Week 4 · Old-Book Foundations and va_list

The first three rounds took you from recursion through language details into move semantics; this one comes back to the foundations of C itself. The first four problems come from owollz4's contributions, and the closer was composed by us around the same book. That book is Kenneth Reek's Pointers on C, an old book that lays the foundations of pointers, functions, and bytes down good and solid. All five problems share one thread: how arguments get passed in C, and how bytes get counted. gcd works out the greatest common divisor recursively; lower-to-upper walks the IO one character at a time through getchar and putchar; max_list has you read a varying number of arguments with va_list; checksum takes the running sum down to the byte level of signed char; and the closing mini sprintf pulls those threads into a single problem: this time you don't call printf, you implement one yourself.

Difficulty runs from one star to three, and the card order is the suggested tackling order: gcd as the warm-up, a pass over character IO, a first hands-on with va_list, the checksum down at the byte level, and a closer that moves you from the person calling va_arg to the person setting the rules. No format mixing this week. The free-response and bug-hunt problems that showed up in the past two rounds take the week off; all five are online-judge problems, and every one of them asks you to actually type the code in and run it.

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Where the problems come from: The first four problems this week were contributed by owollz4; the problem statements and reference solutions are his original drafts, and the judge integration was our work. The closer was composed by us around Chapter 7 of the same book. gcd, max_list, and the mini sprintf all trace back to the after-chapter programming exercises in Chapter 7 of Pointers on C; in the chapter7 folder of the public solutions repository DragScorpio/Pointers-On-C-Solutions you can find the matching implementations, gcd.c, maxlist.c, and implementPrintf. gcd also happens to be a classic that has been making the rounds forever, so this one problem carries two pedigrees. Lower-to-upper and checksum were both flagged by the contributor as coming from the same book. One honest note from when we landed the problems: the original checksum draft omitted the "copy the input to the output character by character" that the problem statement asks for; the putchar(ch) line in the judge version was added by us. The closer's footing is in Chapter 7 too — that chapter's exercises already include "implement a printf yourself"; we trimmed it to the four conversions %d, %s, %c, and %%, and retooled the judging into an online buffer comparison. Widening the lens for a moment, C++ has since replaced both jobs: formatting went to std::format and std::print, and variable arguments went to variadic templates. Every judge case was verified by local testing on two compilers (GCC 16 / Clang 22).

Week 4 · Old-Book Foundations and va_list ​

Week 4 · Old-Book Foundations and va_list