a) A Computer with specifically C++ compiler installed b) Sumita arora (Text book) c) Herbert Scheldt fundamentals of c++

Transcription

1 BLOOM PUBLIC SCHOOL Vasant Kunj, New Delhi Lesson Plan Class XI Subject Computer Science Month July-August No of Periods 19 Chapter (Programming methodology and Flow of control) TTT 7 WT 12 Contents Learning Objectives Resources At the end of this chapter the students will be able to Classify C++ Statements into Selection Statements, Sequential Statements, Iterative Statements Write Selection Statements with various Flow Control Structures ( if, switch ) Write Iterative Statements with various Looping Structures ( for, while, do-while ) Apply Jump Statements ( goto, break, continue, exit( )) a) A Computer with specifically C++ compiler installed b) Sumita arora (Text book) c) Herbert Scheldt fundamentals of c++ Assessment Class Test Period wise Plan Period 1 and 2 Statements The teacher explains that Statement are the instructions given to the computer to perform any kind of action, be it data movement, be it making decision or be it repeating action. The simplest statement is empty or null statement. It takes the following form // It is a null statement These are the statements where syntax requires presence but logic of the program does not. 1

2 Compound Statement ( Block ) A compound statement in C++ is a sequence of statements enclosed by a pair of braces. For instance,... Represents a compound statement. It is also called a block Period 3 and 4 Statement Flow Control In a program, statement may be executed sequentially, selectively or iteratively. Sequence The sequence constructs means the statements are executed sequentially the sequence in which they have been written. This is the default flow of statement. Statement Statement Every C++ program begins with the first statement of main ( ) and ends with the last statement of main ( ). Therefore main is called the driver function. Statement Selection The selection construct means the execution of statements depending upon a conditiontest. If the condition test evaluates to true, a course of action is followed otherwise another course of action is followed. The construct is also called decision-making construct. One course of action Condition? true Statement 1 Statement 2 false Statement 1 Period 5 and 6 Another course of action Statement 2 Iteration The Iteration construct means repetition of a set of statements depending upon a condition. Till the time the condition is true (or false depending upon loop), a set of statements is repeated. As soon as the condition becomes false ( or true), the repetition stops. The iteration construct is called looping construct. Condition? false The exit condition true Statement 1 Statement 2 The loop body 2

3 The set of statements executed again and again is called the body of the loop. The condition on which the execution or exit of the loop depends is called the exit condition or test condition In C++ non-zero value is a true value including negative numbers. A false value is 0. Period 7 and 8 Selection Statements C++ provided two types of selection statements if and switch. In addition, in certain circumstances? operator can be used as an alternative to if statement. The selection statements are also called conditional statements. The syntax is as shown below if ( expression ) statement; where a statement may consist of a single statement, a compound statement, or nothing (in case of empty statement ). The expression must be enclosed in parenthesis. If the expression evaluates to true i.e. a nonzero value, the statement is executed, otherwise ignored. For instance, the following code fragment. If ( ch == ) spaces++ ; checks whether the character variable ch stores a space or not; if it does, the number of spaces are incremented by 1. Consider another example illustrating the use of if statement char ch; cin>>ch; if ( ch == ) cout<< You entered a space << \n ; if ( ch>= 0 && ch <= 9 ) cout<< You entered a digit << \n ; The above example reads a character. If the character input is a space, it flashes a message specifying it. If the character input is a digit, it flashes a message specifying it. The following example also makes use of an if statement. int A, B, C; cin>> A >> B ; if ( A > 10 && B < 15 ) C = ( A B ) * ( A + B ) ; cout<< The result is << C<< \n ; The above statement uses a compound statement (or a block) in the body of it. All the examples allow to execute a set of statements if a condition or expression evaluates to true. What if there is another course of action to be followed if the expression evaluates to false. The other form of if is if ( expression ) if the expression is true a non zero value, the statement 1 is executed, otherwise statement 2 is executed. 3

4 Test false Test expression false body of if true body of if true body of Period 9 and 10 Nested ifs A nested if is an if that has another if in its if s body or in its s body. The nested if can have one of the following three forms 1. if (expression1) If ( expression 2) [ Statement 2;] body of ; 2. if (expression 1) body of if; if ( expression 2) [ ] 3. if (expression 1) If ( expression 2 ) Statement 1; [ Statement 2;] if (expression 3) statement3; [ statement 4;] 4

5 The nested if- statement introduces a source of potential ambiguity referred to as dangling problem. This problem arises when in a nested if statement number of ifs is more than the number of clauses. In the example below if (ch >= A ) if ( ch <= Z ) ++upcase; ++others; The indentation indicates that programmer wants the to be with the outer of. However, C++ matches an with the preceding unmatched if. In this case, the actual evaluation of the if statement will be shown below If (ch >= A ) If (ch<= Z ) ++upcase; ++others; That is, if the inner expression is false, i.e. zero then the clause gets executed. In nested if statement, a dangling statement goes with the preceding unmatched if statement. In an expression such as this If (expr 1) If ( expr 2) the last statement goes with the immediately preceding if statement does not already have an statement. Here statement 2 is executed if expr 2 evaluates to false. However, if we have a code as follows if (expr 1) if ( expr 2) statement 3; the inner goes with immediately preceding unmatched if which is the inner if. The outer goes with immediately preceding unmatched if which is now outer if as the inner if is unmatched. Thus, statement 3 gets executed if exp1 is false. What if you want to override the default dangling- matching? That is if you have a code as shown below If( expr 1) If ( expr 2) Statement 1; Statement; 5

6 And you want the to go with the outer if ( By default it will go with the inner if ) then use braces to override rge default dangling matching. If (expr 1) If( expr 2) Statement1; Statement2; ANSI standard specifies that at least 15 levels of nesting must be supported by a compiler The if--if ladder It is commonly called the if--if staircase because of its appearance. if ( expression 1 ) if ( expression 2 ) if ( expression 3 ) statement 3; statement n; The? alternative to if As studied in Chapter 6, the conditional operator? can be used to replace if- statements of the general form If ( expression 1) Statement 2; This can be alternatively written using? as follows Expression 1? statement 1 statement 2 Period 11 and 12 Comparing if and? 1. Compared to if- sequence,? offer more concise, clean and compact code, but it is obvious as compared to if. 2. Another difference is that the? operator produces an expression, hence a single value can be assigned or incorporated into larger expression, whereas, if is more flexible. 3. When? operator is used in its nested form, it becomes complex and difficult to understand 4.? has a very low precedence. The switch statement C++ provides a multiple branch selection known as switch. This selection statement successively tests the value of an expression against a list of integer or character constants. When a match if found, the statements associated with that constant are executed. switch ( expression ) case constant1 statement sequence 1; 6

7 case constant 2 statement sequence 1; case constant n-1 statement sequence 1; [ default statement sequence n;] The expression is evaluated and its values are matched against the values of the constants specified in the case statements. When a match is found, the statement sequence associated with the case is executed until the break statement or the end of switch statement is reached. The default statement gets executed when no match is found. The default statement is optional and, if it is missing, no action takes place if all matched fail. ANSI standard specifies that at a switch can have upto 257 case statements; however you must limit the number of case statements to a smaller amount for the sake of efficiency. A case statement cannot exist by itself, outside of a switch. The break statement is a jump statement ( we will discuss later in the same chapter ) The switch Vs. if- The switch and. if- both are selection statements and they both let you select an alternative out of given many alternatives by testing an expression. 1. The switch can only test for equality whereas if can evaluate a relational or logical expression i.e. multiple conditions. 2. The switch statement selects its branches by testing the value of same variable (against a set of constants) whereas the if- construction lets you use a series of expressions that may involve unrelated variables and complex expressions. 3. The if- is more versatile of the two statements. For instance, if- can handle ranges whereas switch cannot. Each switch case label must be a single value. 4. the if- statement can handle floating-point tests also apart from handling integer and character tests whereas a switch cannot handle floating-point tests. The case labels of switch must be an integer ( which includes char also ) 5. The switch label must be a constant. So, if two or more variables are to be compared, use if-. 6. The switch statement is more efficient choice in terms of code used in situation that supports the nature of witch operation. Period 11 and 12 The Nested Switch Like if statements, a switch statement can also be nested. There can be a switch as part of the statement sequence of another switch. For instance, switch ( a ) case 1 switch ( b ) case 0 cout<< Divide by zero Error!! case 1 res = a / b; breakl case 2 7

8 Some important things to know about switch 1. A switch statement can only work for equality comparisons. 2. No two case labels in the same switch can have identical values. In case of nested switch statements the case constants of the inner and outer switch can contain common values. 3. If character constants are used in the switch statement, they are automatically converted to their integers( ASCII codes ) 4. The switch statement is more efficient than if in a situation that supports the nature of switch operation. 5. Switch is more efficient than nested if- statement. Iteration Statements Every loop has four elements that have different purposes. These are a) Initialization Expression(s) Before entering a loop, its control variable( s) must be initialized. This initialization of the control variable (s) takes place under initialization expression (s). This gives variable( s) their first value(s). The initialization expression(s) is executed only once, in the beginning. b) Test Expression The test expression is an expression whose truth values decided whether the loop-body will be executed or not. If the test expression evaluates to true, i.e., 1 the loop-body gets executed otherwise the loop is terminated. The entry-controlled loop, the test expression is evaluated before entering into a loop whereas in an exit-controlled loop, the test expression is evaluated before exiting from the loop. In C++, the for loop and while loop are entry-controlled loops and do-while loop is exit-controlled loop. c) Update Expression(s) The update expression(s) change the value(s) of loop variable(s). The update expression(s) is executed; at the end of the loop after the loop-body is executed. d) The body-of-the-loop The statements that are executed repeatedly ( as long as the test expression is non-zero) form the body of the loop. In an entry-controlled loop, first the testexpression is evaluated and if it is non-zero, the body of the loop is executed, if the test-expression evaluates to zero, the loop is terminated. In an exit-controlled loop, the body of the loop is executed and then the test-expression is evaluated. If it evaluates to be zero, the loop is terminated, otherwise repeated. Period 13 and 14 The for Loop The general form (syntax) of the for loop statement is for ( initialization expression(s) ; test-expression ; update expression(s) ) body-of-the-loop; void main( ) int i; for( i =1 ; I <= 10 ; ++ i ) cout << \n << i; Working of the for loop initialization test-expression update expression(s) expression(s) for( i = 1 ; i <= i ) cout << \n << i; 1.Firstly, initialization expression is executed, i.e. i =1 which gives the first value 1 to variable i; 8

9 2.Then, the test-expression evaluated i.e. i <= 10 which results into true i.e. 1; 3. Since the test-expression is true, the body of the loop i.e. cout << \n << i is executed which prints the current value of i on the next line. 4. After executing the loop-body, the update expression i.e. ++ I is executed which increments the value of i. 5. After the update expression is expression is executed, the test-expression is again evaluated. If it is true the sequence is repeated from step 3 otherwise the loop terminates. / followed by Dry Run initialization expression(s) testexpressio False Ex Variations of for Loop True Body of the Loop 1. Multiple initialization and update Expressions 2. Optional Expressions update expression(s) 3. Infinite Loop 4. Empty Loop Scope of a variable A variable declared in a for or while loop can be accessed after the statement because the variable declaration has not taken place within the braces of the loop block, the item will be in scope when the loop terminates. That means the same variable cannot be declared in the same scope. The while Loop This is an entry-controlled loop. The syntax is while ( expression ) loop-body where the loop-body may contain a single statement, a compound statement or an empty statement. The loop iterates while the expression evaluates to true. When the expression becomes fals, the program control passes to the line after the loop-body code. In a while loop, a loop control variable should be initialised before the loop begins as an uninitialised variable can be used in an expression. The loop variable should be updated inside the body of the loop. Variations of for Loop 1.empty loop 2. infinite loop The do-while Loop This is an exit-controlled loop. It evaluates the test-expression at the bottom of the loop after executing its loop-body statements. This means that do-while loop always executes at least once. 9

10 CW Nested Loops A loop may contain another loop in its body. This form is called nested loop. But in a nested loop, the inner must terminate before the outer loop. Comparison of Loops When you know the number of times the loop has to iterate, for loop is the best choice. When you do not want the loop to be executed even once without the condition getting checked, while loop is the preferred choices. When you want that the loop can be executed at least once before the condition being checked, do-while is the preferred choice. Jump Statements These statements unconditionally transfer program control within a function. C++ has four statements that perform an unconditional branch return, goto, break, continue. The goto statement With this statement, we can transfer the program control anywhere in the program. The target destination of a goto statement is marked by a label. The syntax is goto label; label If a label appears just before a closing brace, a null statement must follow the label. A goto statement may not jump forward a variable definition. Period 15 and 16 The break Statement This enables a program o skip over part of the code. A break statement terminates the smallest enclosing while, do-while, for, or switch statement. Execution resumes at the statement immediately following the body of the terminated statement. while ( test expression ) Statement; if ( val > 2000 ) for ( int; expression ; update) Statement 1; If ( val > 2000) Statement 3; do Statement 1; If ( val > 2000) while( test expression); Statement 3; 10

11 Statement 2; Statement 3; A break used in a switch statement will affect only that switch i.e. it will terminate only the very switch it appears in. It does not affect any loop the switch happens to be in. The continue Statement The continue is another jump statement like the break statement. But the continue statement is somewhat different from break. Instead of forcing termination, it forces iteration of the loop to take place, skipping any code in between. while ( test expression ) Statement; if ( condition ) continue; Statement 2; Statement 3; for ( int; expression ; update) Statement 1; If ( condition) continue; Statement 3; do Statement 1; If ( condition) continue; while( test expression); Statement 3; Period 17 and 18 The exit ( ) Function This causes the program to terminate as soon as it is encountered, no matter where it appears in the program listing. This function does not have any return value. Its argument is returned to he operating system. This value can be tested in batch files where ERROR LEVEL gives you the return value provided by exit function. Generally the value 0 signifies a successful termination and any other number indicates some error. This function is defined in a header file process.h, which must be included in a program that uses exit( ) function. Important question covered as a concept Q1. ) what is entry control and exit control loop? Q2.) What is the result of if condition ending with ; semicolon. (Q3.) Explain the if statement? (Q4.) Give an example of a test condition comparing it with a literal using If construct? (Q5.) What is the following segment checking? if (ch=' ') Spaces ++; (Q6.) What is the syntax of the if statement? 11

12 (Q7.) What does iteration mean? (Q8.) How is a block statement written? (Q9.) When is a null or empty statement required? (Q10.) What is nested if? Part 2 Mention two differences between if and conditional operator? (Q2.) What is the Nested if structure? (Q3.) Explain the update expression in the for loop? (Q4.) How can the statements of a program be executed? (3 Marks) (Q5.) Elaborate the structure of a Switch statement? (3 Marks) (Q6.) State any three differences between switch statement and if- statement? (3 Marks) (Q7.) What is the difference between if- and switch? Part 3 (Q1.)List the variations of the While loop? (5 Marks) (Q2.) In the following segment, can the loop mentioned, be used as time delay loop? Lwait=0; while(++lwait<500000) ; ; 12

13 (Q4.) Explain S1, R and R2 in the following for loop statement? for(s1;r;r2) (Q3.) Display the output of the following program. int i=10; while(i>=6) cout << i < < endl ; i - -; i - -; (Q5.) Give examples of 3 test conditions which work with If. a) comparing a literal b) comparing 2 variables c) testing truth value of a variable (Q6.) List any 3 variations in the for loop? (3 Marks) (Q7.) Write a program to evaluate the factorial of a number? Period 19 Class Test 13