Lecture 17

Macros
What are macros?
- they tell the compiler to substitute an identifier with expressions, statements, or group of statements
- you can think of macros as if they are a way to tell the compiler to replace some code with other code
Why are macros important?
- they make your code easier to read and maintain
macro.c:

common_constants.h:

Things to note:
- macros can be in same file or included from separate file
- macros allow you to make a change in one location and have the change reflected everywhere else in the program
- macros are more useful with things that are used repeatedly
- read more at http://en.wikipedia.org/wiki/C_preprocessor (Section Macro Definition and Expansion
- read more at http://gcc.gnu.org/onlinedocs/cpp/Macros.html if you want to learn more advance uses of macros
More Advanced Macros and Compilation Techniques
Macros for expressions and multiline statements
- they can be faster than using functions
- functions are more flexible and easier to maintain
- recommend using functions first and only switch to macros if there's a need for it
- examples:
  - Macro for an expression:
    area.c:
  - Macro for a function:
    count.c:
Conditional Compilation
- compiling some code based on a condition
- useful for inserting debug statements when necessary
- example:
  - Compile a debug statement only when debugging:
    debug.c:
Update net_balance.c to have Conditionally Compiled Debug Statements
Update net_balance.c with conditional debug statements:
- account_balance
- account_type
Assignment
Homework Problem 17.1, at the end of this problem you should submit to Laulima:
- a file named UHusername_17_1.c
- a file named UHusername_17_1_macros.h
- - zhaol_17_1.c
  - zhaol_17_1_marcros.h
- write a program that includes a macro file UHusername_17_1_macros.h and will be similar to common_constants.h)
  - the macro file will define the following:
    - a macro named PI to 2 decimal places
    - a macro expression named CIRCUMFERENCE to calculate the circumference of the circle
    - a macro expression named CIRCLE_AREA to calculate the area of a circle
    - a macro expression named SQUARE_AREA to calculate the area of a square
  - the program will ask for a length and then calculate the following:
    - circumference of a circle
    - area of a circle
    - area of a square
  - Test Case 1:
```
Please enter a length:
[user enters 3]
the circumference of the circle is: 18.84
the area of the circle is: 28.26
the area of the square is: 9.00
```