string_sequences

Lecture 10

Loop Review and Strings as Sequences

8 Ventôse, Year CCXXXI

Song of the day: Ocean Road by Melody's Echo Chamber (2022).

Sections

1. Review: Game Programming 101
2. Strings As Sequences
3. String Comparison
4. The in Operator
5. Special Characters And print's end Parameter

Part 0 (Review): Game Programming 101

In game programming, your standard game loop looks as follows:

INITIALISE the game

WHILE user hasn't QUIT:
    ASK for user
    PROCESS user input
    DISPLAY results of process

SHUT DOWN the game

We're going to create a simple mental math game that will follow this general structure. The goal of the game is for us to be able to calculate the average of a given set of random numbers in our heads. The user will decide:

• The upper limit of the randomly generate numbers (the lower limit will always be 1)
• What the tolerance of their answer should be (i.e. if the tolerance is 1, and the answer is 7, then all answers between 6 and 8 are considered correct)
• How many random numbers will be averaged.

The user will also have the ability to decided whether they want to play the game again after guessing the answer, or simply quit. Here's a sample execution of how the whole game would look like (you can format your output however you like as long as the mechanics of the game work the same):

|————————WELCOME TO THE NUMBER GUESSING GAME————————|
> ENTER THE UPPER LIMIT OF THE GENERATED NUMBERS (must be higher than 0): 10
> ENTER THE TOLERANCE OF YOUR ANSWER: 0.5
> READY? (press any key to begin) 
> HOW MANY NUMBERS WOULD YOU LIKE TO AVERAGE? 5
> ADDING 6 TO THE SUM...
> ADDING 4 TO THE SUM...
> ADDING 1 TO THE SUM...
> ADDING 9 TO THE SUM...
> ADDING 9 TO THE SUM...
> WHAT IS THE AVERAGE OF THESE 5 NUMBERS? 6
CORRECT! THE EXACT ANSWER WAS 5.8. WOULD YOU LIKE TO PLAY AGAIN? (press any key for yes, 'q' for no) 
> HOW MANY NUMBERS WOULD YOU LIKE TO AVERAGE? 2
> ADDING 6 TO THE SUM...
> ADDING 8 TO THE SUM...
> WHAT IS THE AVERAGE OF THESE 2 NUMBERS? 7
CORRECT! THE EXACT ANSWER WAS 7.0. WOULD YOU LIKE TO PLAY AGAIN? (press any key for yes, 'q' for no) q
GAME OVER.

_{Note here that the numbers 6, 4, 1, 9, and 9 in the first run of the game are being automatically generated and added to the sum without input from the user. The correct answer is 5.8 here, and the user entered 6. This is considered correct because the tolerance of the game is 0.5. Therefore, any number between 5.2 and 6.4 is considered correct.}

This program has several parts, so let's tackle it section by section.

Initialising The Game

We want the intialisation process of the game to do the following:

1. Welcome the user to the game
2. Ask what tolerance of the correct answer they want? (you can assume that they will enter a non-zero positive number)
3. Ask the user what the upper limit of the generated numbers will be (you can assume that they will enter an integer)
   • If the user enters a number for the upper limit that is lower than or equal to the 0, your program must ask them to enter another number again until they enter a number that is higher than the 0
4. Ask the user if they are ready to begin, and have them enter any key to begin the game

A sample execution of this section will behave as follows:

|————————WELCOME TO THE NUMBER GUESSING GAME————————|
> ENTER THE UPPER LIMIT OF THE GENERATED NUMBERS (must be higher than 0): 10
> ENTER THE TOLERANCE OF YOUR ANSWER: 0.5
> READY? (press any key to begin) 

Make sure to get this part to work before moving on!

One Run Of The Game

Before we get started with the actual game loop, try to get the game to run just once. So, forget about asking the user if they want to play again—just pretend that they're going to play once, and that's it.

If this is the case, your job is to:

1. Ask the user how many randomly generated numbers they want.
2. Generate that many random numbers and add them together.
3. Calculate the average.
4. Ask the user to guess that average.
   1. If they guess within the plus/minus tolerance of the correct answer, then the game ends (make sure to tell them use that they were correct).
   2. If they don't guess within that range, the game asks them to enter an answer again.

A sample execution of the game up to this point would look like this:

|————————WELCOME TO THE NUMBER GUESSING GAME————————|
> ENTER THE UPPER LIMIT OF THE GENERATED NUMBERS (must be higher than 0): 10
> ENTER THE TOLERANCE OF YOUR ANSWER: 0.5
> READY? (press any key to begin) 
> HOW MANY NUMBERS WOULD YOU LIKE TO AVERAGE? 5
> ADDING 6 TO THE SUM...
> ADDING 4 TO THE SUM...
> ADDING 1 TO THE SUM...
> ADDING 9 TO THE SUM...
> ADDING 9 TO THE SUM...
> WHAT IS THE AVERAGE OF THESE 5 NUMBERS? 6
CORRECT! THE EXACT ANSWER WAS 5.8.

Make sure to get this part to work before moving on!

A proper game loop

Finally, get your program to ask the user if they want to play again. You already know how to make your program run once. So what do we do to code that we want to run endlessly until the user tells us to stop?

Take a look at the first sample execution to see how your game should behave.

Part 1: Strings as Sequences

Remember that a for-loop iterates through every member of a sequence?

for number in range(0, 10):
    print("Sequence member '", number, "'", sep="")

Output:

Sequence member '0'
Sequence member '1'
Sequence member '2'
Sequence member '3'
Sequence member '4'
Sequence member '5'
Sequence member '6'
Sequence member '7'
Sequence member '8'
Sequence member '9'

So, in this case, our sequence is every whole number between and including 0 and 9:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9

It turns out that, in Python, sequences aren't limited to being numerical—they can also be alpha-numerical:

alpha_seq = "Adachi and Shimamura"

for character in alpha_seq:
    print(character)

Output:

A
d
a
c
h
i
 
a
n
d
 
S
h
i
m
a
m
u
r
a

So is a string a sequence? Technically. It can basically be used in sequence-like ways, which makes it very handy for programmers, since a good amount of user-input that we receive is in string form. The ability to be able to consider characters one-by-one is will prove to be indispensable.

---

What are other cool, sequence-like things we can do with strings?

Well, what if we wanted to access any individual letter in a string? Let's say we have the following activity log in a chat-room:

[A]: hey
[A]: whats up
[B]: Please leave me alone

In this case, it's pretty clear that the "A" and "B" characters represent two members of a chat-room. Moreover, we know that both "A" and "B" are always the second character in the whole line. We can use this information to our advantage by using something called indexing.

Let's say we wanted to find out whether user A or user B sent the message:

ID_LOCATION = 1

message = "[B]: Please leave me alone"
user_id = message[ID_LOCATION]

print("User", user_id, "sent this message.")

Output:

User B sent this message.

So, what did we do? The key line here is:

user_id = message[ID_LOCATION]

Since ID_LOCATION is equal 1, we could read this line as:

Take the first character from the string variable message and store it inside a variable called user_id.

Why is it the first character if A and B always appear after the [? General speaking, all programming languages start counting from 0 instead of 1. That's actually why the range() function's default starting value is 0—it's an inherent characteristic of most programming languages. So if you wanted to refer to the [ in the strings above, you'd say:

In the string "[B]: Please leave me alone", the 0th (zeroth) element is [.

So with this knowledge, we can now print each of the characters of a string in two ways:

BOOK_TITLE = "In Search of Lost Time"
LENGTH = 22

# using sequences
for letter in BOOK_TITLE:
    print(letter)

# using indices
for index in range(LENGTH):
    letter = BOOK_TITLE[index]
    print(letter)

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One quick other (very) important thing about strings is that they are immutable. That is, once you define the value of a string, you cannot change it. The only way to do something similar is to create a whole new string using the value of the old string:

first_name = "Élisabeth Louise"
last_name = "Vigée Le Brun"
full_name = first_name + " " + last_name

print(full_name)

Output:

Élisabeth Louise Vigée Le Brun

Here, it is very important to recognize that the string full_name is not first_name with the strings " " and last_name appended to it. Instead, it is a completely different string, existing in a completely different place in memory. This new string will simply happen to have the contents of first_name, " ", and last_name put together in that order—and only because we asked Python to create it in such a way.

Mutability / immutability is a huge topic in this class and computer science in general, so don't forget the words. We'll see them again soon enough.

Part 2: String Comparison

If we have a variable called string, and it has a value of "abc". What does the following expression evaluate to?

>>> string == "abc"

Well, let's check the output:

>>> string == "abc"
True

Makes sense; the comparison operator == checks whether two Python objects are equal in value. Since the value of string is "abc", it is indeed equal to the string "abc".

So, if we can check for the equality of strings, can we check for inequality?

>>> string != "not abc lol"
True

Makes sense; these strings are not at all the same in value. These are both pretty intuitive operations, but what about comparing them using >, <, >=, and <=?

>>> string >= "not abc lol"
False

>>> string < "bcd"
True

Clearly, these operations are not causing errors, so how do they work? When comparing string, Python uses their lexicographical order in order to determine whether one is larger than the other. Lexicographical order simply means applying a value of, say, 1 to "a", 2 to "b", 3 to "c", etc. (the numerical equivalent of "a" is actually 97, but don't worry about that for now).

This means that:

>>> "a" > "b"
False
>>> "a" < "b"
True
>>> "A" < "b"
True

Notice that these operations are not case-sensitive—a very rare exception in programming.

What about comparisons using strings of different lengths? Python basically goes in order:

>>> "Paris" > "Parthenon"
False

1. Is the "P" from "Paris" equal to the "P" from "Parthenon"? Yes, so move on to the next character.
2. Is the "a" from "Paris" equal to the "a" from "Parthenon"? Yes, so move on to the next character.
3. Is the "r" from "Paris" equal to the "r" from "Parthenon"? Yes, so move on to the next character.
4. Is the "i" from "Paris" equal to the "t" from "Parthenon"? No, so apply the > operation.
5. Is "i" > "t"? "i" has a lower lexicoogical value than "t" (i.e. it appears earlier in the alphabet), so this operation evaluates to False.
6. The whole operation evaluates to False

What about this?

>>> "Car" >= "Cartagena"
False

1. Is the "C" from "Car" equal to the "C" from "Cartagena"? Yes, so move on to the next character.
2. Is the "a" from "Car" equal to the "a" from "Cartagena"? Yes, so move on to the next character.
3. Is the "r" from "Car" equal to the "r" from "Cartagena"? Yes, so move on to the next character.
4. Since "Car" has no more values to compare with "Cartagena", "Cartagena" is by default of larger value.
5. The operation, thus, evaluates to False.

Here are more examples:

>>> "Sonny Boy" < "SONNY BOY"
False

>>> "Nozomi" >= "Mizuho"
True

>>> "Napoleon I" > "Napoleon III"
False

>>> "!!!" <= "   "
False

That last one is a bit of a toughie if you don't know about ASCII values and how to find them, but don't worry—it's not important for now. We'll get there eventually.

Part 3: The in Operator

Yay, a new operator! in is actually kind of heaven-sent if you come from a Java/C++ background. in is what is called a membership operator, and it evaluates to either True or False:

>>> "Car" in "Cartagena"
True

>>> "PARIS" in "Parisian"
False

The above operations can be described in English as follows:

The string Car exists as a sub-string of the exact same value somewhere in the string Cartagena.

The string PARIS does not exist as a sub-string of the exact same value somewhere in the string Parisian.

That's why it's called a membership operator: it checks for the membership of an object inside another object. You can also check for non-membership:

>>> "Prague" not in "Czechia"
True

>>> "1" in "onetwothree"
False

>>> "net" not in "onetwothree"
False

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Part 4: Special Characters and print()'s end Parameter.

There are a few "special characters" in programming that we should be aware of. The two that we'll need in this course are "\n" and "\t".

\n is the newline operator:

information = "NAME: Alice Sara Ott\nOCCUPATION: Pianist\nBIRTHPLACE: München, West Germany"
print(information)

Output:

NAME: Alice Sara Ott
OCCUPATION: Pianist
BIRTHPLACE: Munich, West Germany

And \t is the tab, or indentation, operator:

first_half = "Jan\tFeb\tMar\tApr\tMay\tJun\t"
second_half = "Jul\tAug\tSep\tOct\tNov\tDec\t"

print(first_half)
print(second_half)

Output:

Jan	Feb	Mar	Apr	May	Jun	
Jul	Aug	Sep	Oct	Nov	Dec	

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