A set is a collection of distinct elements — no duplicates, order irrelevant. The two operations every course asks you to implement in C are union (A ∪ B: every element that appears in A or B) and intersection (A ∩ B: only the elements that appear in both). This page implements them cleanly over arrays, with a shared …
Exercise 4-8. Suppose that there will never be more than one character of pushback. Modify getch and ungetch accordingly. The original K&R implementation uses an array buf[BUFSIZE] and an index bufp to support multiple pushed-back characters. If only one character of pushback is ever needed, the array becomes unnecessary. A single static int buf suffices …
Exercise 4-7. Write a routine ungets(s) that will push back an entire string onto the input. Should ungets know about buf and bufp, or should it just use ungetch? The answer is: ungets should use ungetch, not access buf and bufp directly. This is the principle of information hiding — getch and ungetch own the …
Exercise 4-6. Add commands for handling variables. (It’s easy to provide for twenty-six variables with single-letter names.) Add a variable for the most recently printed value. Twenty-six variables map naturally to an array var[26] indexed by c – ‘a’. The protocol: a lowercase letter pushes its variable’s value; = assigns the top of the stack …
Exercise 4-5. Add access to library functions like sin, cos, exp, and pow. See <math.h> in Appendix B, Section 4. Math functions need the calculator to recognize words as well as single characters. getop currently returns one character at a time. The fix: if getop sees a letter, it reads the whole word into s[] …
This C program adds 1 to each digit of a given number. When a digit is not 9, it simply increases by 1: 3→4, 5→6. When a digit is 9, it wraps to 0 and carries +1 to the next digit to the left. This carry propagates: if the next digit is also 9, it …