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[-> Notes 02/26](02-26.md)
---
# Large Programs
- When programs are long
- Need to split the file into several files
- Need to compile them together
- Example:
- We have files: main.c, list.c, file.c
- To compile: `gcc -o proj main.c list.c file.c`
- Sharing global variables
- Define types in a `.h` file
- Declare global variables (do not initialize!)
- extern in a .h file
- Declare and initialize global variables
- in one of the .c file (e.g., the main one)
- Include the .h file in all the .c files which use any of the global variables: `#include "proj.h"`
- Example:
- in `link.h`, declare (do not initialize!) global variables
- `extern NODE *head;`
- In main.c, include link.h, and declare and initialize global variables
```
#include "link.h"
NODE *head = (NODE *)NULL;
```
- In all the other .c files, just include link.h
- `#include "link.h"`
- Use `makefile` to automate the compilation process
- Create a "makefile" file with instructions
- Command `make` will parse the makefile file and execute the appropraite command
- `<prompt> make proj1`
- Example 1:
```
proj1: main1.c list1.c io1.c
gcc -o proj1 main1.c list1.c io1.c
./proj1 filename
```
- Example 2:
```
proj2: main2.c list2.c thr2.c
gcc -o proj2 main2.c list2.c thr2.c -lpthread
./proj2 filename1 filename2
```
- The tab is important! (its the syntax)
## Development Tools
- Unix/Linux
- Environment and commands
- Tools - learn to use these!!
- vi
- grep
- gdb
- makefile
- shell script
- Libraries
- Math
- String
- Memory
- Threads
---
# Special Operators
## Condtional Operator
- The conditional operator `?:` takes three operands
- c ? r1 : r2
- The value of the expression using the conditional operator is the value of either its second or third operand, depending on the value of the first operand
- Shorthand for:
```
if(c){
//result value is r1
}
else{
//result value is r2
}
```
- Examples
- In assignment: `x = (a < b) ? a : b;`
- x will be assigned the smallest value between a and b
- Very useful for macros
- Examples:
- `#define MAX(a,b) (((a) > (b)) ? (a) : (b))`
- returns the max between the paramaters assigned to a and b
- `define ISLETTER(c) (((c) >= 'A' && (c) <= 'z') ? 1 : 0)`
- returns 1 if the value of assigned to c is a letter and returns 0 if not
## Sequential Evaluation
- The comma operator
- Evaluates its two operands in sequence, yeilding the value of the second operand as the value of the expression
- The value of the first is discarded
- Example:
- In assignments: `x = (i += 2, a[i]);` -> `i += 2; x = a[i];`
- Parenthesis are important because precedence
## Bitwise Operators
- Positive integers are represented in the computer by standard binary numbers
- Examples:
```
short n = 13;
//in memory - 0000 0000 0000 1101
//2^0 + 2^2 + 2^3 = 13
char c = 5;
//in memory - 0000 0101
//2^0 + 2^2 = 5
```
- Bitwise operators
- take operands of any integer type
- char, short, int, long
- but treat an operand as a collection of bits rather than a single number
### Bitwise Negation
- Bitwise negation
- Operand: `~`
- Application of `~` to an integer produces a value in which each bit of the operand has been replaced by its negation
- 0 -> 1
- 1 -> 0
- Example:
- ` n = 0000 0000 0000 1101`
- `~n = 1111 1111 1111 0010`
### Bitwise Shift
- Shift operators
- shift left: `<<`
- shift right: `>>`
- Take two integers operands
- The value on the left is the number to be shifted
- Viewed as a collection of bits that can move
- To avoid implementation problems, avoid negative numbers when shifting right
- The value on the right is a nonnegative number telling how far to move the bits
- Operand
- `<<` shifts bits left
- `>>` shifts bits right
- The bits that "fall off the end" are lost
- The "emptied" positions are filled with zeros
- Examples:
```
int n; // n is 0000 0000 0000 1101
n << 1 //now n is 0000 0000 0001 1010
n << 4 //now n is 0000 0000 1101 0000
n >> 3 //now n is 0000 0000 0000 0001
n << 4 //now n is 0000 0000 1101 0000
```
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