CSE 232 Lab 3 Notes

As with most labs, these notes focus on concepts that might not
be as clear in the course pack examples.



1. == and =

0 == false, all other values == true.

The following is WRONG, but it will never throw an error.

if (x = 3) {
	...
}

The "if" body will always execute, since "x = 3" returns "3", which counts as
"true".

You must use operator "==" in order to test for equality.



2. AND and OR

"A && B" means A AND B. "A || B" means A OR B (OR both). "and" and "or" may
not work on all compilers. "&" and "|" are bitwise operators.



3. Parentheses

Put parentheses around complex expressions.

	if ((A > B) || (C > D)) {
		...
	}
	
	
	
4. If-else and "if-if"

The following two sets of if statements do NOT do the same thing.

	// block 1

	if (X == A) {
		...
	}
	if (Y > B) {
		...
	}
	if (Y > C) {
		...
	}


	// block 2

	if (X == A) {
		...
	}
	else if (Y > B) {
		...
	}
	else if (Y > C) {
		...
	}

In the first case, all three bodies can potentially be executed, while in the
second, only one will be.



5. Unnecessary "if"s

The following three sets do the same thing, but you should always write it
the last way. There is no reason to do it the first two ways.

	// block 1 (***BAD***)
	if (X <= A) {
		...
	}
	if (X > A) {
		...
	}

	// block 2 (***BAD***)
	if (X <= A) {
		...
	}
	else if (X > A) {
		...
	}

	// block 3 (***GOOD***)
	if (X <= A) {
		...
	}
	else {
		...
	}



6. Boundary conditions

If you want every value in a range to match one case, be careful
with < vs <= and > vs >=

	// handles all positive numbers correctly
	if (0 <= x && x <= 100) {
		...
	}
	else if (x <= 200) {
		...
	}
	else {
		...
	}
	
	

7. Switch-Case

Once the "switch" statement is evaluated, every case from the result to the
end will be executed.

	switch(x) {
		case 3:
			cout << "3" << endl;
		case 2:
			cout << "2" << endl;
		case 1:
			cout << "1" << endl;		
		default:
			cout << "done" << endl;
	}
	
	/* if x == 3, then the output is:
		3
		2
		1
		done
	*/

You must have a "break" at the end of each case if you want only
one case to be executed.

	switch(x) {
		case 3:
			cout << "3" << endl;
			break;
		case 2:
			cout << "2" << endl;
			break;
		case 1:
			cout << "1" << endl;
			break;
		default:
			cout << "not 1/2/3" << endl;
			break; // not strictly necessary
	}
	
Note that each "case" must be an integer value (char/short/int/unsigned), not
a variable.



8. While and For

In C++, "for" is a shortcut for "while". the following two loops
do the same thing.

	int i = 0;
	while (i < 10) {
		cout << "i" << endl;
		i++;
	}
	
	for (int i = 0; i < 10; i++) {
		cout << "i" << endl;
	}

When you write loops like these, always use the "for" version. Use "while"
for middle-tested loops and other loops that don't fit the "for" format well.



9. Loop conditions

Standard practice is to start from 0, and use < in the end condition. For
iterating through arrays, this is the only sane way to do it (arrays
covered in lab 6).



10. Top, bottom, and middle tested loops

"While" and "for" are usually top-tested loops. Each iteration is

	check condition (stop?) -> execute body
	
"Do-while" is rarely used, but does the reverse. Because of this, the
body is always executed at least once.

	do {
		// body
	} while (cond);
	
	// execute body -> check condition (stop?)
	
"Middle-tested" loops are also very common. Create an infinite loop
with a break condition somewhere in the middle.

	while (true) {  // OR: for(;;) {
		// fetch data
		
		if (cond) break;
		
		// process data
	}
	
See the course pack for lots of examples.