Decision Making -Branching. Class Incharge: S. Sasirekha

Transcription

1 Decision Making -Branching Class Incharge: S. Sasirekha

2 Branching The C language programs presented until now follows a sequential form of execution of statements. Many times it is required to alter the flow of the sequence of instructions. C language provides statements that can alter the flow of a sequence of instructions. These statements are called control statements. These statements help to jump from one part of the program to another. The control transfer may be conditional or unconditional.

3 if Statement Use Depending on the result of a condition the program will either branch and execute some code, then continue with the rest of the program. OR continue with the rest of the program. Syntax 1. if ( condition ) statement 1; 2. if ( condition ) statement 1; The additional line is white space, therefore there is no ; after if ( condition ) Warning: Inserting a ; may be accepted by the compiler but will produce unexpected results.

4 Although not required for single statement if constructs, It is recommend that braces are used as follows, so that there is no confusion to what is included within the construct. if ( condition ) statement 1; When more than 1 statement is used then braces MUST be used. if ( condition ) statement 1; statement 2; Description Use if to implement a conditional statement. When condition evaluates to be nonzero, statement 1 executes. If condition is false (0), statement 2 executes.

5 Example if (count < 50) count++; This code will add 1 to the value of count is < (less than) 50

6 Example program Calculate the absolute value of an integer # include <stdio.h> //Include the stdio.h file void main () // start of the program int numbers; printf ("Type a number:"); scanf ("%d", &number); if (number < 0) number = -number; printf ("The absolute value is %d \n", number);

7 If else If choices are required then an additional if... else pair is required. Syntax if ( condition ) statement 1; else statement 2;

8 Description When condition evaluates to be nonzero, statement 1 executes. If condition is false (0), statement 2 executes. Example if (x < y) z = x; else z = y; Explanation If the value of x is less than the value of y then z is assigned the value of x If the value of x is NOT less than the value of y then z is assigned the value of y

9 #if and #else preprocessor statements The #if and #else pre-processor statements (directives) look similar to the if and else statements, but have very different effects. #define TURBOC #ifdef TURBOC #define INT_SIZE 16 #else #define INT_SIZE 32 #endif

10 Example program Program find whether a number is negative or positive #include <stdio.h> void main () int num; printf ("Enter the number"); scanf ("%d", &num); if (num < 0) printf ("The number is negative"); else printf ("The number is positive");

11 Nested if Statement The if statement may itself contain another if statement is known as nested if statement Syntax: if (condition1) if (condition2) statement-1; else statement-2; else statement-3;

12 The if statement may be nested as deeply as you need to nest it. One block of code will only be executed if two conditions are true. Condition 1 is tested first and then condition 2 is tested. The second if condition is nested in the first. The second if condition is tested only when the first condition is true else the program flow will skip to the corresponding else statement

13 Nested if Statement Example #include <stdio.h> main () int a,b,c,big; printf ("Enter three numbers"); scanf ("%d %d %d", &a, &b, &c); if (a > b) if (a > c) big = a ; else big = c ; else if (b > c) big = b; else big = c; printf ("Largest of %d, %d & %d = %d", a,b,c,big); er

14 The ELSE If Ladder When a series of many conditions have to be checked we may use the ladder else if statement which takes the following general form. Syntax if (condition1) statement 1; else if (condition2) statement2; else if (condition3) statement3; else if (condition) statement n; else default statement; statement-x;

15 This construct is known as if else construct or ladder. The conditions are evaluated from the top of the ladder to downwards. As soon on the true condition is found, the statement associated with it is executed and the control is transferred to the statement x (skipping the rest of the ladder). When all the condition becomes false, the final else containing the default statement will be executed.

16 Example program using If else ladder to grade the student according to the following rules.

17 #include <stdio.h> void main () int marks; printf ("Enter marks\n"); scanf ("%d", &marks); if (marks <= 100 && marks >= 70) printf("\n Distinction"); else if (marks >= 60) printf("\n First class"); else if (marks >= 50) printf ("\n second class"); else if (marks >= 35) printf ("\n pass class"); else printf ("Fail");

18 The Switch Statement Switch statement allows a program to select one statement for execution out of a set of alternatives. During the execution of the switch statement only one of the possible statements will be executed the remaining statements will be skipped. The switch statement removes these disadvantages by using a simple and straight forward approach. Syntax Switch (expression) Case case-label-1; Case case-label-2; Case case-label-n; Case default

19 When the switch statement is executed the control expression is evaluated first and the value is compared with the case label values in the given order. If the label matches with the value of the expression then the control is transferred directly to the group of statements which follow the label. If none of the statements matches then the statement against the default is executed. The default statement is optional in switch statement in case if any default statement is not given and if none of the condition matches then no action takes place in this case the control transfers to the next statement of the if else statement.

20 A program to stimulate the four arithmetic operations using switch #include <stdio.h> void main () int num1, num2, result; char operator; printf ("Enter two numbers"); scanf ("%d %d", &num1, &num2); printf ("Enter an operator"); scanf ("%c", &operator); ; switch (operator) case '+': result = num1 + num2; break;

21 case '-': result = num1 - num2; break case '*': result = num1 * num2; break; case '/': if (num2!= 0) result = num1 / num2; else printf ("warning : division by zero \n"); result = 0; break; default: printf ("\n unknown operator"); result = 0; break; printf ("%d", result);

22 The GOTO statement The goto statement is simple statement used to transfer the program control unconditionally from one statement to another statement. Although it might not be essential to use the goto statement in a highly structured language like C, there may be occasions when the use of goto is desirable. Syntax a> goto label; Label; Statement; b> label; goto label;

23 The goto requires a label in order to identify the place where the branch is to be made. A label is a valid variable name followed by a colon. The label is placed immediately before the statement where the control is to be transformed. A program may contain several goto statements that transferred to the same place when a program. The label must be unique.

24 A program to find the sum of n natural numbers using goto statement #include <stdio.h> main () int n, sum = 0, i = 0; printf ("Enter a number"); scanf ("%d", &n); loop: i++; sum += i; if (i < n) goto loop; printf ("\n sum of %d natural numbers = %d", n, sum);