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Notes/01_Introduction/03_Numbers.md
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@@ -1,5 +1,9 @@
[Contents](../Contents) \| [Previous (1.2 A First Program)](02_Hello_world) \| [Next (1.4 Strings)](04_Strings)
# 1.3 Numbers # 1.3 Numbers
This section discusses mathematical calculations.
### Types of Numbers ### Types of Numbers
Python has 4 types of numbers: Python has 4 types of numbers:
@@ -27,7 +31,7 @@ if d == 0:
print('d is False') print('d is False')
``` ```
*Don't write code like that. It would be odd.* *But, don't write code like that. It would be odd.*
### Integers (int) ### Integers (int)
@@ -47,9 +51,9 @@ Common operations:
x + y Add x + y Add
x - y Subtract x - y Subtract
x * y Multiply x * y Multiply
x / y Divide x / y Divide (produces a float)
x // y Floor Divide x // y Floor Divide (produces an integer)
x % y Modulo x % y Modulo (remainder)
x ** y Power x ** y Power
x << n Bit shift left x << n Bit shift left
x >> n Bit shift right x >> n Bit shift right
@@ -165,12 +169,18 @@ Try it out.
## Exercises ## Exercises
Reminder: These exercises assume you are working in the `practical-python/Work` directory. Look
for the file `mortgage.py`.
### Exercise 1.7: Dave's mortgage ### Exercise 1.7: Dave's mortgage
Dave has decided to take out a 30-year fixed rate mortgage of $500,000 with Guido’s Mortgage, Stock Investment, and Bitcoin trading corporation. Dave has decided to take out a 30-year fixed rate mortgage of $500,000
The interest rate is 5% and the monthly payment is $2684.11. with Guido’s Mortgage, Stock Investment, and Bitcoin trading
corporation. The interest rate is 5% and the monthly payment is
$2684.11.
Here is a program that calculates the total amount that Dave will have to pay over the life of the mortgage: Here is a program that calculates the total amount that Dave will have
to pay over the life of the mortgage:
```python ```python
# mortgage.py # mortgage.py
@@ -238,7 +248,7 @@ While you’re at it, fix the program to correct the for overpayment that occurs
`int()` and `float()` can be used to convert numbers. For example, `int()` and `float()` can be used to convert numbers. For example,
```pycon ```python
>>> int("123") >>> int("123")
123 123
>>> float("1.23") >>> float("1.23")
@@ -248,7 +258,7 @@ While you’re at it, fix the program to correct the for overpayment that occurs
With that in mind, can you explain this behavior? With that in mind, can you explain this behavior?
```pycon ```python
>>> bool("False") >>> bool("False")
True True
>>> >>>

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Notes/01_Introduction/04_Strings.md
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@@ -1,8 +1,12 @@
[Contents](../Contents) \| [Previous (1.3 Numbers)](03_Numbers) \| [Next (1.5 Lists)](05_Lists)
# 1.4 Strings # 1.4 Strings
### Representing Text This section introduces way to work with text.
String are text literals written in programs with quotes. ### Representing Literal Text
String literals are written in programs with quotes.
```python ```python
# Single quote # Single quote
@@ -20,12 +24,17 @@ look into my eyes, you're under.
''' '''
``` ```
Triple quotes capture all text enclosed in multiple lines. Normally strings may only span a single line. Triple quotes capture all text enclosed across multiple lines
including all formatting.
There is no difference between using single (') versus double (")
quotes. The same type of quote used to start a string must be used to
terminate it.
### String escape codes ### String escape codes
Escape codes are used to represent control characters and characters that can't be easily typed Escape codes are used to represent control characters and characters that can't be easily typed
at the keyboard. Here are some common escape codes: directly at the keyboard. Here are some common escape codes:
``` ```
'\n' Line feed '\n' Line feed
@@ -38,8 +47,8 @@ at the keyboard. Here are some common escape codes:
### String Representation ### String Representation
The characters in a string are Unicode and represent a so-called "code-point". You can Each character in a string is stored internally as a so-called Unicode "code-point" which is
specify an exact code-point using the following escape sequences: an integer. You can specify an exact code-point value using the following escape sequences:
```python ```python
a = '\xf1' # a = 'ñ' a = '\xf1' # a = 'ñ'
@@ -54,6 +63,7 @@ available character codes.
### String Indexing ### String Indexing
Strings work like an array for accessing individual characters. You use an integer index, starting at 0. Strings work like an array for accessing individual characters. You use an integer index, starting at 0.
Negative indices specify a position relative to the end of the string.
```python ```python
a = 'Hello world' a = 'Hello world'
@@ -62,7 +72,7 @@ c = a[4] # 'o'
d = a[-1] # 'd' (end of string) d = a[-1] # 'd' (end of string)
``` ```
You can also slice or select substrings with `:`. You can also slice or select substrings specifying a range of indices with `:`.
```python ```python
d = a[:5] # 'Hello' d = a[:5] # 'Hello'
@@ -71,6 +81,8 @@ f = a[3:8] # 'lowo'
g = a[-5:] # 'world' g = a[-5:] # 'world'
``` ```
The character at the ending index is not included. Missing indices assume the beginning or ending of the string.
### String operations ### String operations
Concatenation, length, membership and replication. Concatenation, length, membership and replication.
@@ -161,7 +173,8 @@ TypeError: 'str' object does not support item assignment
### String Conversions ### String Conversions
Use `str()` to convert any value to a string suitable for printing. Use `str()` to convert any value to a string. The result is a string holding the
same text that would have been produced by the `print()` statement.
```python ```python
>>> x = 42 >>> x = 42
@@ -172,7 +185,7 @@ Use `str()` to convert any value to a string suitable for printing.
### Byte Strings ### Byte Strings
A string of 8-bit bytes, commonly encountered with low-level I/O. A string of 8-bit bytes, commonly encountered with low-level I/O, is written as follows:
```python ```python
data = b'Hello World\r\n' data = b'Hello World\r\n'
@@ -201,9 +214,13 @@ text = data.decode('utf-8') # bytes -> text
data = text.encode('utf-8') # text -> bytes data = text.encode('utf-8') # text -> bytes
``` ```
The `'utf-8'` argument specifies a character encoding. Other common
values include `'ascii'` and `'latin1'`.
### Raw Strings ### Raw Strings
Raw strings are string literals with an uninterpreted backslash. They specified by prefixing the initial quote with a lowercase "r". Raw strings are string literals with an uninterpreted backslash. They
are specified by prefixing the initial quote with a lowercase "r".
```python ```python
>>> rs = r'c:\newdata\test' # Raw (uninterpreted backslash) >>> rs = r'c:\newdata\test' # Raw (uninterpreted backslash)
@@ -237,9 +254,9 @@ is covered later.
## Exercises ## Exercises
In these exercises, you experiment with operations on Python's string type. In these exercises, you'll experiment with operations on Python's
You should do this at the Python interactive prompt where you can easily see the results. string type. You should do this at the Python interactive prompt
Important note: where you can easily see the results. Important note:
> In exercises where you are supposed to interact with the interpreter, > In exercises where you are supposed to interact with the interpreter,
> `>>>` is the interpreter prompt that you get when Python wants > `>>>` is the interpreter prompt that you get when Python wants
@@ -250,7 +267,7 @@ Important note:
Start by defining a string containing a series of stock ticker symbols like this: Start by defining a string containing a series of stock ticker symbols like this:
```pycon ```python
>>> symbols = 'AAPL,IBM,MSFT,YHOO,SCO' >>> symbols = 'AAPL,IBM,MSFT,YHOO,SCO'
>>> >>>
``` ```
@@ -259,7 +276,7 @@ Start by defining a string containing a series of stock ticker symbols like this
Strings are arrays of characters. Try extracting a few characters: Strings are arrays of characters. Try extracting a few characters:
```pycon ```python
>>> symbols[0] >>> symbols[0]
? ?
>>> symbols[1] >>> symbols[1]
@@ -273,8 +290,6 @@ Strings are arrays of characters. Try extracting a few characters:
>>> >>>
``` ```
### Exercise 1.14: Strings as read-only objects
In Python, strings are read-only. In Python, strings are read-only.
Verify this by trying to change the first character of `symbols` to a lower-case 'a'. Verify this by trying to change the first character of `symbols` to a lower-case 'a'.
@@ -287,7 +302,7 @@ TypeError: 'str' object does not support item assignment
>>> >>>
``` ```
### Exercise 1.15: String concatenation ### Exercise 1.14: String concatenation
Although string data is read-only, you can always reassign a variable Although string data is read-only, you can always reassign a variable
to a newly created string. to a newly created string.
@@ -295,14 +310,21 @@ to a newly created string.
Try the following statement which concatenates a new symbol "GOOG" to Try the following statement which concatenates a new symbol "GOOG" to
the end of `symbols`: the end of `symbols`:
```pycon ```python
>>> symbols = symbols + 'GOOG' >>> symbols = symbols + 'GOOG'
>>> symbols >>> symbols
'AAPL,IBM,MSFT,YHOO,SCOGOOG' 'AAPL,IBM,MSFT,YHOO,SCOGOOG'
>>> >>>
``` ```
Oops! That's not what you wanted. Fix it so that the `symbols` variable holds the value `'HPQ,AAPL,IBM,MSFT,YHOO,SCO,GOOG'`. Oops! That's not what you wanted. Fix it so that the `symbols` variable holds the value `'AAPL,IBM,MSFT,YHOO,SCO,GOOG'`.
```python
>>> symbols = ?
>>> symbols
'AAPL,IBM,MSFT,YHOO,SCO,GOOG'
>>>
```
In these examples, it might look like the original string is being In these examples, it might look like the original string is being
modified, in an apparent violation of strings being read only. Not modified, in an apparent violation of strings being read only. Not
@@ -311,12 +333,12 @@ time. When the variable name `symbols` is reassigned, it points to the
newly created string. Afterwards, the old string is destroyed since newly created string. Afterwards, the old string is destroyed since
it's not being used anymore. it's not being used anymore.
### Exercise 1.16: Membership testing (substring testing) ### Exercise 1.15: Membership testing (substring testing)
Experiment with the `in` operator to check for substrings. At the Experiment with the `in` operator to check for substrings. At the
interactive prompt, try these operations: interactive prompt, try these operations:
```pycon ```python
>>> 'IBM' in symbols >>> 'IBM' in symbols
? ?
>>> 'AA' in symbols >>> 'AA' in symbols
@@ -326,13 +348,13 @@ True
>>> >>>
``` ```
*Why did the check for "AA" return `True`?* *Why did the check for `'AA'` return `True`?*
### Exercise 1.17: String Methods ### Exercise 1.16: String Methods
At the Python interactive prompt, try experimenting with some of the string methods. At the Python interactive prompt, try experimenting with some of the string methods.
```pycon ```python
>>> symbols.lower() >>> symbols.lower()
? ?
>>> symbols >>> symbols
@@ -342,14 +364,14 @@ At the Python interactive prompt, try experimenting with some of the string meth
Remember, strings are always read-only. If you want to save the result of an operation, you need to place it in a variable: Remember, strings are always read-only. If you want to save the result of an operation, you need to place it in a variable:
```pycon ```python
>>> lowersyms = symbols.lower() >>> lowersyms = symbols.lower()
>>> >>>
``` ```
Try some more operations: Try some more operations:
```pycon ```python
>>> symbols.find('MSFT') >>> symbols.find('MSFT')
? ?
>>> symbols[13:17] >>> symbols[13:17]
@@ -364,14 +386,14 @@ Try some more operations:
>>> >>>
``` ```
### Exercise 1.18: f-strings ### Exercise 1.17: f-strings
Sometimes you want to create a string and embed the values of Sometimes you want to create a string and embed the values of
variables into it. variables into it.
To do that, use an f-string. For example: To do that, use an f-string. For example:
```pycon ```python
>>> name = 'IBM' >>> name = 'IBM'
>>> shares = 100 >>> shares = 100
>>> price = 91.1 >>> price = 91.1
@@ -383,6 +405,31 @@ To do that, use an f-string. For example:
Modify the `mortgage.py` program from [Exercise 1.10](03_Numbers) to create its output using f-strings. Modify the `mortgage.py` program from [Exercise 1.10](03_Numbers) to create its output using f-strings.
Try to make it so that output is nicely aligned. Try to make it so that output is nicely aligned.
### Exercise 1.18: Regular Expressions
One limitation of the basic string operations is that they don't
support any kind of advanced pattern matching. For that, you
need to turn to Python's `re` module and regular expressions.
Regular expression handling is a big topic, but here is a short
example:
```python
>>> text = 'Today is 3/27/2018. Tomorrow is 3/28/2018.'
>>> # Find all occurrences of a date
>>> import re
>>> re.findall(r'\d+/\d+/\d+', text)
['3/27/2018', '3/28/2018']
>>> # Replace all occurrences of a date with replacement text
>>> re.sub(r'(\d+)/(\d+)/(\d+)', r'\3-\1-\2', text)
'Today is 2018-3-27. Tomorrow is 2018-3-28.'
>>>
```
For more information about the `re` module, see the official documentation at
[https://docs.python.org/library/re.html](https://docs.python.org/3/library/re.html).
### Commentary ### Commentary
As you start to experiment with the interpreter, you often want to As you start to experiment with the interpreter, you often want to