DATA STRUCTURES AND ALGORITHMSI (DSA010).pdf
- 1. DATA STRUCTURES AND ALGORITHMSI (DSA010) CONTROL STRUCTURES
- 2. CONTROL STRUCTURES 1.1 Structured Program Theorem It is possible to write any computer program by using only three basic control structures: Executing one subprogram, and then another subprogram (sequence) Executing one of two subprograms according to the value of a boolean expression (selection) Executing a subprogram until a boolean expression is true (iteration or repetition) Type of control structure sequence Selection (iteration or repetition)
- 3. SELECTION CONTROL STRUCTURE The selection structures are of very high importance, because they enable programs to branch into different blocks of instructions depending on particular conditions. This way they provide a kind of “intelligence” to programs. A control structure is a block of programming that analyzes variables and chooses a direction in which to go based on given parameters. • The term flow control details the direction the program takes (which way program control "flows"). Hence it is the basic decision-making process in computing; flow control determines how a computer will respond when given certain conditions and parameters.
- 4. Basic Terminologies Those initial conditions and parameters are called preconditions. Preconditions are the state of variables before entering a control structure. Based on those preconditions, the computer runs an algorithm (the control structure) to determine what to do. The result is called a postcondition. Postconditions are the state of variables after the algorithm is run. An Example Let us analyze flow control by using traffic flow as a model.
- 5. A vehicle is arriving at an intersection. Thus, the precondition is the vehicle is in motion. Suppose the traffic light at the intersection is red. The control structure must determine the proper course of action to assign to the vehicle. - Precondition: The vehicle is in motion. - Control Structure Is the traffic light green? If so, then the vehicle may stay in motion. Is the traffic light red? If so, then the vehicle must stop. End of Control Structure
- 6. Type Description Best Use Case if Executes when condition is true Single decision if-else Executes one of two blocks Two outcomes if-elif-else Checks multiple conditions Multiple outcomes Nested if if inside another if Dependent decisions switch-case Matches against multiple constant values Clean multiple-choice selection (non-Python) SELECTION CONTROL STRUCTURE:
- 7. 1. Simple If Statement Executes a block of code only when a specified condition is true. 2. If-Else Statement Provides two paths: one block runs if the condition is true, the other runs if the condition is false. 3. Nested If Statement Contains an if or if-else statement inside another if or else block, allowing multiple conditions to be checked in a hierarchy. 4. If-Else If Ladder Checks multiple conditions sequentially; the first true condition's block is executed. 5. Case (or Switch) Statement Evaluates a single expression and executes one block of code among many alternatives, based on matching values. SELECTION CONTROL STRUCTURE:
- 8. 1. Simple if Statement : A simple if statement is a basic control structure used in programming to execute a block of code only if a certain condition is true. If the condition is false, the code inside the if block is skipped.
- 9. START int number1 int number2 IF number1 > number2 THEN PRINT "The largest number is", number1 ELSE PRINT "The largest number is", number2 ENDIF END Example : Write an Algorithm to find the Largest of Two Numbers
- 10. if-else Statement : Chooses between two blocks of code based on whether the condition is true or false.
- 11. if-elif-else Statement Checks multiple conditions in sequence and executes the block where the first true condition is found.
- 12. Case Statement A case statement (also known as a switch statement in many programming languages) is a control structure that allows a program to execute one block of code from multiple options, based on the value of a single variable or expression. It is used as an alternative to writing many if...else if statements, making the code more organized and readable when handling multiple possible values of the same variable. Example 1: Grade Evaluation
- 13. Write a CASE statement in pseudocode for Grade Feedback Based on Letter Grade INPUT grade CASE grade OF "A": Display "Excellent!" "B": Display "Very Good!" "C": Display "Good" "D": Display "You passed" "F": Display "Failed" OTHERWISE: Display "Invalid grade" ENDCASE Write a CASE statement in pseudocode for Student Year Classification. INPUT year Level CASE yearLevel OF 1: Display "You are a First-Year student" 2: Display "You are a Second-Year student" 3: Display "You are a Third-Year student" 4: Display "You are a Final-Year student" OTHERWISE: Display "Invalid year level" ENDCASE
- 14. • General Example: Convert the following linear if statement into nested if statement. linear if statement IF a > 5 AND b < 7 THEN statement(s)_A ELSE IF c > 6 THEN statement(s)_B ELSE statement(s)_C ENDIF nested if statement IF a > 5 THEN IF b < 7 THEN statement(s)_A ELSE IF c > 6 THEN statement(s)_B ELSE statement(s)_C ENDIF ELSE IF c > 6 THEN statement(s)_B ELSE statement(s)_C ENDIF
- 15. Symbol Name Meaning = Assignment / Equal (varies by context) Often used to assign a value (e.g., x = 5) == Equal to Checks if two values are equal != Not equal to Checks if two values are not equal < Less than Example: x < 10 > Greater than Example: x > 5 <= Less than or equal to Example: x <= 10 >= Greater than or equal to Example: x >= 5 Comparison (Relational) Operators (used for comparing values)
- 16. SET i = 1 WHILE i <= 3 DO // Comparison Operator: <= SET j = 1 WHILE j <= 3 DO // Comparison Operator: <= DISPLAY i + " x " + j + " = " + (i * j) // Arithmetic Operator: * SET j TO j + 1 // Arithmetic Operator: + END WHILE SET i TO i + 1 // Arithmetic Operator: + END WHILE Pseudocode with Operators
- 17. Operator Type Description <= Comparison Operator Checks if the left value is less or equal + Arithmetic Operator Adds numbers or strings (concatenation) * Arithmetic Operator Multiplies numbers = Assignment Operator Assigns value to a variable Operators Used Pseudocode
- 18. SET number = 7 IF number % 2 == 0 THEN // % is Modulus Operator (remainder) DISPLAY "Even Number" ELSE DISPLAY "Odd Number" END IF Check if a number is even or odd Operators Used: •% → Arithmetic Operator (modulus) •== → Comparison Operator (equal to) •= → Assignment Operator
- 19. SET a = 12 SET b = 8 SET c = 15 IF a > b AND a > c THEN // Logical Operator: AND DISPLAY "a is the largest" ELSE IF b > a AND b > c THEN DISPLAY "b is the largest" ELSE DISPLAY "c is the largest" END IF Find the largest of three numbers