# Python - Basic Types & Type Conversion

OverviewQuestions:

What kinds of data do programs store?

How can I convert one type to another?

Objectives:

Explain key differences between integers and floating point numbers.

Explain key differences between numbers and character strings.

Use built-in functions to convert between integers, floating point numbers, and strings.

Requirements:

Time estimation:30 minutesLevel:Introductory IntroductorySupporting Materials:Published:Apr 25, 2022Last modification:Feb 13, 2023License:Tutorial Content is licensed under Creative Commons Attribution 4.0 International License. The GTN Framework is licensed under MITpurl PURL:https://gxy.io/GTN:T00099version Revision:3

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Python is a typed language, data has a type, and different types of data cannot always be connected immediately and might need some conversion step before they can be used together. For instance if you add a number to a number, what should happen? If you add a number to a message, what do you expect will happen?

AgendaIn this tutorial, we will cover:

## Types

Every value in a program has a specific type.

Name | Python Code | Represents |
---|---|---|

Integer | `int` |
represents positive or negative whole numbers like 3 or -512. |

Floating point number | `float` |
represents real numbers like 3.14159 or -2.5. |

Character string | `str` |
text, written with either `'` or `"` quotes (they must match) |

### Checking the Type

Use the built-in function `type`

to find out what type a value has. This works on values as well as variables. But remember: the *value* has the type — the *variable* is just a label.

Check the type of values with the `type()`

function:

```
print(type(52))
print(type(3.14159))
```

You can also check the types of variables

```
fitness = 'average'
print(type(fitness))
```

### Methods

A value’s type determines what the program can do to it. Some operations may work

```
print(5 - 3)
```

And some operations may not work:

```
print('hello' - 'h')
```

For instance, you can use the `+`

and `*`

operators on strings.

```
full_name = 'Ahmed' + ' ' + 'Walsh'
print(full_name)
separator = '=' * 10
print(separator)
```

Some methods only accept specific types, or only work on specific types.

The built-in function `len`

returns the length of your data. Which of the following would you expect to work? `len(string)`

? `len(int)`

?

```
print(len(full_name))
print(len(52))
```

### Matching Types

Not all types support all operations, adding an integer to a string doesn’t make much sense:

```
print(1 + '2')
```

This does not work because it’s ambiguous: should `1 + '2'`

be `3`

(a number) or `'12'`

(a string)? Some types can be converted to other types by using the type name as a function.

```
print(1 + int('2'))
print(str(1) + '2')
```

### Operation Support

Here is a quick chart showing which operations are allowed for each pair:

Left\Right | `int` |
`float` |
`str` |
---|---|---|---|

`int` |
`+-*/` |
`+-*/` |
`*` |

`float` |
`+-*/` |
`+-*/` |
`` |

`str` |
`*` |
`` | `+` |

As you can see you can do `3 * "test"`

and `"test" * 3`

, but it doesn’t work with floats.

## Can mix integers and floats freely in operations.

Integers and floating-point numbers can be mixed in arithmetic. Python 3 (which we use) automatically converts integers to floats as needed.

```
print(f'half is {1 / 2.0}')
print(f'three squared is {3.0 ** 2}')
```

## Variables only change value when something is assigned to them.

If we make one cell in a spreadsheet depend on another, and update the latter,
the former updates automatically. However, this does **not** happen in programming languages.

```
variable_one = 1
variable_two = 5 * variable_one
variable_one = 2
print(f'first is {variable_one} and second is {variable_two}')
```

The computer reads the value of `first`

when doing the multiplication, creates
a new value, and assigns it to `second`

. After that, `second`

does not remember
where it came from. Every computation happens line-by-line.

Question: FractionsWhat type of value is 3.14159? How can you find out?

It is a floating-point number (often abbreviated “float”). It is possible to find out by using the built-in function

`type()`

.`print(type(3.14159)) <class 'float'>`

```
# Test out solutions here!
```

Question: Automatic Type ConversionWhat type of value is the result of (3.25 + 4)?

It is a float: integers are automatically converted to floats as necessary.

`result = 3.25 + 4 print(f'result is {type(result)}')`

`7.25 is <class 'float'>`

```
# Test out solutions here!
```

Question: Choose a TypeWhat type of value (integer, floating point number, or character string) would you use to represent each of the following? Try to come up with more than one good answer for each problem. For example, in # 1, when would counting days with a floating point variable make more sense than using an integer?

- Number of days since the start of the year.
- Time elapsed from the start of the year until now in days.
- Serial number of a piece of lab equipment.
- A lab specimen’s age
- Current population of a city.
- Average population of a city over time.
The answers to the questions are:

- Integer, since the number of days would lie between 1 and 365.
- Floating point, since fractional days are required
- Character string if serial number contains letters and numbers, otherwise integer if the serial number consists only of numerals
- This will vary! How do you define a specimen’s age? whole days since collection (integer)? date and time (string)?
- Choose floating point to represent population as large aggregates (eg millions), or integer to represent population in units of individuals.
- Floating point number, since an average is likely to have a fractional part.

Question: Division TypesIn Python 3, the

`//`

operator performs integer (whole-number) floor division, the`/`

operator performs floating-point division, and the`%`

(ormodulo) operator calculates and returns the remainder from integer division:`print(f'5 // 3: {5 // 3}') print(f'5 / 3: {5 / 3}') print(f'5 % 3: {5 % 3}')`

`5 // 3: 1 5 / 3: 1.6666666666666667 5 % 3: 2`

If

`num_subjects`

is the number of subjects taking part in a study, and`num_per_survey`

is the number that can take part in a single survey, write an expression that calculates the number of surveys needed to reach everyone once.We want the minimum number of surveys that reaches everyone once, which is the rounded up value of

`num_subjects/ num_per_survey`

. This is equivalent to performing a floor division with`//`

and adding 1. Before the division we need to subtract 1 from the number of subjects to deal with the case where`num_subjects`

is evenly divisible by`num_per_survey`

.`num_subjects = 600 num_per_survey = 42 num_surveys = (num_subjects - 1) // num_per_survey + 1 print(num_subjects, 'subjects,', num_per_survey, 'per survey:', num_surveys)`

`600 subjects, 42 per survey: 15`

```
# Test out solutions here!
```

Question: Strings to NumbersWhere reasonable,

`float()`

will convert a string to a floating point number, and`int()`

will convert a floating point number to an integer:`print("string to float:", float("3.4")) print("float to int:", int(3.4))`

`string to float: 3.4 float to int: 3`

If the conversion doesn’t make sense, however, an error message will occur.

Input: Python`print("string to float:", float("Hello world!"))`

Output`Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: could not convert string to float: 'Hello world!'`

Given this information, what do you expect the following program to do?

What does it actually do?

Why do you think it does that?

`print("fractional string to int:", int("3.4"))`

What do you expect this program to do? It would not be so unreasonable to expect the Python 3

`int`

command to convert the string “3.4” to 3.4 and an additional type conversion to 3. After all, Python 3 performs a lot of other magic - isn’t that part of its charm?`int("3.4")`

However, Python 3 throws an error.

`Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: invalid literal for int() with base 10: '3.4'`

Why? To be consistent, possibly. If you ask Python to perform two consecutive typecasts, you must convert it explicitly in code.

`int(float("3.4"))`

`3`

```
# Test out solutions here!
```

Question: Arithmetic with Different TypesWhich of the following will return the floating point number

`2.0`

? Note: there may be more than one right answer.`first = 1.0 second = "1" third = "1.1"`

`first + float(second)`

`float(second) + float(third)`

`first + int(third)`

`first + int(float(third))`

`int(first) + int(float(third))`

`2.0 * second`

Answer: 1 and 4 give exactly 2.0. Answer 5 gives the value

`2`

which may be considered equivalent, but is not returning a float specifically.

```
# Test out solutions here!
```