K&R C Chapter 5 Exercise Solutions — Pointers and Arrays

Chapter 5: Pointers and Arrays is the chapter C beginners fear most — and with reason. K&R covers pointer arithmetic, the equivalence between pointers and arrays, pointers to functions, and multi-dimensional arrays. The exercises build real utilities: getfloat, strncpy/strncat/strncmp from scratch, a tail command, a sort program with -r/-n/-f/-d flags, and a full C declaration parser (dcl/undcl).

These are worked solutions to all 20 exercises. Each solution compiles cleanly with gcc -ansi -Wall.

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

Exercises

1. Exercise 5-1. As written, getint treats a + or - not followed by a digit as a valid representation of zero. Fix it to push such a character back on the input.

   Solution to Exercise 5-1

2. Exercise 5-2. Write getfloat, the floating-point analog of getint. What type does getfloat return as its function value?

   Solution to Exercise 5-2

3. Exercise 5-3. Write a pointer version of the function strcat that we showed in Chapter 2: strcat(s,t) copies the string t to the end of s.

   Solution to Exercise 5-3

4. Exercise 5-4. Write the function strend(s,t), which returns 1 if the string t occurs at the end of the string s, and zero otherwise.

   Solution to Exercise 5-4

5. Exercise 5-5. Write versions of the library functions strncpy, strncat, and strncmp, which operate on at most the first n characters of their argument strings.

   Solution to Exercise 5-5

6. Exercise 5-6. Rewrite appropriate programs from earlier chapters and exercises with pointers instead of array indexing. Good possibilities include getline, atoi, itoa, reverse, strindex, and getop.

   Solution to Exercise 5-6

7. Exercise 5-7. Rewrite readlines to store lines in an array supplied by main, rather than calling alloc to maintain storage. How much faster is the program?

   Solution to Exercise 5-7

8. Exercise 5-8. There is no error-checking in day_of_year or month_day. Remedy this defect.

   Solution to Exercise 5-8

9. Exercise 5-9. Rewrite the routines day_of_year and month_day with pointers instead of indexing.

   Solution to Exercise 5-9

10. Exercise 5-10. Write the program expr, which evaluates a reverse Polish expression from the command line, where each operator or operand is a separate argument. For example, expr 2 3 4 + * evaluates 2×(3+4).

    Solution to Exercise 5-10

11. Exercise 5-11. Modify the programs entab and detab (written as exercises in Chapter 1) to accept a list of tab stops as arguments. Use the default tab settings if there are no arguments.

    Solution to Exercise 5-11

12. Exercise 5-12. Extend entab and detab to accept the shorthand entab -m +n to mean tab stops every n columns, starting at column m. Choose convenient (for the user) default behavior.

    Solution to Exercise 5-12

13. Exercise 5-13. Write the program tail, which prints the last n lines of its input. By default, n is 10, but it can be changed by an optional argument, so that tail -n prints the last n lines. The program should behave rationally no matter how unreasonable the input or the value of n.

    Solution to Exercise 5-13

14. Exercise 5-14. Modify the sort program to handle a -r flag, which indicates sorting in reverse (decreasing) order. Be sure that -r works with -n.

    Solution to Exercise 5-14

15. Exercise 5-15. Add the option -f to fold upper and lower case together, so that case distinctions are not made during sorting; for example, a and A compare equal.

    Solution to Exercise 5-15

16. Exercise 5-16. Add the -d (“directory order”) option, which makes comparisons only on letters, numbers and blanks. Make sure it works in conjunction with -f.

    Solution to Exercise 5-16

17. Exercise 5-17. Add a field-handling capability, so sorting may be done on fields within lines, each field sorted according to an independent set of options.

    Solution to Exercise 5-17

18. Exercise 5-18. Make dcl recover from input errors.

    Solution to Exercise 5-18

19. Exercise 5-19. Modify undcl so that it does not add redundant parentheses to declarations.

    Solution to Exercise 5-19

20. Exercise 5-20. Expand dcl to handle declarations with function argument types, qualifiers like const, and so on.

    Solution to Exercise 5-20

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