Extend atof to hadle scientific notation of the form
123.45e-6
where a floating-point numver may be followed by e or E and an optionaly signed exponent.

Our getch and ungetch do not handle a pushed-back EOF correctly. Decide what their properties ought to be i an EOF is pushed back, then implement your desing.

Make Source Code compliant with the TDD environment Test

Given the basic framework, it's straightforward to extend the calculator. Add the modulus (%) operator and provision for negative numbers.

Adapt the ideas of pinttd to write a recursive version o itoa; that is, convert an integer into a string by calling a recursive routine.

Add access to library functions like sin, exp and pow. See <nath.h> in Appendix B, Section 4.

Add commnds for handling variables. (Itś easy to provide twenty-six variables with single-letter names.) Add a variable for the most recently printed value.

Write the funtion strrindex(s, t), which eturns the position of the rightmost occurrence of t in s, or -1 if there is none.

An alternate organization uses getline to read an entire input line, this makes getch and ungetch unnecessary. Revise calculator to use this pproach.

Define a macro swap(t, x, y) that interchanges two arguments of type t. (Block structure will help.)

Modify getop so that it doesn't need to use ungetch. Hint: use an internal static variable

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?

Add commands to print the top element o the stack without poping, to duplicate it, and to swap the top two elements. Add a command to clear the stack.

Suppose that there will never be more than one character of pushback. Modify getch and ungetch accordingly.

Write a recursive version of the function reverse(s), which reverses the string s in place