

```python
help("keywords")
```

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Variables </p>
.pull-left[
- Which names are valid?
  - `a`
  - `a1`
  - `a_b_c___95`
  - `_abc`
  - `_1a`
  - `1`
  - `1a`
  - `1_`
  - `name!`
  - `another-name`
]

.pull-right[

```python
a
a1
a_b_c___95
_abc
_1a
1
1a
1_
name!
another-name
```

]

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Assignment </p>

- In Python, we use `=` to assign a value to a variable.

```python
# Here we assign the integer value 5 to the variable x.
x = 5

# Now we can use the variable x in the next line.
y = x + 12  
y
```

- In math, `=` means *equality* of both sides.
- In programs, `=` means **assignment**: *assign the value on the right side to the variable on the left side*.

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Assignment </p>

- In programs, everything on the right side needs to have a value.
  - The right side can be a literal value, or a variable that has already been assigned a value, or a combination.
  
- When Python reads `y = x + 12`, it does the following:
  - Sees the `=` in the middle.
  - Knows that this is an assignment.
  - Calculates the right side (gets the value of the object referred to by `x` and adds it to `12`).
  - Assigns the result to the left-side variable, `y`.

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Variables Are Names, Not Places </p>

- In Python, *variables* are just names.

<div class="figure" style="text-align: center">
<img src="../lec_figs/int-py-fig2-3.png" alt="Names point to objects (variable `a` points to an integer object with value `7`)" width="29%" />
<p class="caption">Names point to objects (variable `a` points to an integer object with value `7`)</p>
</div>

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Variables Are Names, Not Places </p>

- If we want to print the value of anything (a variable (e.g., `x`, `y`) or a literal value (e.g., `5`, `12`, `abcd`)), we can use `print(thing)` or type `thing`.

- If we want to know the type of anything, we can use `type(anything)`.

- If we want to check whether a variable points to an object of a specific type, use `isinstance(type)`:

.pull-left[

```python
type(7)
isinstance(7, int)
a = 7
print(a)
b = a
```
]
.pull-right[

```python
type(a)
type(b)
type(50)
type(99.9)
type('abcd')
```
]

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Copying </p>

- Assignment does not copy a value; 
  - It just attaches a name to the object that contains the data.

- What value is *assigned* to variable `y` at the end of the following code?

```python
# Here we assign the integer value 5 to the variable x.
x = 5

# Now we can use the variable x in the next line.
y = x + 12  
y

x = 8
```

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Copying </p>

- Assigning an existing variable `a` to a new variable named `b`
just makes `b` point to the same object that `a` does.

<div class="figure" style="text-align: center">
<img src="../lec_figs/int-py-fig2-4.png" alt="Copying a name (now variable `b` also points to the same integer object)" width="29%" />
<p class="caption">Copying a name (now variable `b` also points to the same integer object)</p>
</div>

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Copying </p>

- What value is *assigned* to variable `y` at the end of the following code?

```python
x = 5   
y = x 
x = 29
```

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Copying </p>

- A **list** is a *mutable* array of values.

- In the code below, `a` and `b` each points to a list with three integer members:

```python
a = [2, 4, 6]
b = a
a[1] = 0
```

- List members (`a[0]`, `a[1]`, and `a[2]`) are themselves like names, pointing to integer objects with the values 2, 4, and 6.

- If both names point to a mutable object, we can change the object’s value via either name, and we’ll see the changed value when we use either name.

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Choose Good Variable Names </p>

- It’s surprising how important it is to choose good names for your variables.

- In many of the code examples so far, I’ve been using throwaway names like `a` and `x`.

- In real programs, you’ll have many more variables to keep track of at once, and you’ll need to balance *brevity* and *clarity*.

- For example, it’s faster to type `num_loons` rather than `number_of_loons` or `blueberry_inventory`, but it’s more explanatory than `n`.

---
# Data: Types, Values, Variables, and Names
### <p style="color:#00449E"> Class Exercises </p>
1. Assign the integer value 33 to the variable `queen`, and print it.

2. What type is the value `3`?

3. What type is the value `3.0`?

4. What type is the expression `8 + 3.0`?

5. Modify the following line of code, so that a variable of string type data (`<class 'str'>`) is created.

```python
var_created = 1.5
print( "The data type of var_created is: ", type(var_created) )
```

---
class: inverse, center, middle

# Numbers
<html><div style='float:left'></div><hr color='#EB811B' size=1px width=796px></html>

---
# Numbers

- Let's look at the following Python’s simplest built-in data types in detail:

- **Booleans** (which have the value `True` or `False`)
  
  - **Integers** (whole numbers such as `10` and `28`)
  
  - **Floats** (numbers with decimal points such as `3.141592`, or sometimes exponents like `1.0e8`, which means one times ten to the eighth power, or `100000000.0`)

---
# Numbers
### <p style="color:#00449E"> Booleans </p>

- The only values for the boolean data type are `True` and `False`.

- The special Python function `bool()` can convert any Python data
type to a boolean.
  - The `bool()` function takes any value as its argument and returns the boolean equivalent.
  
.pull-left[

```python
bool(True)
bool(1)
bool(4)
bool(-4)
```
]

.pull-right[

```python
bool(False)
bool(0)
bool(0.0)
```
]

---
# Numbers
### <p style="color:#00449E"> Integers </p>

- Any sequence of digits in Python represents *a literal integer*:

.pull-left[
- We can’t have an initial 0 followed by a digit between 1 and 9:

```python
0
01
```
]

.pull-right[
- We can’t have any commas as a digit separator in the integer.
- We can use the underscore (`_`) character as a digit separator

```python
1,000,000   # We get a tuple!
billion = 1_000_000_000
billion
3_2_1
```

]

---
# Numbers
### <p style="color:#00449E"> Integer Operations </p>

.pull-left[
- Python can be a simple calculator.

- Here is a table of the math operators:

]
.pull-right[
- Dividing by zero with either kind of division causes a Python exception

```python
12 + 8
12 - 8
13 * 1.2
2 + 3 * 4
2 ** 3
2 / 0
2 // 0
```
]

---
# Numbers
### <p style="color:#00449E"> Integers and Variables </p>

- In Python, the expression on the right side of the `=` is calculated first, and then assigned to the variable on the left side.

.pull-left[
- Subtract `2` from `a`.
- Assign the result of that subtraction to a temporary variable, `tmp`.
- Assign the value of the temporary variable to `a`.

]
.pull-right[

```python
a = 72
tmp = a - 2
a = tmp
```
]

---
# Numbers
### <p style="color:#00449E"> Integers and Variables </p>

- We can combine the arithmetic operators with assignment by putting the operator before the `=`.

.pull-left[

```python
a = 72
a -= 2   # This is like a = a - 2
```

]

.pull-right[

```python
b = 62
b += 2    # This is like b = b + 2
```
]

.pull-left[

```python
c = 72
c *= 2   # This is like c = c * 2
```

]

.pull-right[

```python
d = 62
d /= 2    # This is like d = d / 2
```

]

---
# Numbers
### <p style="color:#00449E"> Type Conversions </p>

- To change other Python data types to an integer, use the `int()` function.
  - `int()` keeps the whole number and discard any fractional part.

- `True` and `False` are converted to integer values `1` and `0`.

.pull-left[

```python
int(True)
int(False)
```

]

.pull-right[

```python
bool(1)
bool(0)
```
]

.pull-left[

```python
int(72.3)
int(1.0e4)
```

]

.pull-right[

```python
bool(1.0)
bool(0.0)
```

]

---
# Numbers
### <p style="color:#00449E"> Floats </p>

- Integers are whole numbers, but floating-point numbers (called **floats** in Python) have decimal points:

.pull-left[

```python
4.
4.0
04.0
```

]

.pull-right[

```python
4e0
4e1
4.0e1
4.0 * (10 ** 1)
```
]

---
# Numbers
### <p style="color:#00449E"> Floats </p>

- To convert other types to floats, we use the `float()` function.

.pull-left[

```python
float(True)
float(False)
```

]

.pull-right[

```python
float(88)
float('89')
```
]

---
# Numbers
### <p style="color:#00449E"> Class Exercises </p>
1. Multiply the number of seconds in a minute (`60`) by the number of minutes in an hour (also `60`) using Python.

2. Assign the result from the previous task (seconds in an hour) to a variable called `seconds_per_hour`.

3. How many seconds are in a day? Use your `seconds_per_hour` variable.

4. Calculate seconds per day again, but this time save the result in a variable called `seconds_per_day`

5. Divide `seconds_per_day` by `seconds_per_hour`. Use floating-point (`/`) division.