Strings#

Previous Lecture#

In the previous lecture we covered functions.
- Functions are small, self-contained set of instructions that can be called at anytime.
We covered how to use built-in functions.
We covered how to write your own functions

Lecture Objectives#

In this lecture we will take a deep-dive into the str or string data type. You will learn
- What is the meaning of arithmetic operators on strings.
- How to convert strings to numbers and numbers to strings.
- How to use special characters inside strings.
- How to print results to the screen.
- How to get information from the user.
- How to format strings.
- What methods are available to manipulate strings.

Strings Basics#

In Python, text is represented as a string, which is a sequence of characters (letters, digits, and symbols).
In Python, we indicate that a value is a string by putting either single or double quotes around it.

Operations on Strings#

len() – to get length of a string
+ – can add strings, but not str and type float or int
* – to repeat a string
+= – add to another string and save value
int(3)
float(3.4)

Simple examples#

Single Quotes vs Double Quotes#

'Aristotle'
"Issac Newton"

Triple Quotes for String Literal#

'''Should you want a string
​ 	that crosses multiple lines,
​ 	Use matched triple quotes.'''

"""
This is a string literal. 
        Spaces are printed as is. 
  Hello there!"""

Determining the length of a string#

scientist = 'Issac Newton'
print(len(scientist))

Concatenating Strings#

'Alan Turning' + ' ' + 'Grace Hopper'

'NH' + 3 # This will not work. You cannot add a str and int type. 

'Four score and ' + str(7) + ' years ago'

Converting Strings to Numbers#

int('0')
int('11')
int('-324')
float('-324.40')

Repeating Strings#

'AT' * 5
'-' * 5

Using Special Characters in Strings#

How would you put a single quote inside a string that is declared using a single quote?
'\\' – how would you print /\/\
'\n'
'\''
'\"'
'\t' – useful for parsing TSV files

Printing in Python#

print('abbcd', 2, 3)
print('abbcd', 2, 3, sep='\n') # default separator is space

Getting information from the Keyboard#

number = input('Please enter a number: ')

Converting Numeric Values#

number = int(input('Please enter a number: '))

def convert_celsius(fahrenheit):
    return (fahrenheit - 32.0) * 5.0 / 9.0    

def convert_celsius():
    fahrenheit = float(input('Please enter temperature in Fahrenheit: '))
    celsius =  (fahrenheit - 32.0) * 5.0 / 9.0  
    print('The temperature in celsius is: ', celsius) 

String Formatting#

The fastest and latest way to do string formatting is using the F-strings (https://realpython.com/python-f-strings/)
```
PI = 3.14159265359 
print(f'{PI:.2f}')
```
But you have to know the other ways so you can read older code or use these ways if you have to use an older version of Python

Oldest Method#

 PI = 3.14159265359 
 name = 'PI'
 print('%s is %.2f' % (name, PI))  # oldest way format specifier is <width>.<precision><type>

Newer Method#

Still used for creating string templates

PI = 3.14159265359 
name = 'PI'
#{<index>:<format-specifier>} where the format specifier is <width>.<precision><type>
print(('{0} is {1:.2f}'.format('PI', PI)) ) # 

Newest Method#

Newest and fastest method for string formatting

PI = 3.14159265359 
name = 'PI'
# {<name_of_variable>:<format-specifier>} where the format specifier is <width>.<precision><type>
print(f'{name} is {PI:.2f}') # newest way

Reference: https://pyformat.info/

course_number = 'EAS503'
class_size = 113
class_average = 92.3

my_str * 10

x = '-'
x * 10

my_str = 'EAS503'
my_int = 113
my_float = 92.3

line1 = 'This is the first line.\n'
line2 = 'This is the second line.'
lines = line1 + line2

line = 'This is the first line.\n'
line += 'This is the second line.'

Example 1#

str_format = '{}'.format(course_number)
f_string = f'{course_number}'

print(str_format)
print(f_string)

Example 2#

str_format = 'The course number is {}.'.format(course_number)
f_string = f'The course number is {course_number}.'

print(str_format)
print(f_string)

Example 3 use index#

str_format = 'The course number is {}. It has {} students.'.format(course_number, class_size)
str_format = 'The course number is {0}. It has {1} students.'.format(course_number, class_size)
f_string = f'The course number is {course_number}. It has {class_size} students.'

print(str_format)
print(f_string)

Example 4 change index#

str_format = 'The course number is {1}. It has {0} students.'.format(course_number, class_size)
f_string = f'The course number is {class_size}. It has {course_number} students.'

print(str_format)
print(f_string)

Example 5 adding a float#

str_format = 'The course number is {0}. It has {1} students. The class average is {2}'.format(course_number, class_size, class_average)
f_string = f'The course number is {class_size}. It has {course_number} students. The class average is {class_average}.'

print(str_format)
print(f_string)

Example 6 specify number of spaces to use – width#

str_format = 'The course number is {0:10}. It has {1:10} students. The class average is {2:10}'.format(course_number, class_size, class_average)
f_string = f'The course number is {course_number:10}. It has {class_size:10} students. The class average is {class_average:10}.'

print(str_format)
print(f_string)

Example 7 right align#

str_format = 'The course number is {0:>10}. It has {1:>10} students. The class average is {2:>10}'.format(course_number, class_size, class_average)
f_string = f'The course number is {class_size:>10}. It has {class_size:>10} students. The class average is {class_average:>10}.'

print(str_format)
print(f_string)

Example 8 left align#

str_format = 'The course number is {0:<10}. It has {1:<10} students. The class average is {2:<10}'.format(course_number, class_size, class_average)
f_string = f'The course number is {course_number:<10}. It has {class_size:<10} students. The class average is {class_average:<10}.'

print(str_format)
print(f_string)

Example 9 center align#

str_format = 'The course number is {0:^10}. It has {1:^10} students. The class average is {2:^10}'.format(course_number, class_size, class_average)
f_string = f'The course number is {course_number:^10}. It has {class_size:^10} students. The class average is {class_average:^10}.'

print(str_format)
print(f_string)

Example 10 Padding with zeros#

student_id = 223333

str_format = 'The number padded {} padded with zeros {:08}'.format(student_id, student_id)
f_string = f'The number padded {student_id} padded with zeros {student_id:08}'

print(str_format)
print(f_string)

Example 11 Padding with dashes#

student_id = 223333

str_format = 'The number padded {} padded with zeros {:->8}'.format(student_id, student_id)
f_string = f'The number padded {student_id} padded with zeros {student_id:->8}'

print(str_format)
print(f_string)

https://scipython.com/book/chapter-2-the-core-python-language-i/string-representation-of-integers-with-comma-separated-thousands/

title = '|' + '{:^51}'.format('Cereal Yields (kg/ha)') + '|'
line = '+' + '-'*15 + '+' + ('-'*8 + '+')*4
row = '| {:<13} |' + ' {:6,d} |'*4
header = '| {:^13s} |'.format('Country') + (' {:^6d} |'*4).format(1980, 1990,
                                                                  2000, 2010)
print('+' + '-'*(len(title)-2) + '+',
      title,
      line,
      header,
      line,
      row.format('China', 2937, 4321, 4752, 5527),
      row.format('Germany', 4225, 5411, 6453, 6718),
      row.format('United States', 3772, 4755, 5854, 6988),
      line,
      sep='\n')

String methods#

We have already encountered functions: built-in functions and functions we have defined. A method is another kind of function that is attached to a particular type. This section covers the methods that are attached to string types.
Method calls in this form—'browning'.capitalize()—are shorthand for this: str.capitalize('browning').

String Methods

Summary#

Python uses type str to represent text as sequences of characters.
Strings are created by placing pairs of single quotes ’ or double quotes " around the text.

'''Should you want a string
    ​ 	that crosses multiple lines,
    ​ 	Use matched triple quotes.'''

Special characters like newline (\n) and tab (\t) are represented using escape sequences that begin with a backslash. For example, "this string\nspans\nthree lines".
Values can be printed using built-in function print, and input can be provided by the user using built-in function input.
Methods are like functions, except that the first argument must be an object of the class in which the method is defined.

EAS503

Strings

Contents

Strings#

Previous Lecture#

Lecture Objectives#

Strings Basics#

Operations on Strings#

Simple examples#

Single Quotes vs Double Quotes#

Triple Quotes for String Literal#

Determining the length of a string#

Concatenating Strings#

Converting Strings to Numbers#

Repeating Strings#

Using Special Characters in Strings#

Printing in Python#

Getting information from the Keyboard#

Converting Numeric Values#

String Formatting#

Oldest Method#

Newer Method#

Newest Method#

Example 1#

Example 2#

Example 3 use index#

Example 4 change index#

Example 5 adding a float#

Example 6 specify number of spaces to use – width#

Example 7 right align#

Example 8 left align#

Example 9 center align#

Example 10 Padding with zeros#

Example 11 Padding with dashes#

String methods#

Summary#