K&R C Chapter 3 Exercise Solutions — Control Flow

Chapter 3: Control Flow covers C’s branching and looping constructs. It is short (6 exercises) but the exercises are non-trivial: exercise 3-3 implements expand(s1,s2) to expand shorthand like a-z into the full alphabet; 3-4 handles itoa for the most negative integer (a classic overflow trap); 3-5 converts integers to arbitrary bases.

These are worked solutions to all 6 exercises. Each solution compiles cleanly with gcc -ansi -Wall.

Book: The C Programming Language, 2nd Ed — Kernighan & Ritchie  |  All chapters index

---

Exercises

1. Exercise 3-1. Our binary search makes two tests inside the loop, when one would suffice (at the price of more tests outside). Write a version with only one test inside the loop and measure the difference in run-time.

   Solution to Exercise 3-1

2. Exercise 3-2. Write a function escape(s,t) that converts characters like newline and tab into visible escape sequences like \n and \t as it copies the string t to s. Use a switch. Write a function for the other direction as well, converting escape sequences into the real characters.

   Solution to Exercise 3-2

3. Exercise 3-3. Write a function expand(s1,s2) that expands shorthand notations like a-z in the string s1 into the equivalent complete list abc...xyz in s2. Allow for letters of either case and digits, and be prepared to handle cases like a-b-c and a-z0-9 and -a-z.

   Solution to Exercise 3-3

4. Exercise 3-4. In a two’s complement number representation, our version of itoa does not handle the largest negative number, that is, the value of n equal to -(2^(wordsize-1)). Explain why not. Modify it to print that value correctly, regardless of the machine it runs on.

   Solution to Exercise 3-4

5. Exercise 3-5. Write the function itob(n,s,b) that converts the integer n into a base b character representation in the string s. In particular, itob(n,s,16) formats n as a hexadecimal integer in s.

   Solution to Exercise 3-5

6. Exercise 3-6. Write a version of itoa that accepts three arguments instead of two. The third argument is a minimum field width; the converted number must be padded with blanks on the left if necessary to make it wide enough.

   Solution to Exercise 3-6

See also: K&R C — All Chapters Index | Complete C Programs List