Checkpoint — Strings

Q: What does std::string::find return when nothing matches?

It returns std::string::npos, the largest std::size_t. Test with pos != std::string::npos; find(x) >= 0 is always true, because the result is unsigned.

Q: What is the difference between std::stoi("42abc") and std::from_chars on the same text?

std::stoi("42abc") returns 42 and ignores the rest unless you check how many characters it consumed. std::from_chars also parses 42 but leaves ptr pointing at the a, so testing ptr == last reports that the whole token was not a number.

Q: What does std::setprecision(2) print for 1234.5 without std::fixed?

It prints 1.2e+03. Without std::fixed the precision counts significant digits, and two significant digits of 1234.5 need scientific notation. With std::fixed the same setting prints 1234.50.

25 min · +25 XP · 3 exercises · 14 questions

Exercises0/3

Quiz · 70% to clear14 q

This checkpoint covers the whole module: std::string as a value type, the find family and npos, slicing and replacing, characters as small integers and the <cctype> cast, stoi and std::from_chars, splitting and validating input, manipulators and std::format, and std::string_view with its lifetime rule.

How it works. Fourteen questions and three programs. You need 70% on the questions and every program accepted to clear the module. You can retake it as often as you like; your best score counts.

Before you start, make sure you can answer these from memory:

What does std::string::find return when nothing matches, and how do you test for it?
Why must a char be cast to unsigned char before calling std::isalpha?
What is the difference between std::stoi("42abc") and std::from_chars on the same text?
Which stream manipulators are sticky, and which one applies to the next insertion only?
What does std::setprecision(2) print for 1234.5 without std::fixed?
When does a std::string_view dangle, and what is the one rule that prevents it?
Why does a replace-all loop advance by the replacement's length rather than the match's?

The three programs are a word-frequency count, a fixed-width report and a run-length codec; get the reading idiom right before the logic.

Common questions

What does `std::string::find` return when nothing matches?

It returns std::string::npos, the largest std::size_t. Test with pos != std::string::npos; find(x) >= 0 is always true, because the result is unsigned.

What is the difference between `std::stoi("42abc")` and `std::from_chars` on the same text?

std::stoi("42abc") returns 42 and ignores the rest unless you check how many characters it consumed. std::from_chars also parses 42 but leaves ptr pointing at the a, so testing ptr == last reports that the whole token was not a number.

What does `std::setprecision(2)` print for `1234.5` without `std::fixed`?

It prints 1.2e+03. Without std::fixed the precision counts significant digits, and two significant digits of 1234.5 need scientific notation. With std::fixed the same setting prints 1234.50.

Why does a replace-all loop advance by the replacement's length rather than the match's?

So the next search starts after the text just inserted. If the replacement contains the search string — replacing "a" with "aa" — restarting at the match position finds the insertion again and the loop never ends.

C++ Strings Quiz: find, stoi, Formatting and String View

Test your C++ string skills with 14 questions and three programs on find and npos, substr, char conversions, stoi, parsing input, formatting and string view.

Course: C++ study plan
Module: Strings
Kind: Checkpoint — cleared at 70%
Reading time: 25 min
Runtime: Clang 18 · C++20

Checkpoint — Strings is the checkpoint that closes the Strings module: a graded quiz and whole-program exercises, passed at 70%.

Instructions

How it works. Fourteen questions and three programs. You need 70% on the questions and every program accepted to clear the module. You can retake it as often as you like; your best score counts.

Before you start, make sure you can answer these from memory:

What does std::string::find return when nothing matches, and how do you test for it?
Why must a char be cast to unsigned char before calling std::isalpha?
What is the difference between std::stoi("42abc") and std::from_chars on the same text?
Which stream manipulators are sticky, and which one applies to the next insertion only?
What does std::setprecision(2) print for 1234.5 without std::fixed?
When does a std::string_view dangle, and what is the one rule that prevents it?
Why does a replace-all loop advance by the replacement's length rather than the match's?

The three programs are a word-frequency count, a fixed-width report and a run-length codec; get the reading idiom right before the logic.

Common questions

What does `std::string::find` return when nothing matches?

It returns std::string::npos, the largest std::size_t. Test with pos != std::string::npos; find(x) >= 0 is always true, because the result is unsigned.

What is the difference between `std::stoi("42abc")` and `std::from_chars` on the same text?

What does `std::setprecision(2)` print for `1234.5` without `std::fixed`?

It prints 1.2e+03. Without std::fixed the precision counts significant digits, and two significant digits of 1234.5 need scientific notation. With std::fixed the same setting prints 1234.50.

Why does a replace-all loop advance by the replacement's length rather than the match's?

Exercises

Word frequency

Read all of standard input. A word is a maximal run of letters and digits (std::isalnum with the unsigned char cast), converted to lower case; every other character separates words. Print each distinct word with its count as <word> <count>, one per line, in ascending std::string order, then total=<n> distinct=<m>.

Input: any number of lines. Output: one line per distinct word, then the summary.

Example: input

Hello, hello! HELLO world.
World 42 42

→ output

42 2
hello 3
world 2
total=7 distinct=3

A std::map<std::string, int> keeps the words in order for you.

Temperature report

Read an integer N (at least 1) and then N lines. Each line is a city name — one or more words separated by single spaces — followed by two decimals, the maximum and minimum temperature. Parse from the right: the last two tokens are the numbers and everything before them is the name.

Print a header and one row per city with std::format: header {:<14}{:>7}{:>7}{:>7} with City, Max, Min, Range; rows {:<14}{:>7.1f}{:>7.1f}{:>7.1f} with the name, max, min and range (max − min). Then print Widest range: <city> (<range with one decimal>) — the first city on a tie.

Input: N, then N lines. Output: N + 2 lines.

Example: input

3
New York 28.5 19.0
Oslo 12.3 -3.5
Lima 21 15

→ output

City              Max    Min  Range
New York         28.5   19.0    9.5
Oslo             12.3   -3.5   15.8
Lima             21.0   15.0    6.0
Widest range: Oslo (15.8)

Run-length codec

Read lines until end of input. Each line is encode <text> or decode <text>; the text contains no spaces.

encode: the text is letters only. Replace every maximal run of one character with <count><char>: aaabcc → 3a1b2c.
decode: the text should be a sequence of <count><char> groups where the count is one or more digits and the char is a letter; expand it: 3a1b2c → aaabcc. Counts may have several digits (12x). If the text is not well-formed — a count with no letter after it, a letter with no count before it, a count of zero, or an empty text — print invalid.

Use c - '0' to accumulate counts, std::to_string to emit them, and the <cctype> functions with the unsigned char cast.

Input: any number of command lines. Output: one line per command.

Example: input

encode aaabcc
decode 3a1b2c
decode 3

→ output

3a1b2c
aaabcc
invalid

In this module: Strings

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