IACS Computes! 2019
IACS Computes! High School summer camp
Day 1
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7
Day 8
Day 9
Strings
What is a string? How is it different from another data type (ex. int)?
Basics
Let’s define a string and save it to a variable.
a = "Woof"
a = "Woof"
b = "Meow"
c = "Squirrel"
We can print the contents of a string in a simple and straightforward manner:
print(a)
Woof
Python strings can be written using ‘ or “, as long as they match.
print('Hello')
print("Goodbye")
Hello
Goodbye
We can add two strings together (this process is commonly called concatenation).
"Woof" + "Meow"
'WoofMeow'
We can also do this with strings stored in variables.
a = "Woof"
b = "Meow"
a + b
'WoofMeow'
Can I use multiplication instead of addition? How about division?
a * 5
'WoofWoofWoofWoofWoof'
Challenge: Take the strings “Hello” and “ World”, and print the sum twice back-to-back.
print(("Hello" + " World") * 2)
Hello WorldHello World
# First sum the two strings
my_string = "Hello" + " World"
# Print the new string twice by multiplying it times 2
print(my_string * 2)
Hello WorldHello World
Strings are basically lists
Yesterday, we learned about lists. Python sees strings as a list of characters. So, we can access characters in strings just like we do with elements in lists.
c = "Squirrel"
c[1]
'q'
c[10]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-11-ea0c20b3b68b> in <module>
----> 1 c[10]
IndexError: string index out of range
Just like in normal lists in Python, we also use negative indices.
c[-1]
'l'
Challenge: How would we find the sixth character of the string ‘race car’?
"race car"[6]
'a'
What if I want to include some quotation marks in my string? If I write them as part of the string, Python gets confused:
newstring0 = "And then I told her, "Well you better go catch it!""
File "<ipython-input-14-10b748bdfde9>", line 1
newstring0 = "And then I told her, "Well you better go catch it!""
^
SyntaxError: invalid syntax
How can I fix this problem?? Hint: I have to tell Python somehow that I don’t want this character to be special.
newstring1 = "And then I told her, \"Well you better go catch it!\""
print(newstring1)
And then I told her, "Well you better go catch it!"
newstring2 = "Here is\na new line"
print(newstring2)
Here is
a new line
newstring3 = "Here is a\ttab"
print(newstring3)
Here is a tab
newstring4 = "And I put a backslash \\ into this string."
print(newstring4)
And I put a backslash \ into this string.
Important Functions
So what are some built-in things we can do with strings? Just like with lists, we can use the len( ) function to find the length of the string.
a = "Woof"
len(a)
4
c = "Squirrel"
len(c)
8
We can remove the white space surrounding a string using a function called strip().
line = " Lots of spaces "
print(line.strip() + ".")
print(line.lstrip() + ".")
print(line.rstrip() + ".")
Lots of spaces.
Lots of spaces .
Lots of spaces.
We can also get rid of all the white space in a string and save each word into as a seperate element of a list using a function called split( ).
line = "So many spaces everywhere!"
new_list = line.split()
print(new_list) # note that this is a list of strings, not a string itself!
['So', 'many', 'spaces', 'everywhere!']
We can turn the new list of strings back into one big string.
print(" ".join(new_list))
So many spaces everywhere!
Challenge: Break the following sentence into individual words: “This is a boring sentence.” Then print each word, one per line.
s = "This is a boring sentence.".split()
for word in s:
print(word)
This
is
a
boring
sentence.
We can replace characters too!
print("cat".replace("c","h"))
hat
How do I get only the words out of this screen name?
a = "harry_james_potter"
a.split("_")
['harry', 'james', 'potter']
Oh but this is a name! We should really capitalize the first letter of each word.
name_list = a.split("_")
for i, name in enumerate(name_list):
name_list[i] = name.title()
print(name_list)
['Harry', 'James', 'Potter']
Similarly, we can change letters to upper and lower case with .upper() and .lower().
print('MY SHOUTY SENTENCE'.lower())
print('my shift key has broken'.upper())
my shouty sentence
MY SHIFT KEY HAS BROKEN
Testing Strings
We can test if a character or even a string is inside another string using the “in” keyword.
"a" in "car"
True
"cat" in "The cat is over there."
True
"cat" in "The quick brown fox jumped over the lazy dog's back."
False
Note that the in keyword is case senstive.
word = "Hello World"
print(word.count('l'))
3
print(word.find("H"))
0
print(word.index("World"))
6
"2334".isdigit()
True
print(word.isalnum()) # check if all char are alphanumeric
print(word.isalpha()) # check if all char in the string are alphabetic
print(word.isdigit()) # test if all char in string are digits
print(word.istitle()) # test if string contains title words
print(word.isupper()) # test if string contains upper case
print(word.islower()) # test if string contains lower case
print(word.isspace()) # test if string contains spaces
print(word.endswith('d')) # test if string endswith a d
print(word.startswith('H')) # test if string startswith H
False
False
False
True
False
False
False
True
True
Python III: Return of the Slice
So how would we go about changing a letter in a string? Unfortunately we can’t just use an equals sign as you would do for a list.
my_string = "Hello World!"
my_string[0] = "h" # Let's try to change the H to an h
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-39-3a8e474667eb> in <module>
1 my_string = "Hello World!"
2
----> 3 my_string[0] = "h" # Let's try to change the H to an h
TypeError: 'str' object does not support item assignment
What happened here? Just like with tuples, strings are immutable, which means that we cannot change characters in a string once it has been created. So how would we do it? Instead of changing the letter in the existing string, we need to create a new string made up of the old string along with any modifications we make. We can achieve this through slicing.
Let’s try changing the first letter again:
my_string = "Hello World!"
new_string = "h" + my_string[1:]
print(new_string)
hello World!
What if we want to change a letter in the middle of our string, say change the ‘w’ to a ‘W’? For this we’d slice up to the letter we want to change, add in our changed letter, then add the slice from the letter after that up to the end.
new_string = my_string[:6] + 'w' + my_string[7:]
print(new_string)
Hello world!
Practice Problems
Create a list containing some strings. Then add all of the strings together and print the result.
my_list = ["rabbit", "crocodile", "butterfly"]
" ".join(my_list)
'rabbit crocodile butterfly'
Write some code that determines how many words are in a string. (Assume that the string is a sentence where words are all seperated by a space).
my_string = "The quick brown fox jumps over the lazy dog"
len(my_string.split())
9
Write some code that doubles every letter. For example, ‘Help’ would become ‘HHeellpp’. Store the elongated string as a new string, and print it.
my_string = "Help"
new_string = ""
for letter in my_string:
new_string += letter * 2
print(new_string)
HHeellpp
Write some code that takes a string as input and prints the string, written in reverse. For example, if given the string ‘coconut” the cell would print ‘tunococ’.
"coconut"[::-1]
'tunococ'
Write some code that inserts the character ‘h’ at a given location inside another string. For example, create an integer and initialize it to 5. Then insert ‘h’ after the 5th position of the string ‘She sells seashells’.
my_string = "She sells seashells"
n = 5
my_string[:n] + 'h' + my_string[n:]
'She shells seashells'
Advanced Problems
Numbers: Write some code that determines if a string contains any number (any of the following: 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9) and returns True or False.
result = False
my_string = "blah3"
for l in my_string:
if l.isdigit():
result = True
print(result)
True
Vowels: Write some code that determines the index (or indices) in a string of all the vowels.
vowels = ["a", "o", "i", "u", "e"]
vowel_indices = []
my_string = "Hello world!"
# use enumerate to keep track of the index
# as we iterate over the letters
for i, letter in enumerate(my_string):
if letter in vowels:
vowel_indices.append(i)
print(vowel_indices)
[1, 4, 7]
Palindrome:
From Wikipedia (https://en.wikipedia.org/wiki/Palindrome):
A palindrome is a word, phrase, number, or other sequence of characters which reads the same backward or forward. Allowances may be made for adjustments to capital letters, punctuation, and word dividers. Examples in English include ‘A man, a plan, a canal, Panama!’, ‘Amor, Roma’, ‘race car’, ‘stack cats’, ‘step on no pets’, ‘taco cat’, ‘put it up’, and ‘Was it a car or a cat I saw?’.
Write some code that determines if a string is a palindrome. Make it case insensitive. Deal with punctuation as you choose (or not at all).
string = "tacocat"
# first we are going to strip the string of any punctuation
new_string = ""
for letter in string:
if letter.isalpha():
new_string += letter
# find the length of the string / 2 (rounded down to the nearest integer)
half_length = len(new_string) // 2
# now test whether the reversed string starts with the first half of the original string
palindrome = (new_string[::-1].startswith(new_string[:half_length]))
print(palindrome)
True