Exercise 4-11. Modify getop so that it doesn’t need to use ungetch. Hint: use an internal static variable. In the original calculator, getop calls ungetch once: when it reads a non-digit character to terminate a number, it pushes that character back so the next call to getop will see it. The fix: instead of pushing …
Exercise 4-10. An alternate organization uses getline to read an entire input line; this makes getch and ungetch unnecessary. Revise the calculator to use this approach. The original calculator reads the input one character at a time via getch/ungetch. This exercise replaces that machinery with a line-buffer approach: read the whole line into a static …
Exercise 4-9. Our getch and ungetch do not handle a pushed-back EOF correctly. Decide what their properties should be if an EOF is pushed back, then implement your decision. The bug: K&R’s original implementation uses char buf[BUFSIZE]. When ungetch(EOF) is called, EOF (typically -1) is stored as char. On platforms where char is unsigned, -1 …
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 …