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Exersixe 4-1. Write the function strrindex(s,t) , which returns the position of the rightmost occurrence of t in s , or -1 if there is none.
 
Exersixe 4-2. Extend atof to handle scientific notation of the form 123.45e-6 where a floating-point number may be followed by e or E and an optionally signed exponent.
 
Exersixe 4-3. Given the basic framework, it's straightforward to extend the calculator. Add the modulus ( % ) operator and provisions for negative numbers.
 
Exersixe 4-4. Add commands to print the top element of the stack without popping, to duplicate it, and to swap the top two elements. Add a command to clear the stack.
 
Exersixe 4-5. Add access to library functions like sin , exp , and pow . See <math.h> in Appendix B, Section 4.
 
Exersixe 4-6. Add commands for handling variables. (It's easy to provide twenty-six variables with single-letter names.) Add a variable for the most recently printed value.
 
Exersixe 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 ?
 
Exersixe 4-8. Suppose that there will never be more than one character of pushback. Modify getch and ungetch accordingly.
 
Exersixe 4-9. Our getch and ungetch do not handle a pushed-back EOF correctly. Decide what their properties ought to be if an EOF is pushed back, then implement your design.
 
Exersixe 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.
 
Exersixe 4-11. Modify getop so that it doesn't need to use ungetch. Hint: use an internal static variable.
 
Exersixe 4-12. Adapt the ideas of printd to write a recursive version of itoa ; that is, convert an integer into a string by calling a recursive routine.
 
Exersixe 4-13. Write a recursive version of the function reverse(s) , which reverses the string s in place.
 
Exersixe 4-14. Define a macro swap(t,x,y) that interchanges two arguments of type t . (Block structure will help.)