Programming Fundamental Instructor Name: Lecture-8 Today’s Lecture Repetition Structure (Loops) continued Do-while loop 2 Repetition Structure Property of While Loop The property of while loop is that the body of while loop may execute zero times or more. The reason is that the condition of while loop is first evaluated and then the body of while loop executes. But we may face situations where we need to perform some task (body of while loop) at-least once. Guess some situations ? 3 Repetition Structure do-while Statement Let's consider a scenario where we want to take an integer input from user until user have entered a positive number. In this case we will use a dowhile loop like this. do { TakeInputFromUser() } while(input < 0) In this case we have to run loop at-least once because we want input from user at-least once. This loop will continue running until user enters a positive number. 4 Repetition Structure do-while Statement – Flow Chart 5 Repetition Structure do-while Statement Syntax do statement while( condition ); VS do { statement } while( condition ); It is good programming practice to always including braces in a do...while statement helps eliminate ambiguity between the while statement and the do...while statement containing one statement. 6 Repetition Structure do-while Statement Consider another scenario where a computer program has a character stored from a-z. It gives to user five chances or tries to guess the character. In this case, the task of guessing the character must be performed at least once. To ensure that a block of statements is executed at least once, do while loop is perfect choice In this case we have to run loop at-least once because we want input (guess) from user to check whether the character is guessed or not. Broadly speaking, in while loop, the condition is tested at the beginning of the loop before the body of the loop is performed. Whereas in do-while loop, the condition is tested after the loop body is performed. 8 Therefore, in do-while loop, the body of the loop is executed at least once. Repetition Structure Problem Analysis A character in the program to be guessed by User Lets call this character is ‘z’ User is allowed to guess it in maximum five (5) attempts, so we need to track the number of attempts a user made to guess the character We need a variable to store the number of tries, lets call it tryNum. It would be of type int. Program prompts user to guess a character, we will store this character in a variable, lets call it c. it would be of type char. A character can be assigned as c=‘a’; 9 Repetition Structure Problem Analysis After getting the character in variable c we need to compare it with our character that is ‘z’ in the program To keep track of number of attempts we will use if/else structure If character is same as we have in our program a message will appear to congratulate the user other wise in the else clause we will add 1 in the counter variable In the while condition we will check the counter value if it is less than or equals to 5 we will ask user to guess character again Once the character is matched we must exit the loop Lets have a look on Flow chart of program 10 Repetition Structure Problem/Scenario – Flow Chart 11 Repetition Structure Program Code using namespace std; #include<iostream> void main ( ) { int tryNum = 0 ; char c ; do{ cout << “Please enter a character between a-z for guessing : “ ; cin >> c ; if ( c == ‘z’){ cout << “Congratulations, Your guess is correct” ; tryNum = 6; } else { tryNum = tryNum + 1; } } while ( tryNum <= 5); } 12 Repetition Structure Problem Solution There is an elegant way to exit the loop when the correct number is guessed A change in the while condition is required to exit program elegantly The condition will check whether the number of tries is less than or equals to five and the variable c is not equals to ‘z’ Our while condition will now become a compound statement by using logical && When one of the condition in compound condition will become false the control will exit the loop The change code will look like this: 13 Repetition Structure Program Code using namespace std; #include<iostream> void main ( ) { int tryNum = 0 ; char c ; do{ cout << “Please enter a character between a-z for guessing : “ ; cin >> c ; if ( c == ‘z’){ cout << “Congratulations, Your guess is correct” ; } else { tryNum = tryNum + 1; } } while ( tryNum <= 5 && c != ‘z’); } 14 Increment Operator What is Increment Operator An increment operator is used to increment the value of a variable by one (1). We have seen an increment statement in the previous slide like this: tryNum = tryNum + 1; This form of statement is very common and it is used in every repetition structure The C++ provides a unary operator that increased the value of its operand by 1 This operator is called increment operator and is written as ++ The above statement can be replaced with the statement tryNum++; 15 Decrement Operator What is Decrement Operator The statement tryNum++ increments the value of variable tryNum by 1 There is another operator called decrement operator and it used the operator - A decrement operator is used to decrement the value of a variable by one (1). For example we have a statement that decrements the value of variable x is written as : x = x - 1; The above statement can be replaced with the statement x--; 16 Increment Operator Categories of Increment Operator An increment operator is categories as Prefix increment Post fix increment Similarly for decrement we have prefix decrement and postfix decrement Apparently the difference is that in case of pre increment the operator ++ is placed before variable while in case of post increment operator ++ is placed after variable Both of these cases are equivalent if we are simply incrementing the value of a variable. Then it doesn’t matter that we are doing pre increment or post increment 17 Increment Operator Difference of Pre & Post Increment The difference of pre and post increment matters when we used them in a expression where it is used to assign value to anther variable Here the used of these operators is very tricky and you must use them carefully Let suppose we have a variable j and we use pre increment (++j) , the value of j is first increased by 1 and then it will be used in the expression If we use post increment (j++), the value of j is used in expression and after than the value of j is incremented by 1. If j=5 , we write the expression x=++j; What would be the value of x? 18 Increment Operator Difference of Pre & Post Increment 19 Increment/Decrement Operator Things to remember Increment /Decrement operator should not be used on constant We cannot use increment /decrement operator on the constant values because increment /decrement operator operates on only variables. It increments /decrements the value of the variable by 1 and stores the incremented/decremented value back to the variable Increment and Decrement operator only increases or decreases the value of variable by 1. If you want to increase or decrease the value by a number other than one for example you want j=j+5 C++ provides operators to perform such kind of tasks in short. These operators are called compound assignment operator. Compound Assignment Operator do two things they perform an action (addition, subtraction, multiplication, division, etc) and do some assignment. 20 Increment/Decrement Operator Problems What would the value of x if, x=0,y=0 x=++y; x=y++; x+=y++; x+=++y; x=y--; x=y++ + ++y + y + y--; x=--y + --y + --y + --y + --y; x=++y + y++ + ++y + ++y + y++; x*=y++ + ++y; x=x + x++ + y++ + ++y + ++x + y++; 21 22
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