Exercises: Strings

A club badge needs two initials, a three-character code piece, and the code length.

Specification: Define badge_line(first, last, code). Use position 0 from first and last. Use the slice code[2:5]. Return one string in the shape initials | piece | length. Build it with string concatenation, and use str(len(code)) for the final part.

Worked sample calls:

badge_line("Mira", "Stone", "AX742Q") -> "MS | 742 | 6"
badge_line("Leo", "Park", "ZZcat9") -> "LP | cat | 6"
badge_line("Nia", "Young", "12SUN") -> "NY | SUN | 5"

A route board receives labels with spaces at the ends and underscores between words.

Specification: Define clean_route(text). Remove spaces from the ends, replace each underscore with a single space, and return the result in uppercase. Use only the taught string methods.

Worked sample calls:

clean_route("  north_gate " ) -> "NORTH GATE"
clean_route("moon_base") -> "MOON BASE"
clean_route("  red_fox_2  " ) -> "RED FOX 2"

A word gate opens when its text reads the same from both ends.

Specification: Define is_palindrome(text). Make a lowercase string with spaces replaced by empty text. Use exactly for i in range(len(cleaned) // 2). Compare cleaned[i] with cleaned[len(cleaned) - 1 - i]. Return False inside the loop on a mismatch, and return True after the loop.

Worked sample calls:

is_palindrome("Taco cat") -> True
is_palindrome("python") -> False
is_palindrome("Level") -> True

A message checker labels each character as a vowel, consonant, or other character.

Specification: Define classify_char(ch). Lowercase ch. If it is in "aeiou", return "vowel". Elif it is in "abcdefghijklmnopqrstuvwxyz", return "consonant". Else return "other".

Worked sample calls:

classify_char("U") -> "vowel"
classify_char("m") -> "consonant"
classify_char("7") -> "other"

A puzzle display hides vowels but leaves every other character unchanged.

Specification: Define mask_vowels(text). Start a new string as empty. Visit each character in text. If its lowercase form is in "aeiou", add "*" to the new string; else add the original character. Return the completed new string. Do not change the original string.

Worked sample calls:

mask_vowels("Rocket") -> "R*ck*t"
mask_vowels("SKY") -> "SKY"
mask_vowels("A-OK!") -> "*-*K!"

A decoder needs the first position where a chosen character appears.

Specification: Define position(text, ch). Loop through range(len(text)). Return i inside the loop at the first position where text[i] == ch. Return -1 after the loop when no position matched. The return value is an integer.

Worked sample calls:

position("banana", "n") -> 2
position("cocoa", "o") -> 1
position("planet", "x") -> -1

A word lab needs a printed table showing the total for every lowercase alphabet letter.

Specification: First define count_char(text, ch). Start an integer counter at 0, visit every character, add 1 when the character equals ch, and return the counter. Then define print_alphabet_scan(text). Lowercase text, print the heading letter | total, loop through "abcdefghijklmnopqrstuvwxyz", call count_char for each letter, and print each row with an f-string. Do not return the table.

Worked sample calls:

count_char("pepper", "p") -> 3
count_char("pepper", "z") -> 0

print_alphabet_scan("Bee") ->
letter | total
a | 0
b | 1
c | 0
d | 0
e | 2
f | 0
g | 0
h | 0
i | 0
j | 0
k | 0
l | 0
m | 0
n | 0
o | 0
p | 0
q | 0
r | 0
s | 0
t | 0
u | 0
v | 0
w | 0
x | 0
y | 0
z | 0

A launch report needs its number of words.

Specification: Define word_count(line). Add a purpose comment. Start a counter at 0 and increment it for each word in line.split(). Return the count; do not index the split result. Print the stand-in report with f"Words: {count}".

Worked sample calls:

word_count("rockets need fuel to fly") -> 5
print(f"Words: {word_count('rockets need fuel to fly')}") -> Words: 5

A crew roster turns full names into badge initials.

Specification: Define initials(line). Add a purpose comment. For each word in line.split(), add word[0].upper() to a result string. Return the result. Here word is a string; do not index the split result.

Worked sample calls:

initials("grace brewster hopper") -> "GBH"

A lock accepts exactly four digit characters.

Specification: Define is_valid_pin(pin). Add a purpose comment. Use if len(pin) != 4: to return False, elif not pin.isdigit(): to return False, and else: to return True. Negative numbers are not accepted.

Worked sample calls:

is_valid_pin("4071") -> True
is_valid_pin("40a1") -> False
is_valid_pin("407") -> False

A headline editor needs the first longest word.

Specification: Define longest_word(line). Visit words with for word in line.split():, keep the first word whose length is strictly greater than the saved best, and return it. Use f"Longest: {word}" for the stand-in report. Use an empty string when the line has no words.

Worked sample calls:

longest_word("nova rover launch pad") -> "launch"
print(f"Longest: {longest_word('nova rover launch pad')}") -> Longest: launch

A phrase checker ignores spaces and letter case.

Specification: Define is_sentence_palindrome(text). Remove spaces with replace, lowercase the result, then compare that cleaned string with its reversing slice [::-1]. Return the boolean.

Worked sample calls:

is_sentence_palindrome("Was it a car or a cat I saw") -> True

A transmitter compresses each run of matching characters.

Specification: Define rle(text). Walk the string in order, count consecutive copies of the current character, and append that character plus str(count) when a run ends. Return the encoded string. For an empty input, return an empty string.

Worked sample calls:

rle("aaabcc") -> "a3b1c2"

An alphabet mirror swaps lowercase letters from opposite ends.

Specification: Define atbash(text). Set alphabet = "abcdefghijklmnopqrstuvwxyz". For each character, use if ch in alphabet: and append alphabet[25 - alphabet.find(ch)]; otherwise append ch unchanged. Keep capitals, spaces, and punctuation unchanged.

Worked sample calls:

atbash("abc xyz!") -> "zyx cba!"
atbash("Abc!") -> "Ayx!"

A file shelf labels Python and text filenames.

Specification: Define file_type(name). Use endswith and an if/elif/else chain: .py returns "python", .txt returns "text", and any other ending returns "other".

Worked sample calls:

file_type("game.py") -> "python"
file_type("notes.txt") -> "text"
file_type("image.png") -> "other"

A title grows one character per line.

Specification: Define word_triangle(word). Build and return a single string with rows word[:i] for i from 1 through len(word), separated by "\n". Do not leave a newline after the last row.

Worked sample calls:

word_triangle("CODE") -> "C\nCO\nCOD\nCODE"

A display puts a title inside a fixed-width field.

Specification: Define center_text(text, width). Set left = (width - len(text)) // 2 and right = width - len(text) - left. Return " " * left + text + " " * right. Here the right padding is deliberate; do not strip it.

Worked sample calls:

center_text("gnat", 10) -> "   gnat   "
center_text("gnat", 9) -> "  gnat   "

A label puts a dash between its letters.

Specification: Define spaced(text). Use "-".join(text) and return the resulting string.

Worked sample calls:

spaced("ROBOT") -> "R-O-B-O-T"

A code wheel moves its first letter to the end.

Specification: Define rotate_left(word). Return word[1:] + word[0] for a nonempty word.

Worked sample calls:

rotate_left("nova") -> "ovan"

A compact label shows the first and last few letters.

Specification: Define ends(word, k). Return word[:k] + "..." + word[-k:]. Use a positive k that is no larger than the word length.

Worked sample calls:

ends("telescope", 3) -> "tel...ope"

A message filter looks for two warning words.

Specification: Define is_spammy(text). Lowercase the message, then return whether "win" in cleaned or "free" in cleaned.

Worked sample calls:

is_spammy("You WIN a prize") -> True
is_spammy("See you at noon") -> False

A puzzle keeps shared letters in first-word order.

Specification: Define common_letters(first, second). For each character in first, add it to the result only when it is in second and not already in the result. Return the result.

Worked sample calls:

common_letters("thunder", "under") -> "under"

A word becomes the top, sides, and bottom of a frame.

Specification: Define word_box(word) for a word of at least two letters. Return the first row word, then each inside row word[i] + " " * (len(word) - 2) + word[-1 - i] for i from 1 to len(word) - 2, then the last row word[::-1]. Join rows with "\n" and leave no trailing spaces or final newline.

Worked sample calls:

word_box("CODE") -> "CODE\nO  D\nD  O\nEDOC"

A decoder needs the earliest vowel position.

Specification: Define first_vowel(text). Use for i in range(len(text)); return i immediately when text[i].lower() in "aeiou". Return -1 after the loop if none appears.

Worked sample calls:

first_vowel("rhythm and blues") -> 7
first_vowel("rhythm") -> -1

A label checker stops at its first digit.

Specification: Define has_digit(text). Visit characters in order. Return True immediately at the first ch.isdigit(). Return False after the loop if no digit appears.

Worked sample calls:

has_digit("launch42") -> True
has_digit("nova") -> False

A sign painter alternates uppercase and lowercase characters by position.

Specification: Define alternating_case(text). Start an empty result string. Loop through range(len(text)). Add text[i].upper() when i % 2 == 0; else add text[i].lower(). Return the completed string. Spaces and punctuation stay in their positions.

Worked sample calls:

alternating_case("python") -> "PyThOn"
alternating_case("moon base") -> "MoOn bAsE"
alternating_case("A1b2!") -> "A1B2!"

A puzzle transmitter shifts letters forward and wraps around the alphabet.

Specification: First define position(alphabet, ch) with the same left-to-right search as Exercise 6: return i inside the loop for a match and -1 after the loop. Then define shift(message, k) for k >= 0. Set alphabet to "abcdefghijklmnopqrstuvwxyz", lowercase the message, and build a new string character by character. For each character, call position(alphabet, ch). If it returns -1, add the unchanged character. Else use (pos + k) % 26 and alphabet indexing to add the shifted letter. Return the new string.

Worked sample calls:

position("abcdefghijklmnopqrstuvwxyz", "q") -> 16
shift("Code 9!", 2) -> "eqfg 9!"
shift("XYZ", 3) -> "abc"
shift("stay", 0) -> "stay"

A display fits whole words within a width.

Specification: Define wrap(line, width). Visit words with for word in line.split(): and build one current line plus a result string. The first word gets no leading space. Join a later word when len(current) + 1 + len(word) <= width; otherwise close the current line with "\n" and start a new line. A word longer than width sits alone. Return the wrapped string with no trailing newline.

Worked sample calls:

wrap("the quick brown fox jumps", 10) -> "the quick\nbrown fox\njumps"

A numeric field may contain non-digits.

Specification: Define to_number(text). If text.isdigit() is true, return int(text); otherwise return -1. Negative text such as "-3" is not accepted.

Worked sample calls:

to_number("42") -> 42
to_number("4x") -> -1
to_number("-3") -> -1