Week 7: Preparation

1:22

Before you continue with this week’s preparation, please visit Week 6: Closure and make sure that you:

• know the concepts listed in the syllabus, and

• have completed the checkpoint exercises.

Reading Material

In the Think Python (TP) book, strings are covered in the first 5 sections of Chapter 8 Strings and Regular Expressions. Additionally, we also cover f-strings, which the book introduces in the second section of Chapter 13 Files and Databases.

Copy-and-Run

Prep 7.1: Basics of Strings

Run the following code to be reminded what you already know about strings.

s1 = "DTU"
year = 1829
s = s1 + " " + "is founded in " + str(year)
print(s)
print(len(s))

You have also seen string comparison before. Can you predict the output of the following code?

check1 = 'Maja' == 'Maja'
check2 = 'Maja' == 'maja'
check3 = 'Maja' == 'Maja '
check4 = 'Maja' == "Maja"

print(check1, check2, check3, check4)

And lastly, check once more how len() works with strings.

len1 = len('Maja')
len2 = len('Maja ')
len3 = len('')
len4 = len('   ')

print(len1, len2, len3, len4)

Prep 7.2: String Indexing

Now run the following code.

bm_string = "batmand"
print(bm_string[0])
print(bm_string[3])
print(bm_string[0:3])
print(bm_string[3:])
print(bm_string[::2])
print(bm_string[::-1])

As you can see, you can access individual characters in a string by using square brackets [] just as you indexed lists. You can use slicing as well, making it even more important to get familiar with the syntax [start:stop:step] and how default values work. Look at the illustration below and use slicing to access:

• last three characters of the string (for batmand this would be and),

• all characters except the first and the last one (for batmand this would be atman),

• every second character but starting from the second one (for batmand this would be amn),

• in reversed order every second character starting from the last one (for batmand this would be datb).

When working with lists last week, you saw that list indexing returned elements, while list slicing returned lists, and the two could not be concatenated. Let’s see how this works with strings.

my_string = "I am a student at DTU"
my_character = my_string[0]
my_substring = my_string[6:14]

check = my_character + my_substring
print(check)

As you can see, this is different with strings. Individual elements are also strings, and you can concatenate them with other strings.

Prep 7.3: Strings Are Immutable

When working with lists last week, you saw that you can change individual elements of a list. Let’s see how this works with strings.

my_string = "i am a student at DTU."
my_string[0] = "I"
print(my_string)

As you can see, Python tells you that this is not possible. You cannot change individual characters in a string. We say that strings are immutable, which means that you cannot modify a string. Remember, you can always use reassignment where you use the old string value to construct a new one. Run the following code to see how this can be done.

my_string = "i am a student at DTU."
my_string = "I" + my_string[1:]
print(my_string)

Prep 7.4: Traversing Strings

Look at the following code. By combining your understanding of for-loops and string indexing, you should be able to predict the output of the code. Run the code to check if you were right.

my_string = 'Engineering is fun!'
for i in range(len(my_string)):
    print('Character with index', i, 'is', my_string[i])

Notice that a space and an exclamation mark are also characters, and can be accessed in the same way as any other character. Now try this for-loop.

for char in "Engineering is fun!":
    print(char)

As with lists, you can use the for-loop to directly traverse string elements. Think about what the following piece of code does before running it.

alphabet = "abcdefghijklmnopqrstuvwxyz"
vowels = "aeiou"
consonants = ""
for char in alphabet:
    if char not in vowels:
        consonants = consonants + char
print(consonants)

Prep 7.5: String Methods

Try running the following code snippet.

title = "02002 Computer Programming"
print(title)
title = title.upper()
print(title)
title = title.lower()
print(title)

Similarly to lists, strings have many built-in methods for common operations. However, since strings are immutable, none of these methods change the string. Look back at the code above and confirm that both lines with method invocation are reassigning the returned value to the variable. Look back at how you used the sort() method with lists to see what method invocation looks like without reassignment.

Run the code snippets below and experiment with string methods count() and find().

my_string = "Mississippi River is a river in the United States."

count1 = my_string.count("s")
count2 = my_string.count("S")
count3 = my_string.count("i")
count4 = my_string.count("x")
count5 = my_string.count("iss")

print(count1, count2, count3, count4, count5)

my_string = "One two three four five six seven eight nine ten"

index1 = my_string.find("e")
index2 = my_string.find("two")
index3 = my_string.find("ten")
print(index1, index2, index3)

index4 = my_string.find("f")
index5 = my_string.find("f", index4 + 1)
index6 = my_string.find("f", index5 + 1)

print(index4, index5, index6)

index7 = my_string.find("g")
index8 = my_string.find("g", 0, 30)
print(index7, index8)

In some assignments, we give the find() method one additional argument, an index to start searching from. In the last assignment, we provided an additional argument to end the search before a specific index. Experiment with these arguments and see how they affect the output.

As you can see, the find() method works almost exactly as index(), but it returns -1 if the substring is not found, instead of raising an error.

Copy the code that you just tried, and replace find() with index(). Run the code and check which lines produce the same output and which produce an error.

Now, run this code snippet:

s = "Zinc is a metal"
s = s.replace("Zinc", "Iron")
print(s)

You might be thinking that by using indexing, slicing, and looping, you could achieve the same behavior as with string methods. That is correct. Therefore, it is not crucial to remember all the string methods. There are many that we do not cover in this course; you can look them up. Nevertheless, it is good to know that they exist and can simplify your code.

You can find the most useful string methods on the W3Schools page and the full list of string methods in the Python documentation.

Prep 7.6: String Methods split() and join()

Run this code.

physicists_string = "Bohr Heisenberg Schrödinger Dirac Pauli Fermi"
physicists = physicists_string.split()
print(physicists)

Re-run the code or make small modifications to answer the following questions.

• What is the type of the variable returned by the split() method? If you are in doubt, run print(type(physicists)) to check.

• How would the method work if there were more than one space between some of the words?

• What would happen if there were one or more spaces at the beginning or the end of the string?

Now run this code to see other uses of the split() method.

spaces_string = "many   spaces"
print(spaces_string.split())
print(spaces_string.split(" "))

ratings = "Great, Good, Poor, Okay, Poor, Good, Great, Okay"
print(ratings.split(", "))

question = "How much wood would a woodchuck chuck if a woodchuck could chuck wood?"
print(question.split("chuck"))
print(question.split("wood"))

Now run this code.

physicists = ['Bohr', 'Heisenberg', 'Schrödinger', 'Dirac', 'Pauli', 'Fermi']
separator = " "
together = separator.join(physicists)
print(together)

Now look at the line where the method join() is invoked, and answer the following questions. If needed, make small modifications to the code to check your answers.

• What is the type of the variable returned by the join() method? If you are in doubt, run print(type()) to check.

• What is the type of the variable given to the join() method as a parameter? That is, what is the type of the variable in the parentheses?

• What is the type of the variable the method is invoked on? That is, what is the type of the variable before the dot?

Now run this code to see other uses of the join() method.

physicists = ['Bohr', 'Heisenberg', 'Schrödinger', 'Dirac', 'Pauli', 'Fermi']
joint = ", ".join(physicists)
print(joint)
print(" - . - ".join(physicists))

Prep 7.7: The in Operator for Strings

Run the following code and observe the output.

dna_seq = 'TTAACGCATGCCATAGGACGGTTAGGCTCAGAACCCGCAACCAATACACGTGATTTTCTCGTCCCCTG'
pattern = 'CTCG'
match = pattern in dna_seq
print(match)

And run this code to figure out whether the in operator is case-sensitive.

print("i" in "Mississippi")
print("I" in "Mississippi")
print("kansas" in "arkansas")
print("Kansas" in "arkansas")

Prep 7.8: Everybody Loves f-strings

In this course, we have used print statements to track what our code is doing. Look now at this example.

planet = 'Mars'
distance = 1.5
message = f"{planet} is {distance} AU from the Sun"
print(message)

You have learned how to construct such a message using string concatenation and by casting integers to strings. However, Python has f-strings that make this much easier. Everything inside the curly braces {} is evaluated and converted to a string.

Look how f-strings work in the following code.

for name in ["Anders", "Vedrana"]:
    if name=="Vedrana":
        s = "is"  
    else:
        s = "is NOT"
    print(f"{name} {s} teaching this course.")

import math
print(f"Pi to 3 decimal places is {math.pi:.3f}")
print(f"Two thirds is {2/3}")
print(f"Two thirds with 5 decimal places is {2/3:.5f}")

for i in range(1, 11):
    s = ""
    for j in range(1, 11):
        s = s + f'{i*j:4}' 
    print(s)    

hours = 9
minutes = 5
print(f"Time is {hours:02}:{minutes:02}")

As you can see, using f-strings, you can decide how numbers are to be printed: the number of decimal places, the space occupied by the number, and whether to use leading zeros.

Below we see some common ways to use format specifiers for float and int variables (using 216 and 3.141592653589793 as examples). The letter d refers to an integer, and the letter f refers to a float.

Data type	Format specifier description	Example	Output
int	-	f"{216}"	216
int	[int] (d)	f"{216:d}"	216
int	[width][int] (5d)	f"{216:5d}"	​ ​ 216
int	[leading_zeros][width][int] (05d)	f"{216:05d}"	00216
int	[float] (f)	f"{216:f}"	216.000000
float	[int] (d)	f"{pi:d}"	ValueError
float	-	f"{pi}"	3.141592653589793
float	[float] (f)	f"{pi:f}"	3.141593
float	.[precision][float] (.2f)	f"{pi:.2f}"	3.14
float	[width].[precision][float] (8.2f)	f"{pi:8.2f}"	​ ​ ​ ​ 3.14
float	[leading_zeros][width].[precision][float] (08.2f)	f"{pi:08.2f}"	00003.14

To show the power of f-strings, consider the following snippets of code, which are very cumbersome to write without f-strings.

def greatest_common_divisor(a, b):
    divisor = 1
    for i in range(2, min(a, b)+1):
        if a % i == 0 and b % i == 0:
            divisor = i
    return divisor

a = 12
b = 18
gcd = greatest_common_divisor(a, b)
print(f"Simplifying ratio {a}/{b}:\n {a}/{b} = ({a}/{gcd})/({b}/{gcd}) = {a//gcd}/{b//gcd} = {a/b:.3f}")

max_iter = 1000
pi = 0
for i in range(1, max_iter+1):
    pi = pi + 4 * (-1)**(i+1) / (2*i-1)
    if i % 100 == 0 or i in [1, 10, 20, 50]:
        print(f"Approximating pi step {i:04}/{max_iter}: {pi:.5f}")

Prep 7.9: Newline \n and Other Escape Characters

Run this code.

my_string = "This is the first line.\nThis is the second line."
print(my_string)

The combination \n is called an escape character. It represents a special character in a string — in this case, a new line. The most commonly used escape character is \n, which represents a new line. Try how this works in the following code.

message = "Hello" + 12*"\n-" + "\nWorld"
print(message)

What do you think the length of the string message from the code above is? Run print(len(message)). What is the length of \n?

Some other escape characters are:

• \t: Tab

• \\: Backslash (useful for writing file paths on Windows)

• \': Single quote (useful when a string is enclosed in single quotes)

• \": Double quote (useful when a string is enclosed in double quotes)

Try some of these escape characters in the following code.

print("One\ttwo\tthree\tfour")
print("En\tto\ttre\tfire")

print("C:\\Users\\username\\Documents\\file.txt")

When you print a string, you see the effect of the escape characters, but not the escape characters themselves. Now run this code.

text = "En\tto\ntre\tfire"
print(text)
print(repr(text))

What is the difference between the two printed outputs?

As you can see, the function repr() gives you the string as you would write it in your code, with quotes and escape characters. This is useful when you want to see which escape characters a string contains.

Run this code and compare the two strings.

text = "En\tto\ntre\tfire"
text_repr = repr(text)
print(text_repr)
print(len(text))
print(len(text_repr))

Why is text_repr longer than text? Notice that repr() returns a string, so you need print() to see it, just as with any other string.

Now run this code.

text = "First line\nSecond line\nThird line"
lines = text.split('\n')
print(lines)

How many elements does lines have? What happened to the \n characters? Use repr() on text and on each element of lines if you are not sure.

Prep 7.10: Fix String Errors

Below are snippets of code that cause an error. Figure out what is wrong and fix it.

print(upper("dtu"))

Solution

The upper() is not a function; it is a method and should be invoked using dot-notation.

import math
print(f"The value of pi is {.5f:math.pi}")

Solution

The order of the parts inside curly braces is wrong. The value comes first, then a colon, then the format specifier.

last_name = "Hannemose"
print("The last letter of my last name is", last_name[9])

Solution

Strings use zero-based indexing, so the last letter of the string is at index 8. Even better, you should use negative indexing to access the last character.

alphabet = "abcdefghijklmnopqrstuvwxyz"
print("first letter of the alphabet:", alphabet(0))

Solution

You should use square brackets [] to index a string, not parentheses ().

path = "C:\Documents\User\File"

Solution

A single backslash character \ is used in escape characters. To use a backslash as a normal character, you need to escape it with another backslash.

print("It's a sunny day")
print('It's a rainy day')

Solution

The first line works because the string is enclosed in double quotes ("), so the single quote (') inside it is a normal character. The second line does not work because the string is enclosed in single quotes, so the single quote in It's ends the string. Enclose the string in double quotes, or escape the single quote as \'.

Prep 7.11: Predict the Output

For each of the following snippets of code, try to predict the output before running them.

print("i'm not angry!".upper())

lyrics = "Looks like it's gonna be a great day today"
print(lyrics.split()[6])

print("I am a string".replace("a", "another"))

stans = "kazakhstan, uzbekistan, stanistan, turkmenistan"
print(stans.replace("stan", "land").split(", "))

print(("O"*20+"\n")*10)

print(f"{2002} Computer Programming")
print(f"{2002:5d} Computer Programming")
print(f"{2002:05d} Computer Programming")

fum = 5
bum = 7
print(f"{fum}+{bum}={fum+bum}")

Self Quiz

Question 7.1

What is the value of apple_pie after the following code is executed?

apple_pie = 'apple' + 'pie'

a)

'applepie'

b)

'apple pie'

c)

'apple_pie'

d)

apple+pie

Question 7.2

What are values of check1 and check2 after the following code is executed?

check1 = 'Hello My Friend' == 'hello my friend'
check2 = 'No way!' == "No way!"

a)

True and False

b)

False and True

c)

False and False

d)

True and True

Question 7.3

What is printed by the following code?

s = 'abcdefgh'
print(s[::2])

a)

bdfh

b)

gh

c)

aceg

d)

ab

Question 7.4

What happens when executing the following code?

s = 'Python programming!'
s[-1] = '?'
print(s)    

a)

It prints Python programming?

b)

It prints Python programming!

c)

It prints Python programming!?

d)

A TypeError occurs

Question 7.5

What happens when executing the following code?

s = 'Python programming!'
a = s[:6]
b = s[-1]
print(a + b)    

a)

It prints Python programming!

b)

It prints Python !

c)

It prints Python!

d)

A TypeError occurs

Question 7.6

How many lines does the following code print?

s = 'Python!!!'
for char in s:
    print(char)

a)

0

b)

1

c)

5

d)

6

e)

9

Question 7.7

What is printed by the following code?

s = 'Python programming!'
s.upper()
print(s)    

a)

Python programming!

b)

Python Programming!

c)

PYTHON PROGRAMMING!

d)

python programming!

Question 7.8

What is the value of c after the following code is executed?

s = 'Aabenraa'
c = s.count('a')

a)

-1

b)

0

c)

1

d)

2

e)

3

f)

Larger than 3

Question 7.9

What is the value of c after the following code is executed?

s = 'Aabenraa'
c = s.find('a')

a)

-1

b)

0

c)

1

d)

2

e)

3

f)

Larger than 3

Question 7.10

What is the value of c after the following code is executed?

s = 'Aabenraa'
c = s.find('a', 2)

a)

-1

b)

0

c)

1

d)

2

e)

3

f)

Larger than 3

Question 7.11

What is the value of n after the following code is executed?

text = "He didn't understand. Restless. Confused."
sentences = text.split('.')
n = len(sentences)

a)

0

b)

1

c)

2

d)

3

e)

4

Question 7.12

What is printed by the following code?

words = ['Python', 'is', 'fun']
sentence = ' '.join(words)
print(sentence)

a)

Python is fun

b)

Pythonisfun

c)

Python, is, fun

d)

Python_is_fun

Question 7.13

What is the value of check after the following code is executed?

check = 'A' in 'The quick brown fox jumps over the lazy dog'

a)

-1

b)

None

c)

True

d)

False

Question 7.14

What gets printed by the following code?

if 'in' in 'inside':
    print('Yes')
elif 'out' in 'outside':
    print('No')

a)

Nothing

b)

Yes

c)

No

d)

Yes and No

Question 7.15

What gets printed by the following code?

month = 6
day = 5
year = 2025
print(f'{day}/{month}-{year}')

a)

-2023.8

b)

5/6-2025

c)

05/06-2025

d)

05/06-25

e)

5/6-25

Question 7.16

What is printed by the following code?

string = 'One\nTwo'
print(string)

a)

One\nTwo

b)

OneTwo

c)

One

d)

Two

e)

One
Two

Question 7.17

What is the value of string_length after the following code is executed?

string_length = len('One\nTwo')

a)

6

b)

7

c)

8

d)

9

e)

10

Question 7.18

What is the value of c after the following code is executed?

days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']
c = 0
for day in days:
    if day[0] == 'T':
        c = c + 1

a)

0

b)

1

c)

2

d)

3

e)

4

Question 7.19

You want to print the fraction 1/3 as a decimal number with 2 decimal places. Which of the following code snippets will achieve this?

a)

print(f'{1/3:02}')

b)

print(f'{1/3:.2f}')

c)

print(f'{1/3:2d}')

d)

print(f'{1/3:2}')

Question 7.20

How many lines does the following code print?

beaches = 'Acapulco, Boulders, Copacabana, Navagio, Tulum, Waikiki'
for beach in beaches:
    print(beach)

a)

0

b)

1

c)

6

d)

7

e)

More than 7

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