The term **object** has many meanings in programming, just as it does in real-life. The first and most fundamental definition is **an object is a typed unit of data in memory**.

**Memory is your canvas and objects are your paint.** Understanding the fundamental types of data you can work with, and their capabilities, is the first step on your journey toward programming!

Let’s zoom in a bit to explore memory more closely, before zooming our way out toward thinking in terms of *objects* in memory.

Your computer’s memory is comprised of billions of electronic cells. These cells, called *transistors* are now the most frequently manufactured device in history, with over 13 sextillion produced^{1}! Each transistor has two states, *off* or *on*. This is represented numerically as a **bit** whose value is either `0`

or `1`

. Despite `1`

being the maximal value of a bit, notice there are *two* possible values thanks to `0`

. Unlike humans, **bits are binary**.

You can form many unique patterns by *stringing* bits together. For example, with two bits strung together, you can form four different **bit patterns**: `00`

, `01`

, `10`

, and `11`

. With three bits strung together, you can form eight different combinations: `000`

, `001`

, `010`

, `011`

, `100`

, `101`

, `110`

, `111`

. How many **bit patterns** can you form with four bits?

Each time you extend a bit pattern by another bit, you double the number of possible patterns. Thus, given any number of bits, you can calculate the possible unique patterns with the mathematical expression 2^{bits}. With 8 bits, called a **byte** there are 2^{8}, which *evaluates to* 256, different patterns. Exponential growth is a powerful force! With only 64 bits, there are 2^{64} patterns, which *evaluates to* 18,446,744,073,709,551,616 possibilities.

Humanity has settled on the decimal system as its defacto numbering system. Languages have their own alphabets for textual data. There is an emergent universal *character set* in emoji 🤠. As all of memory is the binary system of `0`

s and `1`

s, how can computers process these widely different * types of data*?

A symbol will follow this question, what does the symbol mean? `V`

If you classify it as an English character, it *represents* the letter “vee”. Classified as a Roman Numeral, though, it *represents* the number five. If you read it in the phrase, “I’m V excited!” it’s an abbreviation of the word “very”. Choosing to classify, and *interpret*, the simple symbol `V`

as different types of data allows for *the same symbol* to **represent** very different ideas.

Programmers decide to *classify* each block of memory they use, or each *object*, as a specific **type** of data: numerical data, textual data, sequences of data, compound groupings of many types of data, and so on. In doing so, the same underlying bit patterns are interpretted to **represent** different ideas.

Binary Bit Pattern | Number (Integer) | Text (Character) |
---|---|---|

`00000000` |
`0` |
`NULL (invisible)` |

`00000001` |
`1` |
`Start of Heading (invisible)` |

`00000010` |
`2` |
`Start of Text (invisible)` |

`00110000` |
`48` |
`0` |

`00110001` |
`49` |
`1` |

`00110010` |
`50` |
`2` |

`01000001` |
`65` |
`A` |

`01000010` |
`66` |
`B` |

`01000011` |
`67` |
`C` |

You can poke at this concept directly in an interactive Python session. To follow along, **open a new terminal session** and run the `python`

program by typing it in and pressing enter. Each of the lines beginning with `>>>`

is one where you will type in an expression and press enter for the Python interpreter to **evaluate**.

```
>>> int(0b01000001)
65
>>> chr(0b01000001)
'A'
>>> int(0b01000010)
66
>>> chr(0b01000010)
'B'
>>> int(0b01000011)
67
>>> chr(0b01000011)
'C'
```

You are encouraged to try other bit patterns! When you are done, evaluate the function call expression `quit()`

in the interactive Python program.

Each line had its own *function call expression*. A

`int`

and `chr`

. The word `int`

is short for `chr`

short for `0b`

followed by the pattern of 0s and 1s. Each expression you wrote was Note: When working in an interactive Python interpreter, the evaluation of each expression is displayed, or “printed”, back to you automatically. This is

greatfor messing around! Thiswill notbe the case when writing stored programs. There you would need to surround any expression you want displayed in a`print(chr(0b01000011))`

.

**What an object’s bit pattern means or represents is decided entirely by the type of data you decide to classify it as.** Thus, when programming in a high level language like Python, it is more important to understand the types of data you are working with than exactly how they are represented as bit patterns. This is one of the most important examples of data

Programming languages offer many built-in data types for you to work with, typically including:

- numerical
- integers
- decimal numbers
- complex numbers

- textual
- logical
- collections of many objects
- sequences
- sets
- dictionaries

The names programming languages choose for these common data types, thier capabilities, and their limitations, are usually very similar. However, there will be subtle differences specific to each language. **When learning a new programming language, start by spending time understanding and exploring its built-in data types and their capabilities.**

Let’s explore these common data types in the context of Python, starting with *primitive* types which are some of the simplest available to you in a programming language.

`type`

inspectionHow do you express the integer number one hundred ten in Python? Quite *literally*! `110`

What about fifty percent represented as a decimal? Quite *literally*! `0.50`

What about the textual word *programming*? Quite *literally*! `"programming"`

The built-in, *primitive* types of a programming language often have *literal* syntax for constructing and representing objects directly in your programs. *(Notice: none of these examples involve any binary! Though, previously, you wrote a binary literal without knowing it. Abstraction for the win!)* These are examples of **literal expressions**.

To learn the type *classification* of any object in Python, you can use the built-in `type`

function. Try it by starting `python`

in a terminal, if you don’t already have one running, and following along:

```
>>> 110
110
>>> type(110)
<class 'int'>
>>> type(211)
<class 'int'>
>>> 0.5
0.5
>>> type(0.5)
<class 'float'>
>>> "programming"
'programming'
>>> type("programming")
<class 'str'>
```

Each literal expression evaluates to *literally* itself. There is no further evaluation necessary or possible in these expressions. You also certainly noticed the classifications of each object’s type have peculiar names such as `int`

, `float`

, and `str`

. Let’s address each individually.

`int`

- IntegersThe built-in type `int`

is short for *integer*. Integers are useful for counting and programs count more things than you’d expect! For example, the number of likes on your last social media post, the number of words in your paper, the number of walking steps in your activity tracker, the number of points scored in games, and so on.

In Python, an `int`

literal is either a zero *or* a nonzero digit followed by zero or more digits. So, `0`

, `1`

, `987654321`

, and `2020`

are all example `int`

literals, but `0110`

is not. For large numbers you cannot use commas to denote places, such as in *1,000*, but you can use underscores which are completely ignored. Thus, `1_000_000`

is the same `int`

value as `1000000`

.

We’ll explore the kinds of *capabilities* an `int`

object has shortly, but to connect them with your understanding of calculations, try asking the Python interpreter to *evaluate* some *integer arithmetic expressions*:

```
>>> 110 * 2
220
>>> 1 + 2 * 3
7
>>> (1 + 2) * 3
9
>>> 1_234 + 1
1235
```

`float`

- Floating-point “Decimal” NumbersWhy not classify the *type* of a decimal numbers, such as `0.25`

, with a name like `dec`

or `decimal`

? Because they’re not *exactly* decimal numbers, if you need to be *very precise* about it. The name `float`

is short for *floating-point*.

Here’s a perplexing, motivating example of *very subtle* limitations of floating-point arithmetic versus true decimal arithmetic. Can you spot any *miniscule* errors?

```
>>> 0.25 + 0.25
0.5
>>> 10.0 / 3.0
3.3333333333333335
>>> 0.1 + 0.2
0.30000000000000004
```

The exact reasons for these computations having a slight error, less than a trillionth, are beyond your concerns right now, but the intuition behind it, and for the name *floating-point*, is as follows. Let’s imagine an elementary numbering system where you are given three digit placeholders to form a number with:

*d*.*d* × 10^{x}

For the two *d* digits, you can choose any digit between `0`

and `9`

. For the *x* digit, you can also choose any digit between `0`

and `9`

, *and* can choose for it to be positive or negative. *In reality, a float value has far more digits to work with than this, this is dramatically simplified for illustrative purposes.*

What is the *largest* number you can represent?

9.9 × 10^{9} which is 9,900,000,000!

What is the *smallest* number you can represent?

0.1 × 10^{ − 9} which is 0.0000000001

That’s a massive range of numbers considering you only needed 3 digits and a sign to represent each!

**What’s the catch?** Try describing a limitation of this system on your own.

What is the second largest number you can represent?

The second largest number would be 9.8 × 10^{9} or 9,800,000,000, that means there are 99,999,999 whole numbers between the largest and second largest that are impossible to represent using this system! More precisely, there are only 1,539 unique numbers this system can represent in the range between 0.0000000001 and 9,900,000,000! Your mathematics has taught you there are an infinite number of decimal numbers in this range!

The good news is modern floating-point precision numbers use 64-bits of precision and can thus represent 2^{64} different possible numbers. That’s astronomically more precision (over 18 quintillion possible numbers!) than our toy system above, but it is not *infinite* nor *arbitrary* precision. The motivating examples demonstrated there are often very small round-off errors in floating-point arithmetic that can be ignored. However, if you find yourself writing software that has no room for small errors in precision (rockets, bank software, medical devices, and so on), you will want to invest more time in understanding the trade-offs of *floating-point* numbers versus slower, more specialized options available for handling precise, numerical data.

We will use `float`

for numbers with decimal points without hesitation or concern moving forward. For our purposes in COMP110, and most purposes you will encounter in computing (apps, games, most data science applications) the exceedingly small amounts of “round-off error” pose no problems and will be ignored moving forward.

`str`

- Strings of Characters for “Textual” DataThe “textual” data type in Python is `str`

, short for * string of characters*, which is common terminology across programming languages. Strings are used

Why not simply call it *text*? *Text* often conveys a bias toward thinking in *words* and *visible symbols* like letters, whereas “characters” can be invisible (such as spaces, tabs, and new line breaks), non-“textual” (digits, code, special symbols, and so on), and from many different *character sets* (Arabic, Chinese, English, Emoji, German, and so on).

There are a few different syntaxes for working with `str`

literals, but we will default to choosing double quoted `str`

literals, such as `"hello, world"`

. The characters following the first double-quote character `"`

begin the contents of the string value and the *matched pairing* double quote ends the `str`

value. You can also use single-quotes, such as `'hello, world'`

, and the result is the same. Most programming languages use double-quoted strings, while some allow either or, so we’ll settle on the syntax that’s more generalizable.

If a `str`

begins and ends with a double quote, how can you have a double quote within a string? To solve this thorny issue, programming languages have settled on a technique called *escaping* which means placing a backslash character `\`

before some character with special meaning. This is best demonstrated through tinkering in an interactive Python session:

```
>>> "The students rejoiced, \"we love programming!\""
'The students rejoiced, "we love programming!"`
```

Notice the backslashes disappeared! *Escape codes* in `str`

data only appear in the code you write and when that code is evaluated to become a true `str`

object, the `\"`

is interpreted as simply a `"`

. Without the leading backslashes in this example, there would have been a syntactical error.

It is worth pausing to emphasize the characters of a string have no meaning in our program beyond simply being characters. For example, the string `"110"`

may have only digit characters in it, but **it is not number** and you cannot do numerical computations with it. Try to and you will see some surprising results:

```
>>> "110" + "110"
'110110`
```

The add *operator*, when used with two `str`

values, produces a third `str`

value that is the first `str`

’s characters immediately followed by the second’s. The technical term for this operation is *concatenation* which is a term that originates in the chemical concept of carbon atoms forming chained bonds with other carbon atoms. Here, characters are being “bonded” into a longer sequence.

`str`

is a Through your programming journey you will encounter many types of data that are considered **sequences** of simpler values. In fact, you already have! Notice a `str`

is a sequence of characters. The ordering of these characters is important! There’s a meaningful difference between “please” and “asleep”, even though they share the exact same characters.

We will explore *sequences* in much more depth soon, but while we are on the subject of `str`

values, let’s foreshadow a bit with some examples of how to access individual **items** in a sequence. In a `str`

value, each item is a character.

```
>>> "str!"[0]
's'
>>> "str!"[1]
't'
>>> "str!"[2]
'r'
>>> len("str!")
4
>>> "str!"[len("str!") - 1]
'!'
>>> 110[0]
TypeError: 'int' object is not subscriptable
```

**Subscription** syntax is formed by pairs of square brackets following a compound value. The individual items in a sequence can be accessed by their *index* inside of the square brackets. You can find the number of items in any sequence using the built-in `len`

function. Notice the `str`

value `"str!"`

has a length of 4 because there are three letter characters followed by an exclamation point character. The quotes are not a part of the `str`

’s contents once it is evaluated, they are simply how we denote `str`

literals in our programs.

**Index numbering starts from 0**. This is true in Python, as well as in most general-purpose programming languages. It takes some comfort to get used to! The first *item* in a sequence is always at index 0. In a sequence with 10 items, the last item’s index is 9. Since the `len`

function will return the number of items in a sequence, you can always find the last item’s index by computing `len(sequence) - 1`

.

Integers and floating-point values *are not sequences*, they are singular numerical values. Thus, you cannot subscript them like a `str`

.

A **docstring** is a special kind of string in Python used to document the programs you write. Typically you will write a *docstring* at the top of every file describing its purpose. As we begin defining subprograms, or functions, you will also write *docstrings* in them describing their purposes in plain English.

Docstrings are written for other humans to read, or for you to read in the future to help refresh the purpose of specific files and functions you authored. Rather than beginning and ending with a single double quote `"`

, a docstring stands out because it begins and ends with **three** double quotes strung together. For example: `"""This is a docstring."""`

As your programs become more complex, using docstrings will become more important. For now, when working in a stored program, begin each file with a docstring describing its purpose.

In the opening of this lesson it was said, “an object is a typed unit of data in memory”. There is a lot of information packed into that statement now that you know a bit more about memory and types! Types are important because they decide how data is interpreted and, as you will explore in the next lesson, inform how you can make use of the object and its capabilities. The operations you can perform on numbers, such as adding them or subtracting them, are different than the operations you can perform on strings. This is broadly true both in programming and the real world: when you have different types of objects you’ll want to do different things with them.

That’s 13,000,000,000,000,000,000,000 transistors produced! https://en.wikipedia.org/wiki/MOSFET↩

In later computer science courses you will encounter systems programming languages, as you work closer to the actual machine, and will learn in much more depth the various binary systems behind integer, floating point, ASCII/UTF-8 data, and more.↩