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# MELJUN CORTES - C++ Looping

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MELJUN CORTES - C++ Looping

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• See pr5-01.cpp
• See pr5-02.cpp
• See pr5-03.cpp
• See pr5-04.cpp
• See pr5-18.cpp
• See pr5-04.cpp and pr5-05.cpp
• See pr5-06.cpp
• See pr5-07.cpp
• See pr5-08.cpp
• See pr5-09.cpp
• See pr5-10.cpp
• See pr5-11.cpp
• See pr5-14.cpp
• See pr5-15.cpp
• See pr5-16.cpp
• See pr5-17.cpp and pr5-17B.cpp
• ### MELJUN CORTES - C++ Looping

1. 1. Looping in C++ MELJUN P. CORTES
2. 2.  The Increment and Decrement Operators Introduction to Loops: The while Loop Using the while loop for Input Validation Counters The do-while loop The for loop Keeping a Running Total 5-2
3. 3.  Sentinels Nested Loops Breaking Out of a Loop The continue Statement Creating Good Test Data 5-3
4. 4. • ++ adds one to a variable val++; is the same as val = val + 1;• -- subtracts one from a variable val--; is the same as val = val – 1;• can be used in prefix mode (before) or postfix mode (after) a variable 5-4
5. 5. • ++val and --val increment or decrement the variable, then return the new value of the variable.• It is this returned new value of the variable that is used in any other operations within the same statement 5-5
6. 6. int x = 1, y = 1;x = ++y; // y is incremented to 2 // Then 2 is assigned to xcout << x << " " << y; // Displays 2 2x = --y; // y is decremented to 1 // Then 1 is assigned to xcout << x << " " << y; // Displays 1 1 5-6
7. 7. • val++ and val-- return the old value of the variable, then increment or decrement the variable• It is this returned old value of the variable that is used in any other operations within the same statement 5-7
8. 8. int x = 1, y = 1;x = y++; // y++ returns a 1 first // The 1 is assigned to x // then y is incremented to 2cout << x << " " << y; // Displays 1 2x = y--; // y-- returns a 2 // The 2 is assigned to x // then y is decremented to 1cout << x << " " << y; // Displays 2 1 5-8
9. 9. • Can be used in arithmetic expressions result = num1++ + --num2;• Must be applied to something that has a location in memory. Cannot have result = (num1 + num2)++; // Illegal• Can be used in relational expressions if (++num > limit) prefix- and postfix-operations will cause different comparisons 5-9
10. 10. • Loop: part of program that may execute more than once (i.e., it repeats)• while loop format: while (condition) { statement(s); No ; here }• The {} can be omitted if there is only one statement in the body of the loop 5-10
11. 11. while (condition) { statement(s); }condition is evaluated ◦ if it is true, the statement(s) are executed, and then condition is evaluated again ◦ if it is false, the loop is exited 5-11
12. 12. falsecondition truestatement(s) 5-12
13. 13. int val = 5; while (val >= 0) { cout << val << " "; val--; }produces output: 5 4 3 2 1 0 5-13
14. 14. while is a pretest loop (condition is evaluatedbefore the loop executes)(Entry Control Loop)If the condition is initially false, the statement(s)in the body of the loop are never executedIf the condition is initially true, the statement(s) inthe body continue to be executed until thecondition becomes false 5-14
15. 15. • The loop must contain code to allow condition to eventually become false so the loop can be exited• Otherwise, you have an infinite loop (i.e., a loop that does not stop)• Example infinite loop: x = 5; while (x > 0) // infinite loop because cout << x; // x is always > 0 5-15
16. 16. Loops are an appropriate structure forvalidating user input data1.Prompt and read in the data. Use a while loop to test if data is valid. Enter the loop only if data is not valid. Inside the loop, prompt the user to re-enter the data. The loop will not be exited until valid data has been entered. 5-16
17. 17. cout << "Enter a number (1-100) and" << " I will guess it. ";cin >> number;while (number < 1 || number > 100){ cout << "Number must be between 1 and 100." << " Re-enter your number. "; cin >> number;}// Code to use the valid number goes here. 5-17
18. 18. Counter: variable that is incremented ordecremented each time a loop repeatsCan be used to control execution of the loop (loopcontrol variable)Must be initialized before entering loopMay be incremented/decremented either insidethe loop or in the loop test 5-18
19. 19. Program can be written so that user inputdetermines loop repetitionCan be used when program processes a list ofitems, and user knows the number of itemsUser is prompted before loop. Their input isused to control the number of repetitions 5-19
20. 20. int num, limit;cout << "Table of squaresn";cout << "How high to go? ";cin >> limit;cout << "nnnumber squaren";num = 1; // loop control variablewhile (num <= limit){ cout << setw(5) << num << setw(6) << num*num << endl; num++;} 5-20
21. 21. • do-while: a post test loop (condition is evaluated after the loop executes)• Format: do { 1 or more statements; } while (condition); Notice the required ; 5-21
22. 22. statement(s)true condition false 5-22
23. 23. • Loop always executes at least once• Execution continues as long as condition is true;• The loop is exited when condition becomes false• Useful in menu-driven programs to bring user back to menu to make another choice 5-23
24. 24. • Pretest loop that executes zero or more times• Useful for counter-controlled loop Required ;• Format: for( initialization; test; update ) { 1 or more statements; } No ; goes here 5-24
25. 25. for(initialization; test; update){ // {} may be omitted statement(s); // if loop body contains} // only 1 statement Perform initialization Evaluate test expression: (a) If true, execute statement(s) (b) If false, terminate loop execution Execute update, then re-evaluate test expression 5-25
26. 26. initialization codeupdate code false test true statement(s) 5-26
27. 27. int sum = 0, num;for (num = 1; num <= 10; num++) sum += num;cout << "Sum of numbers 1 – 10 is " << sum << endl; 5-27
28. 28. If test is false the first time it is evaluated, thebody of the loop will not be executedThe update expression can increment ordecrement by any amountVariables used in the initialization section shouldnot be modified in the body of the loop 5-28
29. 29. Can define variables in initialization codeTheir scope is the for loopInitialization code, test, or update code can containmore than one statementSeparate statements with commasExample:for (int sum = 0, num = 1; num <= 10; num++) sum += num; 5-29
30. 30. Can omit initialization if already done int sum = 0, num = 1; for (; num <= 10; num++) sum += num;Can omit update if done in loop for (sum = 0, num = 1; num <= 10;) sum += num++;Can omit test – may cause an infinite loop for (sum = 0, num = 1; ; num++) sum += num; 5-30
31. 31. running total: accumulated sum of numbers fromeach repetition of loopaccumulator: variable that holds running totalint sum = 0, num = 1; // sum is thewhile (num <= 10) // accumulator{ sum += num; num++;}cout << "Sum of numbers 1 – 10 is " << sum << endl; 5-31
32. 32. • Sentinel: value in a list of values that indicates end of data• Special value that cannot be confused with a valid value, e.g., -999 for a test score• Used to terminate input when user may not know how many values will be entered 5-32
33. 33. int total = 0;cout << "Enter points earned " << "(or -1 to quit): ";cin >> points;while (points != -1) // -1 is sentinel{ total += points; cout << "Enter points earned: "; cin >> points;} 5-33
34. 34. while: pretest loop (loop body may not beexecuted at all)do-while: post test loop (loop body will alwaysbe executed at least once) for: pretest loop (loop body may not beexecuted at all); has initialization and updatecode; is useful with counters or if precise numberof repetitions is known 5-34
35. 35. • A nested loop is a loop inside the body of another loop outer loop• Example: for (row = 1; row <= 3; row++) { inner loop for (col = 1; col <= 3; col++) { cout << row * col << endl; } } 5-35
36. 36. Inner loop goes through all its repetitions for eachrepetition of outer loopInner loop repetitions complete sooner than outerloopTotal number of repetitions for inner loop isproduct of number of repetitions of the two loops.In previous example, inner loop repeats 9 times 5-36
37. 37. Can use break to terminate execution of a loopUse sparingly if at all – makes code harder tounderstandWhen used in an inner loop, terminates that looponly and goes back to outer loop 5-37
38. 38. Can use continue to go to end of loop andprepare for next repetition• while and do-while loops go to test and repeat the loop if test condition is true• for loop goes to update step, then tests, and repeats loop if test condition is trueUse sparingly – like break, can makeProgram logic hard to follow 5-38
39. 39. When testing a program, the quality of the testdata is more important than the quantity.Test data should show how different parts of theprogram executeTest data should evaluate how program handles: ◦normal data ◦data that is at the limits of the valid range ◦invalid data 5-39
40. 40. Question 1 Write a Program which takes a single integer "height" and displays a "pyramid" of this height made up of of "*" characters on the screen. The program, should be able to reproduce the following example output:This program prints a pyramid shape of a specified height on the screen.how high would you like the pyramid?: 37Pick another height (must be between 1 and 30):5 ** ****
41. 41. Question 2 "For" loops can always be re-written as "while" loops, and vice-versa. Are the following two programs equivalent, and what is their output? Explain your answer, and run the programs to check. Program (a): Program (b): #include <iostream.h> #include <iostream> int main() int main() { int count = 1; { for (; count <= 5 ; count++) int count = 1; { while (count <= 5) int count = 1; { cout << count << "n"; int count = 1; } cout << count << "n"; return 0; count++;