Ch3 selection

Chapter 3 CONTROL STRUCTURES: SELECTION

Topic Sequence Structure Selection Structures if if…else switch Repetition Structures (looping) while do…while for break , continue statement & exit() function

Structured Programming Sequence structure One command right after another in order Selection structure One execution branch or another (but not both) Iterative (Repetition) structure Repeating one section of command(s) multiple times

Sequence Structure Example Algorithm for calculating the area of a circle: 1. input radius 2. calculate area 3. print area

Sequence Structure Example Statements executed one after the other in the order written

Sequence Structure Example #include <stdio.h> void main() { int radius; float area; printf("Enter a radius of circle: "); scanf ("%d", &radius); area = 3.14 * radius * radius; printf("Area of circle is %.2f", area); }

SELECTION STRUCTURES Diamond symbol (decision symbol) – Indicates decision is to be made – Contains an expression that can be true or false – Test the condition, follow appropriate path C provides 3 types of selection structures 3.2 if selection (single-selection structure) 3.3 if… else selection (double-selection structure) 3.5 switch selection (multiple-selection structure)

if Selection - if selection structures is called a single-selection structure because it selects or ignores a single action. - either performs (selects) an action if a condition is true or skips the action if the condition is false. Syntax: if (condition) statement ; if selection Flowchart

if Selection Examples Example 3.1: if (num1 > num2) printf("%d is greater than %d\n", num1, num2); Example 3.2: if (x == 100) printf("x is 100");

if Selection Examples If more than one statement should be executed if the condition is true, then these may be grouped together inside braces. Example 3.3: if (num1 > num2) { printf("%d is greater than %d\n", num1, num2); printf("%d is less than or equal to %d\n", num2, num1); } Example 3.4: if (x == 100) { printf ("x is "); printf (“%d”, x); }

if…else Selection Specifies an action to be performed both when the condition is true and when it is false Syntax : if (condition) statement1; else statement2;

if…else Selection Examples Example 3.5 if (num1>num2) printf ("%d is greater than %d\n ", num1, num2); else printf (“%d is greater than %d\n ", num2, num1); Example 3.6 if (x==100) printf ("x is 100"); else printf (“x is not 100");

if…else Selection Examples Example 3.7 Based on the value of two variables, num1 and num2, the suitable message will be printed. if (num1 > num2 ) printf("%d is greater than %d\n", num1, num2); else if (num1 < num2) printf("%d is less than %d\n", num1, num2); else printf("%d is equal to %d\n", num1, num2);

if…else Selection Examples Example 3.8 Based on the value of x, message either positive, negative or zero will be printed. if (x > 0) printf("x is positive"); else if (x < 0) printf("x is negative"); else printf("x is 0");

#include <stdio.h> #include <time.h> main() { int num, guess; /* seed & generate a random number */ srand(time(0)); num = rand() % 10; /* get a guess from the user */ printf("Guess the number: "); scanf("%d", &guess); if (guess < num) printf("Too low - the number was %d\n", num); else if (guess > num) printf("Too high - the number was %d\n", num); else printf("Correct - the number was %d\n", num); }

The Conditional Operator (?:) There is another way to write an if…else structure. Syntax: condition ? statement_1 : statement_2 ; Example: cost = (cost>10.00) ? 15.00 : 10.00; if (cost>10.00) cost = 15.00; else cost = 10.00;

Switch Selection switch (expression) { case constant_1 : group_of_statements_1 ; break; case constant_2 : group_of_statements_2 ; break; : : default: default_group_of_statements ; }

The switch statement Multi-branch alternative to if switch ( expression ) { statement-sequence } The statement-sequence contains the branches Each branch begins with the keyword case case integral-value : Each branch ends with the keyword break The constant must evaluate to some integral data type ( char or int ) Floating-point contant are not allowed

Example 3.10 This section of code prints out the roman numeral corresponding to any of the numbers from 0 to 3. switch (num) { case 1: printf("I"); break; case 2: printf("II"); break; case 3: printf("III"); break; default: printf("?"); }

Example 3.11 If the value of x is 1, 2 or 3, the “x is 1, 2 or 3” message will be printed. Else, “x is not 1, 2 nor 3” message will be appeared. switch (x) { case 1: case 2: case 3: printf("x is 1, 2 or 3"); break; default: printf("x is not 1, 2 nor 3"); }

Transformation from switch to if…else statement if (x == ‘a’) printf("x is a"); else if (x == ‘b’) printf("x is b"); else printf("value of x unknown"); switch (x) { case ‘a’: printf("x is a"); break; case ‘b’: printf("x is b"); break; default: printf("value of x unknown"); } if…else statement switch statement

Exercise Write a program to add or multiply 2 integers. You have to ask the user to enter 3 values. The first one will be a 1 or 2 and the last two will be the numbers that you want to add/multiply. If the user enters 1, you have to add 2 numbers, if the user enters 2, you have to multiply 2 numbers. Sample Execution: Input: 1 5 4 Input: 2 5 4 Output: 9 Output: 20

Ch3 selection

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Ch3 selection