C Control Structures: Selection

Control Structures

• Control structures manage the flow of execution in a program.
• The three basic control structures are sequential flow, selection, and repetition.
• Sequential flow is a group of statements bracketed that execute in order.
• Selection control structures choose statements to execute based on a condition.
• Repetition structures execute a code block multiple times.

Sequential Flow

• A compound statement/code block is a group of statements bracketed and specifies sequential flow.
• The main function is surrounded by brackets, and its statements are executed sequentially.
• Function bodies use compound statements.

C Control Structures

• Selection statements in C consist of if, if...else, and switch.
• Repetition statements consist of for, while, and do...while.

Selection Control Structure

• This lecture describes C control structure for selection.
• The selection control structure chooses alternative program statements based on a condition.
• A condition is an expression that is either false (0) or true (1).

Conditions

• Most conditions are performed using relational and logical operators.
• < means less than.
• > means greater than.
• <= means less than or equal to.
• >= means greater than or equal to.
• == means equal.
• != means not equal.
• ! means not.
• && means AND.
• || means OR.

if Statement

• The if statement determines whether to execute a statement or block.
• The syntax is if (Expression) Statement.
• Statement can be a single statement, a null statement, or a block.

Terminating a Program

• return - used when void main() or exit(1) when int main()
• exit function requires #include <stdlib.h>.

Writing English Conditions in C

• Ensure the C condition is logically equivalent to the English statement.
• “x and y are greater than z” is written as (x > z) && (y > z)

if-else Statement

• The if-else statement chooses whether to execute statement A or statement B.
• Statement A or B can be a single statement or a block.
• The syntax is if (Expression) StatementA else StatementB.
• StatementA and StatementB can each be a single statement, a null statement, or a block.
• else braces can be omitted

Example Program: Circle Area and Circumference

• For a non-negative radius input by a user, calculate the circle area and circumference.
• Area = πr².
• Circumference = 2πr.
• For negative radiuses, display an "ERROR" message.

Nested if Statements

• Nested if statements are when one if statement is inside another, to code decisions with multiple alternatives.
• if (Expression1) Statement1 else if (Expression2) Statement2... else if (ExpressionN) StatementN else Statement N+1
• Exactly 1 of these statements will be executed.

Multi-Way Branching Code Example

• The sample code checks a student's credits earned and outputs the year they are in.
• Fourth year: creditsEarned >= 90
• Third year: creditsEarned >= 60
• Second year: creditsEarned >= 30
• First year: all other creditsEarned values

Common if Statements Errors

• No parentheses around the condition such as if crsr_or_frgt == 'C'.
• A semicolon after the condition, such as if (crsr_or_frgt == 'C');.

Switch Statement

• The switch statement is used instead of nested-if statements, to select several alternatives.
• It is based on the value of a single variable or simple expression known as the controlling expression.
• The value of this expression is commonly of type int or char.
• It has limited usage and cannot be used when, for example, checking if x and y are greater than z
• Also cannot be used to know student score given his grade
• The syntax is:

switch (controlling expression) {
    case value1: statements 1;
        break;
    case valueN: statements n;
        break;
    default: statements;
}

Switch Statement Explanation

• The switch statement compares the variable class to each of the cases in a top-down approach.
• It executes each line of code following the matching case, until it finds a break statement or the end of the switch statement.
• If no case is equal to the value of class, then the default case is executed.
• The default case is optional.

Nested-if Compared to Switch

• Advantages of if:
  • It has more general usage than a switch.
  • It can be use with a range of values
  • It can compare doubles
• Advantages of switch:
  • It is more readable.
• Use the switch whenever there are ten or fewer case labels.

Common Programming Errors

• Avoid if (0 <= x <= 4) and instead write (0 <= x && x <= 4)
• Avoid if (x = 10) and write if (x = 10) the = symbol assigns x the value of 10, so is always true, you must use the ==

More Common Errors

• Ensure you parenthesize the condition
• Ensure add { and } if they are needed
• C matches each else with the closest unmatched if
• In switch statements, ensure the controlling expression and case labels are of the same permitted type
• Remember to include the default case for switch statements
• Ensure add { and } for the switch statement.
• Always add break statements