Building strings

Strings cannot be changed in place, so you build a new one. There are two reliable ways:

out = ""
for ch in word:
    out = out + ch * 2      # grow a string piece by piece

parts = []
for ch in word:
    parts.append(ch.upper())
result = "".join(parts)     # collect, then join once

Reach for these tools:

text.split() breaks on any run of spaces and drops empty pieces. text.split(" ") does not: two spaces in a row give an empty piece.
f"{n:02d}" pads a whole number with zeros to 2 digits: 7 becomes "07".
"-".join(list_of_strings) glues pieces with a separator.

When a rule says "keep this character, change that one", loop over the characters and decide for each one.

Watch out grow one piece per pass.

See it run

word = "tree"
out = ""
for ch in word:
    out = out + ch + "-"

A loop that builds a new string, one character per pass.

Practice building strings

Exercise 1

A club writes members' names on name tags using only their initials. Given text, a sentence of words made of letters, return the uppercase first letters of its words joined into one string.

Rules:

Words are separated by one or more spaces. text may start or end with spaces.
Text with no words gives the empty string "".

Constraints: 0 ≤ len(text) ≤ 1000, only letters and spaces.

Example: solution(" grace brewster hopper ") returns "GBH".

Exercise 2

A receipt must hide a card number. Given text, replace every digit with "*" except the last 4 digits, which stay visible. Characters that are not digits (spaces, dashes, letters) stay exactly where they are.

Return the new string. If text has 4 or fewer digits, nothing is hidden.

Constraints: 0 ≤ len(text) ≤ 100. Digits are the characters "0" to "9".

Example: solution("1234-5678-9012") returns "****-****-9012".

Exercise 3

A stopwatch counts whole seconds. Write solution(seconds) to show them as a clock string "H:MM:SS".

Rules:

Hours are not padded and may exceed 24 (a stopwatch does not roll over into days).
Minutes (0-59) and seconds (0-59) are always two digits, padded with a zero.

Constraints: 0 ≤ seconds ≤ 10^7.

Example: solution(3725) returns "1:02:05" (1 hour, 2 minutes, 5 seconds).

Exercise 4

A label printer prints words on lines that are at most width characters wide. Given the list words, place them greedily: put each word on the current line if it fits, otherwise start a new line.

Return the lines as a list of strings.

Rules:

Words on the same line are separated by exactly one space. A line has no leading or trailing spaces.
A word fits when the line, a space and the word together have at most width characters. The first word of a line always goes on it.
Every word is at most width characters long. No words gives [].

Constraints: 0 ≤ len(words) ≤ 1000, 1 ≤ width ≤ 100.

Example: solution(["the", "cat", "sat", "down"], 7) returns ["the cat", "sat", "down"] because "sat down" would be 8 characters.

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Practice

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Memory · names → values

Variables appear here as Python creates them.

Output & plots