Lecture 5; control
------------------

Announce
-----------
- homeworks1 and 2 due, turnins are timestamped
- no lates for homeworks! only for projects, 1/2 credit
- hw1/2 solutions posted after class
- hw3 is posted
- start early, come to office hours tommorrow

Today Outline
--------
- quick review of variables
- type conversion
- arrays
- C control structures: decisions and loops
- expressions
- statements/statement blocks
- variable scope
- functions


Review
---------
- must be declared before they're used
- must have a type
- variable is a placeholder in memory
- size of placeholder defines range of values
- operators: binary, unary, conditional, order of ops
- printf and scanf
	- scanf %f vs. %lf

Constants
-----------
#define FOO 10


Type Conversion
----------------

- can have expression with several types in expression

- C promotion scheme, smaller vars promoted

- can impose conversion with casting

- demotion dangerous, floats are truncated


Arrays
---------------------

- variables covered so far hold one value

- C gives us a way to define more than one placeholder at once by arrays

**
	int birthyears[10];
	float bodytemps[10];
	char firstinitials[10];
	char lastinitials[10];

- each element is like a single variable

- accessed by integer index starting at 0
	birthyears[0] = 1978;
	birthyears[9] = 2002;
	printf("%d\n", birthyear[1]);

- index can be variable

- chunk of contig memory
	a b c d e f g ... 

- ddim arrays
	int foo[3][4];
**
	  0 1 2 3
	0 a b c d
	1 e f g h
	2 i j k l

- still just contiguous chunk of memory
	0	1	2
	a b c d e f g h i j k l
	0 1 2 3 0 1 2 3 0 1 2 3

	- int foo[A][B]
	- foo[a][b]

	- a*B + b

- careful not to overflow the array!
- can cause segmentation fault
	- try to access memory beyond that referenced by current var
declarations

Useful Program Constructs
---------------------------
** 
- we know variables for storing data

- we know input and output

- we know performing operations on data

- what else would we like to do with a program?

- make decisions

- repeat processes

- ex: what does ls do?
	- ls /tmp
	- uses a variable to store "/tmp"
	- performs OP to check whether /tmp exists
	- makes a decision
		- if tmp does not exist, error and exit
		- otherwise, do a loop
			- while files in /tmp
				- output files to screen

Conditional Control Structures
-------------------------------

- used for making decisions in a program

- most take a conditional expression (exp one or more with conditional ops)

- if/else
        if (<expr>)
                <state>
        else
                <state>

	ex:
		scanf("%d", &a);
		b = 10;
		if (a < 10)
			printf("a is less than 10\n");
		else
			printf("a is greater than 10\n");
			
- ex: 
	if (a < 10 || a > 30)
		if (a < 10)
			printf("lt 10\n");
		else
			printf("gt 30\n");

- OR else if, when you want more than branch

	if (a < 10) {

	} else if (a > 30) {

	} else {

	}

- switch/case
	- another way to do multiple branches over one variable

	scanf("%c", &mychar);
	switch(mychar) {
	case 'a':
		...
		break;
	case 'b':
		...
		break;
	case 'c':
	case 'd':
		...
		break;
	default:
		...
		break;
	}

Loops
--------

- powerful use of computers is their ability to repeat processes quickly: loops

- do/while/for

do
	...
while( <expr> );

while( <expr> )
	...

for (expr1; expr2; expr3)
	statement

ex: 
	for (i=0; i<100; i++)
		printf("foo\n");


- break and continue
	- break in loops or switch
	- break gets you out of loops without complicating conditional expression

	- continue useful for skipping code without introducing more conditionals

- nested loops

- printing/indexing/inputting ddim arrays

	int manyints[20][10]
	for (i=0; i < 20; i++) {
		for (j=0; j < 10; j++) {
			manyints[i][j] = 0;
		}
	}