Strings & Text Processing

Strings are immutable

What does this mean? Let's say we have the string:

 s = "brahm"

Now, let's say that Brahm wants to change the string, so that the 'b' in his name is capitalized, like this: "Brahm"

To do that, Brahm might want to write a line of code like this, the same way you would change an element of a list:

 s[0] = "B" 
#this won't work!

But when he does this, Brahm runs into an error message:

TypeError: 'str' object does not support item assignment

This is a key difference between strings and lists: strings can't be mutated. The only way to change a string variable is to reassign it and make a new string.

So, Brahm can create a new string to capitalize the 'b' in his name:

 str = "Brahm"

Immutability guarantees that string parameters won’t change, like in this example below. Click through the slides to see how the strings are stored in the computer's memory:

String Functions

.upper() and .lower()

.upper() returns a string with all the letters in upper case, and .lower() returns a string with all the letters in lower case.

s = "So long and thanks for all the fish"
>>> s.upper()
" SO LONG AND THANKS FOR ALL THE FISH  "

>>> s.lower()
" so long and thanks for all the fish  "

.replace(char1, char2)

.replace(char1, char2) replaces all instances of the first parameter with the second parameter

s = "So long and thanks for all the fish"
>>> s.replace(“a”, “e”)
“ So long end thenks for ell the fish  ”

>>> s.replace(“s”, “”)
“ o long and thank for all the fih  ”

.find(char)

s = "So long and thanks for all the fish"
>>> s.find(“n”)
5

>>> s.find(“x”)
-1

.strip()

s = "      So long and thanks for all the fish       "
>>> s.strip()
“So long and thanks for all the fish”

.split()

s = "So long and thanks for all the fish"
>>> s.split()
[“So”, “long”, “and”, “thanks”, “for”, “all”, “the”, “fish”]

names = “Bruce,Diana,Victor,Barry,Clark,Arthur,Hal”
>>> names.split(“,”)
[“Bruce”, “Diana”, “Victor”, “Barry”, “Clark”, “Arthur”, “Hal”]

Some more string functions:

>>> chant = “wakanda forever”
>>> stop_balrog = “YOU SHALL NOT PASS”
>>> spaces = “   ”
>>> number = “42”

>>> chant.startswith(“wak”)
True
>>> stop_balrog.startswith(“you”)
False
>>> chant.endswith(“ver”)
True

>>> chant.title()
“Wakanda Forever”

>>> chant.islower()
True
>>> spaces.isspace()
True
>>> number.isdigit()
True

Because strings are immutable, these functions don’t change the string and return a new string instead.

How to Process A String

Processing a string involves transforming or inspecting the contents of the string

 for i in range(len(s)):
	char = s[i]
	# process char

 for char in s:
	# process char

Reversing a string

We can use the code we wrote to reverse a string to check if a string is a palindrome! A palindrome is a word that reads the same way from the front and from the back. Here are some examples: kayak, A man, a plan, a canal - Panama! Here's a palindrome in Hindi: कडक

def reverse_string(s):
	reverse = “”
	for ch in s:
	reverse = ch + reverse
	return reverse 

def is_palindrome(str): 
	normalized = normalize(str)
	rev = reverse_string(normalized)
	return normalized == rev

def normalize(str): 
'''
This function removes all spaces and punctuation from a string
'''
	normalized = ''
	for ch in str: 
		if ch.isalpha(): 
			normalized += ch.lower()
	return normalized
 

Solving a Puzzle

Let's return to the story of Bright Simons, who came up with the mPedigree problem. A significant problem people with malaria face is with counterfeit medicine. In 2013, the UN estimated that 700,000 people die each year because they are given counterfeit drugs. Bright Simons wanted to find a way to prevent malaria patients from receiving counterfeit medicine.

Here's how Bright Simon's solution works: Each box of medicine from the manufacturer has a label with a number on it. Someone can then take that number, and text it to mPedigree. If that number comes from a manufacturer, mPedigree will let them know. If that number comes from a counterfeiter who just put a random number on the box, mPedigree can tell the patient to not take the medicine.

You now have all the knowledge needed to implement Bright Simon's solution in Python. How can we write a function that generates these numbers for his solution?