📝 Working with Strings#

from friendly import *
from friendly.jupyter import friendly_tb

friendly_traceback 0.7.61; friendly 0.7.20.
Type 'Friendly' for basic help.

A lot of times you want to relay information to a user or send new instructions to a program. One common method to perform these tasks in Python is to use strings.

Defining Strings#

Recall that to represent a string you wrap it in quotes:

Single quotes ' allows you to embed double quotes "
Double quotes ' allows you to embed single quotes '
Triple quotes """ allows you to embed single and double quotes, and the string can span multiple lines

single_quote = 'Single quote allow you to embed "double" quotes in your string.'
double_quote = "Double quote allow you to embed 'single' quotes in your string."
triple_quote = """Triple quotes allows to embed "double quotes" as well as 'single quotes' in your string. 
And can also span across multiple lines."""

Strings are immutable which means you cannot replace a value in a string. You need to create a new string.

Definition

Immutable object - an object whose state cannot be modified after it is created

triple_quote = """This is triple quoted string using "single" quotes."""
triple_quote[35] = "'"

TypeError: 'str' object does not support item assignment

Traceback (most recent call last):                                                                  
  Code block [3], line 2                                                                            
    triple_quote[35] = "'"                                                                          
TypeError: 'str' object does not support item assignment

A TypeError is usually caused by trying to combine two incompatible   
types of objects, by calling a function with the wrong type of object,
or by trying to do an operation not allowed on a given type of object.

In Python, some objects are known as immutable: once defined, their   
value cannot be changed. You tried change part of such an immutable   
object: a string (str), most likely by using an indexing operation.

Exception raised on line 2 of code block [3].                                                       

       1| triple_quote = """This is triple quoted string using "single" quotes."""
     > 2| triple_quote[35] = "'"

    triple_quote:  'This is triple quoted string using "single" quotes.'                            
    triple_quote[35]:  '"'                                                                          

Friendly warning: you have redefined the python builtin open.

Formatting within a string#

You can add tabs to a string using \t

string = "Drexel \t Engineering"

print(string)

Drexel 	 Engineering

You can add a line break with \n

string = "Drexel \nEngineering"

print(string)

Drexel 
Engineering

Note

Did you notice the '\t', '\n' above? These are called escape characters. They start with a \ (backslash). Internally, they are not interpreted as normal strings, but rather as special characters that represent something else. For example - '\t' represents a tab. There are many more escape characters.

Length of a String#

You can find the length of a string using the built-in len() function:

len(triple_quote)

String Slicing#

Since strings are a sequence of characters, you can access it through slicing and indexing just like you would with Python lists or tuples.
Strings are indexed with respect to each character in the string and the indexing begins at 0

snack = "Chocolate Cookie."

The first index starts at 0

print(snack[0])

The last index is the period at 16

print(snack[16])

You can access the end of a string using [-1] index

print(snack[-1])

Extracting a Substring#

You can extract a substring using range slicing [Start index (included): Stop index (excluded)]

print(snack[10:16])

Cookie

You can also use a combination of positive and negative slicing

print(snack[10:-1])

Cookie

If you do not specify a start or end index it will use the entire array

# Stop value not provided
print(snack[0:])

# Start value not provided (Stop value excluded according to syntax)
print(snack[:-1])

# This is also allowed
print(snack[:])

Chocolate Cookie.
Chocolate Cookie
Chocolate Cookie.

Using a Stride#

It is possible to use a 3rd parameter stride to specify how many characters you would like to move forward.

mixed_string = "D0r0e0x0e0l0 0E0n0g0i0n0e0e0r0i0n0g"
print(mixed_string[::2])

Drexel Engineering

Reversing a String#

Strides can be used to reverse a string

ordered_string = "ABCDEFG"
print(ordered_string[::-1])

GFEDCBA

Concatenation of String#

There are many times where you might want to join or concatenate strings. In python this can be done using the simple addition operator.

string1 = "Drexel"
string2 = "Engineering"

alma_mater = string1 + " " + string2
print(alma_mater)

Drexel Engineering

This will only work if all the datatypes are strings

string1 = "Drexel"
string2 = "Engineering"
string3 = "Class of"
year = 2027

alma_mater = string1 + " " + string2 + " " + string3 + " " + year
print(alma_mater)

TypeError: can only concatenate str (not "int") to str

Traceback (most recent call last):                                                                  
  Code block [17], line 6                                                                           
    alma_mater = string1 + " " + string2 + " " + string3 + " " + year                               
TypeError: can only concatenate str (not "int") to str

A TypeError is usually caused by trying to combine two incompatible   
types of objects, by calling a function with the wrong type of object,
or by trying to do an operation not allowed on a given type of object.

You tried to concatenate (add) two different types of objects: a      
string (str) and an integer (int).

Exception raised on line 6 of code block [17].                                                      

       2| string2 = "Engineering"
       3| string3 = "Class of"
       4| year = 2027
       5| 
     > 6| alma_mater = string1 + " " + string2 + " " + string3 + " " + year
       7| print(alma_mater)

    string1:  'Drexel'                                                                              
    string2:  'Engineering'                                                                         
    string3:  'Class of'                                                                            
    year:  2027                                                                                     
    string1 + " ":  'Drexel '                                                                       
    string1 + " " + string2:  'Drexel Engineering'                                                  
    string1 + " " + string2 + " ":  'Drexel Engineering '                                           
    string1 + " " + string2 + " " + string3:  'Drexel Engineering Class of'                         
    string1 + " " + string2 + " " + string3 + " ":  'Drexel Engineering Class of '

string1 = "Drexel"
string2 = "Engineering"
string3 = "Class of"
year = 2027

alma_mater = string1 + " " + string2 + " " + string3 + " " + str(year)
print(alma_mater)

Drexel Engineering Class of 2027

Repeating Strings#

The multiply operator allows you to repeat strings

single_word = "hip "
line1 = single_word * 2 + "hurray! "
print(line1 * 3)

hip hip hurray! hip hip hurray! hip hip hurray!

Built-in String Methods#

Python provides many built-in methods or helper functions to manipulate strings.

Capitalize a String#

string = "drexel engineering"

print(string.capitalize())

Drexel engineering

Checks if lower case#

string = "drexel engineering"

print(string.islower())

True

Finds Substring#

Returns the lowest index in the string where the substring is found.

print(string.find("eng"))

Count Occurrence#

Counts how many times a substring occurs.

string = "drexel engineering " * 4

print("initial string: " + string)

print(string.count("eng"))

initial string: drexel engineering drexel engineering drexel engineering drexel engineering 
4

Replace Values#

str.replace(substring, new): replaces all occurrences of the substring in string with new. You can also define a third argument max, which replaces at most max occurrences of substring in the string.

string1 = "hip hip hurray! hip hip hurray! hip hip hurray!"
string2 = string1.replace("hip", "Hip")
print(string1)
print(string2)

print("\nIDs")
print(id(string1))
print(id(string2))

hip hip hurray! hip hip hurray! hip hip hurray!
Hip Hip hurray! Hip Hip hurray! Hip Hip hurray!

IDs
<<<<<<< Updated upstream
139974331557776
139974331557584
=======
4556082128
4550706928
>>>>>>> Stashed changes

You can see that the IDs for the string change. This is because they are immutable.

Splitting Strings#

str.split(sep=""): splits the string according to the delimiter (space if not provided) and returns a list of substrings.

string2.split(sep=" ")

[
│   'Hip',
│   'Hip',
│   'hurray!',
│   'Hip',
│   'Hip',
│   'hurray!',
│   'Hip',
│   'Hip',
│   'hurray!'
]

Formatting Strings#

Sometimes you might want to substitute a variable into a string.

Python Version 3.6 Formatter#

print(
    "{0} scored {1} points for the Drexel Dragons!".format("Coletrane Washington", 24)
)  # Accessing values by position

Coletrane Washington scored 24 points for the Drexel Dragons!

print(
    "{player_name} scored {points} points for the Drexel Dragons!".format(
        player_name="Coletrane Washington", points=24
    )
)

Coletrane Washington scored 24 points for the Drexel Dragons!

Python Version >3.6 F-strings#

f-string is a string literal that is prefixed with f or F. You can define identifiers to be used in your string within curly braces { }.

We recommend using F-strings because their beautiful practicality and simplicity.

player_name = "Coletrane Washington"
points = 24

print(f"{player_name} scored {points} points for the Drexel Dragons!")

Coletrane Washington scored 24 points for the Drexel Dragons!

ENGR 131: Winter 2023: Introductory Programming for Engineers

📝 Working with Strings

Contents

📝 Working with Strings#

Defining Strings#

Formatting within a string#

Length of a String#

String Slicing#

Extracting a Substring#

Using a Stride#

Reversing a String#

Concatenation of String#

Repeating Strings#

Built-in String Methods#

Capitalize a String#

Checks if lower case#

Finds Substring#

Count Occurrence#

Replace Values#

Splitting Strings#

Formatting Strings#

Python Version 3.6 Formatter#

Python Version >3.6 F-strings#