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. Directory order ignores punctuation and control characters — only alphanumeric characters and spaces are considered. The comparison function skips non-directory characters in both strings simultaneously, then compares the next significant …
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. The K&R sort from Section 5.11 calls qsort with a comparison function strcmp. Adding -r requires the comparison function to negate its result. The cleanest approach: add a global flag …
Exercise 5-13. Write the program tail, which prints the last n lines of its input. By default, n is 10, say, 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. …
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. Two new argument forms extend the programs from Exercise 5-11: -m sets the starting column and +n sets the interval. So detab -4 +3 …
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. The Chapter 1 versions of entab and detab used a fixed tab width of 8. This version reads tab stop positions from …
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). The K&R RPN calculator from Chapter 4 reads from stdin. This version reads from argv instead — each …