C Selection Statements

Logical Expressions

Used with selection statements

Operator Type	Associativity	Precedence
Relational (<, >, ⇐, >=)	Left	Lower than Arithmetic
Equality (!=, ==)	Left	Lower than Relational
Logical (!)	Right	Same as Unary + and -
Logical (&&, ||)	Left	Lower than Equality
Conditional (Tertiary)	Left	Lower than Logical

All the above operators have higher precedence than assignment operators
Non-zero Operands are True everything else is False

Logical operators can be short-circuited
Irrespective of the precedence of the operators logical expressions are evaluated condition by condition from left

int i = 1, j = 1, k = 1;  
printf("%d ", ++i || ++j && ++k);  
printf("%d %d %d", i, j, k);  
 
/*
(++1) || ++j && ++k  
2 || ++j && ++k 
1
 
i = 2, j = 1, k = 1 
*/

Boolean Datatype

C89 does not have Boolean datatype
C99 added _Bool which is an unsigned int in disguise which can take the values 0 and 1
If any other non-zero value is assigned to _Bool it is saved as 1
The header <stdbool.h> was also added which includes the type bool which is the same as _Bool. The header also defines macros that allow us to use true and false as values.

If Statement

Cascaded If Statement

Cascaded if statements aren’t a special kind of statement
They are ordinary if statements that happens to have another if statement as its else clause (and that if statement has another if statement as its else clause, as so on)

Dangling else Statement

if (y != 0)
	if (x != 0)
		result = x / y;
else
	printf("Error: y is equal to 0\n");

In the above program the else statement belong to the inner if statement
C associates else statements with the nearest if statement that does not have a else

if (y != 0) {
	if (x != 0)
		result = x / y;
} else
	printf("Error: y is equal to 0\n");

In the above example else belongs to the outer if

Switch Statement

When there are many cases switch statement will be faster than cascaded if statement

switch (grade) {
	case 4: 
		printf("Excellent");
		break;
		
	case 3:
		printf("Good");
		break;
		
	case 2:
		printf("Average");
		break;
		
	case 1:
		printf("Poor");
		break;
		
	case 0:
		printf("Failing");
		break;
		
	default:
		printf("Illegal grade");
		break;
}

The expression supplied to the switch statement should be an integer value
Char can also be used since its treated as int by C but floats and strings cannot be used

The expression passed to the case statement must be a constant int (or char)
Variables cannot be used (unless its an macro) since they are not a constant value

Duplicate case labels are not allowed
The order of the case statements does not matter (default does not have to be last)

switch (grade) {
	case 4:
	case 3:
	case 2:
	case 1:
		printf("Passing");
		break;
		
	case 0:
		printf("Failing");
		break;
		
	default:
		printf("Illegal grade");
		break;
}

Switch statement is a type of computed jump
Case statements are markers that represent different positions in the switch statement
When the last statement in a case is executed the control “falls through” to the first statement in the next case, the case label is ignored