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Repetitive Structures

Repetitive Structures. logExample.cpp. // example of log(k) for k = 1,2,..,8 . . . int main() { cout << "log(1) = " << log(1.0) << endl; cout << "log(2) = " << log(2.0) << endl; cout << "log(3) = " << log(3.0) << endl; cout << "log(4) = " << log(4.0) << endl;

benjamin-wise
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Repetitive Structures

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  1. Repetitive Structures The Ohio State University

  2. logExample.cpp // example of log(k) for k = 1,2,..,8 . . . int main() { cout << "log(1) = " << log(1.0) << endl; cout << "log(2) = " << log(2.0) << endl; cout << "log(3) = " << log(3.0) << endl; cout << "log(4) = " << log(4.0) << endl; cout << "log(5) = " << log(5.0) << endl; cout << "log(6) = " << log(6.0) << endl; cout << "log(7) = " << log(7.0) << endl; cout << "log(8) = " << log(8.0) << endl; return 0; } The Ohio State University

  3. logExample.cpp ... cout << "log(1) = " << log(1.0) << endl; cout << "log(2) = " << log(2.0) << endl; cout << "log(3) = " << log(3.0) << endl; cout << "log(4) = " << log(4.0) << endl; cout << "log(5) = " << log(5.0) << endl; cout << "log(6) = " << log(6.0) << endl; cout << "log(7) = " << log(7.0) << endl; cout << "log(8) = " << log(8.0) << endl; ... > logExample.exe log(1) = 0 log(2) = 0.693147 log(3) = 1.09861 log(4) = 1.38629 log(5) = 1.60944 log(6) = 1.79176 log(7) = 1.94591 log(8) = 2.07944 The Ohio State University

  4. Repetition Structures (Loops) • Motivation: Allow repetition of code (e.g., outputting from 1 to 1000 should not involve the programmer to write 1000 lines of cout statements!) The Ohio State University

  5. logWhile.cpp // example of while loop for log(k) for k = 1,2,..,8 . . . int main() { int k(0); k = 1; while (k <= 8) { cout << "log(" << k << ") = " << log(double(k)) << endl; k++; } return 0; } The Ohio State University

  6. logWhile.cpp ... k = 1; while (k <= 8) { cout << "log(" << k << ") = " << log(double(k)) << endl; k++; } ... > logWhile.exe log(1) = 0 log(2) = 0.693147 log(3) = 1.09861 log(4) = 1.38629 log(5) = 1.60944 log(6) = 1.79176 log(7) = 1.94591 log(8) = 2.07944 The Ohio State University

  7. while Loops • The while statement is of the form: while (conditional expression) { statement1; statement2; ... } The Ohio State University

  8. How while Loops Work • First, the conditional expression is tested. If it is true, then the statement(s) within the loop structure is/are executed. • Once the end of those statements is reached, then the process is repeated. • If the expression ever evaluates to false, then the while statement is exited, and the program continues beyond the loop. The Ohio State University

  9. Types of Loops • Pretest Loops checks the looping condition first, then begins execution • while • for • Posttest Loops begins execution first, then checks looping condition • do-while The Ohio State University

  10. Control Flow of a while Loop The Ohio State University

  11. While Example count = 1; while (count <= 10) { cout << count << “ “; count++; //increment count! } • Output is “1 2 3 4 5 6 7 8 9 10” The Ohio State University

  12. Control Flow of the Example Program The Ohio State University

  13. Repetition Structures 2 • Motivation 2: Allow repetition of code based on input (e.g., a program should be able to output n lines of cout statements where n is a user input.) The Ohio State University

  14. logWhile2.cpp ... int main() { int n(0), k(0); cout << "Enter number of logarithms to compute: "; cin >> n; k = 1; while (k <= n) { cout << "log(" << k << ") = " << log(double(k)) << endl; k++; } return 0; } The Ohio State University

  15. logWhile2.cpp ... cout << "Enter number of logarithms to compute: "; cin >> n; k = 1; while (k <= n) { cout << "log(" << k << ") = " << log(double(k)) << endl; k++; } ... > logWhile2.exe Enter number of logarithms to compute: 5 log(1) = 0 log(2) = 0.693147 log(3) = 1.09861 log(4) = 1.38629 log(5) = 1.60944 The Ohio State University

  16. logWhile2.cpp ... cout << "Enter number of logarithms to compute: "; cin >> n; k = 1; while (k <= n) { cout << "log(" << k << ") = " << log(double(k)) << endl; k++; } ... What happens here? > logWhile2.exe Enter number of logarithms to compute: -3 ??? The Ohio State University

  17. Repetition Structures 3 • If a program receives incorrect input, it can repeatedly prompt for the correct input. The Ohio State University

  18. logWhile3.cpp // example of while loop to prompt for correct input ... int main() { double x(0.0); cout << "Enter number: "; cin >> x; while (x <= 0) { cout << "Input must be positive." << endl; cout << "Enter number: "; cin >> x; } cout << "log(" << x << ") = “ << log(x) << endl; return 0; } The Ohio State University

  19. logWhile3.cpp ... cout << "Enter number: "; cin >> x; while (x <= 0) { cout << "Input must be positive." << endl; cout << "Enter number: "; cin >> x; } cout << "log(" << x << ") = " << log(x) << endl; ... > logWhile3.exe Enter number: -4.5 Input must be positive. Enter number: 0 Input must be positive. Enter number: 4.5 log(4.5) = 1.50408 The Ohio State University

  20. logWhileError.cpp // example of a while loop with a logical error . . . int main() { int k(0); k = 1; while (k <= 8) { cout << "log(" << k << ") = " << log(double(k)) endl; } return 0; } Try running this program. The Ohio State University

  21. probability.cpp ... int main() { double p(0.0); cout << "Enter probability player A wins 1 game: "; cin >> p; while (p < 0.0 || p > 1.0) { cout << "Input must be in range [0:1]." << endl; cout << "Enter probability player A wins 1 game: "; cin >> p; } cout << "Probability player A loses all 5 games = " << pow((1-p), 5.0) << endl; return 0; } The Ohio State University

  22. ... while (p < 0.0 || p > 1.0) { cout << "Input must be in range [0:1]." << endl; cout << "Enter probability player A wins 1 game: "; cin >> p; } ... > probability.exe Enter probability player A wins: 2 Input must be in range [0:1]. Enter probability player A wins: -1 Input must be in range [0:1]. Enter probability player A wins: 0.2 Probability player A loses all 5 games = 0.32768 > The Ohio State University

  23. temperature.cpp // print a table converting fahrenheit to celsius ... int fahrenheit(0), min_fahrenheit(0), max_fahrenheit(0); int STEP_SIZE(10); cout << "Enter min and max fahrenheit: "; cin >> min_fahrenheit >> max_fahrenheit; fahrenheit = min_fahrenheit; // loop until fahrenheit is greater than max_fahrenheit while (fahrenheit <= max_fahrenheit) { // convert fahrenheit to celsius float celsius = (fahrenheit - 32.0) * 5.0/9.0; cout << "farenheit = " << fahrenheit << “ celsius = " << celsius << endl; fahrenheit += STEP_SIZE; // increment by STEP_SIZE } ... The Ohio State University

  24. temperature.cpp ... int STEP_SIZE(10); ... fahrenheit = min_fahrenheit; // loop until fahrenheit is greater than max_fahrenheit while (fahrenheit <= max_fahrenheit) { // convert fahrenheit to celsius float celsius = (fahrenheit - 32.0) * 5.0/9.0; cout << "farenheit = " << fahrenheit << “ celsius = " << celsius << endl; fahrenheit += STEP_SIZE; // increment by STEP_SIZE } ... > temperature.exe Enter min and max fahrenheit: 20 60 farenheit = 20 celsius = -6.66667 farenheit = 30 celsius = -1.11111 farenheit = 40 celsius = 4.44444 farenheit = 50 celsius = 10 farenheit = 60 celsius = 15.5556 The Ohio State University

  25. temperature.cpp ... int STEP_SIZE(10); ... fahrenheit = min_fahrenheit; // loop until fahrenheit is greater than max_fahrenheit while (fahrenheit <= max_fahrenheit) { // convert fahrenheit to celsius float celsius = (fahrenheit - 32.0) * 5.0/9.0; cout << "farenheit = " << fahrenheit << “ celsius = " << celsius << endl; fahrenheit += STEP_SIZE; // increment by STEP_SIZE } ... > temperature.exe Enter min and max fahrenheit: 25 60 farenheit = 25 celsius = -3.88889 farenheit = 35 celsius = 1.66667 farenheit = 45 celsius = 7.22222 farenheit = 55 celsius = 12.7778 The Ohio State University

  26. temperature2.cpp ... int STEP_SIZE(5); ... fahrenheit = min_fahrenheit; while (fahrenheit <= max_fahrenheit) { float celsius = (fahrenheit - 32.0) * 5.0/9.0; cout << "farenheit = " << fahrenheit << “ celsius = " << celsius << endl; fahrenheit += STEP_SIZE; // increment by STEP_SIZE } ... > temperature2.exe Enter min and max fahrenheit: 25 60 farenheit = 25 celsius = -3.88889 farenheit = 30 celsius = -1.11111 farenheit = 35 celsius = 1.66667 farenheit = 40 celsius = 4.44444 farenheit = 45 celsius = 7.22222 farenheit = 50 celsius = 10 farenheit = 55 celsius = 12.7778 farenheit = 60 celsius = 15.5556 The Ohio State University

  27. temperatureError.cpp // print a table converting fahrenheit to celsius ... int fahrenheit(0), min_fahrenheit(0), max_fahrenheit(0); int STEP_SIZE(10); cout << "Enter min and max fahrenheit: "; cin >> min_fahrenheit >> max_fahrenheit; fahrenheit = min_fahrenheit; // loop until fahrenheit does not equal max_fahrenheit while (fahrenheit != max_fahrenheit) // Note != instead of <= { // convert fahrenheit to celsius float celsius = (fahrenheit - 32.0) * 5.0/9.0; cout << "farenheit = " << fahrenheit << “ celsius = " << celsius << endl; fahrenheit += STEP_SIZE; // increment by STEP_SIZE } ... The Ohio State University

  28. sinWhile.cpp (Error) … int main() { double x(0.0); double increment(0.1); cout.setf(ios::fixed); while (x != 1.0) { cout << x << ": " << sin(x) << " " << cos(x) << endl; x += increment; } return 0; } The Ohio State University

  29. for Loops The Ohio State University

  30. logFor.cpp // example of for loop for log(k) for k = 1,2,..,8 #include <iostream> #include <cmath> using namespace std; int main() { for (int k = 1; k <= 8; k++) { cout << "log(" << k << ") = " << log(double(k)) << endl; } return 0; } The Ohio State University

  31. logFor.cpp ... for (int k = 1; k <= 8; k++) { cout << "log(" << k << ") = " << log(double(k)) << endl; } ... > logFor.exe log(1) = 0 log(2) = 0.693147 log(3) = 1.09861 log(4) = 1.38629 log(5) = 1.60944 log(6) = 1.79176 log(7) = 1.94591 log(8) = 2.07944 The Ohio State University

  32. for Loop Syntax for (initialize; condition; alter) { statement1; statement2; statement3; statement4; ... } The Ohio State University

  33. for Loop Syntax (2) • Initializing list • A statement to set the starting value(s) of variables (normally a loop counter) • Expression • The looping condition • Altering list • Statement that is executed at the end of every loop traversal • Normally determines how the counter is manipulated after each pass through the loop • Important note: At the end of a pass through the loop, the statements in the altering list is executed BEFORE the loop expression is evaluated. The Ohio State University

  34. How for Loops Work • First, the initialization statements are executed. • Then the conditional expression is tested. If it is true, then the statement(s) within the loop structure is/are executed. • Once the end of those statements is reached, then altering statements are executed, and the process is repeated. • If the expression ever evaluates to false, then the loop statement is exited, and the program continues beyond the loop. The Ohio State University

  35. Control Flow of a for Loop The Ohio State University

  36. for Loop Example for (int k = 1; k <= 8; k++) { cout << "log(" << k << ") = " << log(double(k)) << endl; } // the while-loop equivalent: int k(0); . . . k = 1; while (k <= 8) { cout << "log(" << k << ") = " << log(double(k)) << endl; k++; } The Ohio State University

  37. for Loop Example2 // Compute n logarithms: for (int k = 1; k <= n; k++) { cout << "log(" << k << ") = " << log(double(k)) << endl; } // the while-loop equivalent: int k(0); . . . k = 1; while (k <= n) { cout << "log(" << k << ") = " << log(double(k)) << endl; k++; } The Ohio State University

  38. (Too) Clever for Loop Example double x(0.0); cout << “Enter Number: “; for (cin >> x; x <= 0; cin >> x) // a while loop is better { cout << "Input must be positive." << endl; cout << "Enter number: "; } // the while-loop equivalent: double x(0.0); cout << “Enter number: “; cin >> x; while (x <= 0) { cout << "Input must be positive." << endl; cout << "Enter number: "; cin >> x; } cout << "log(" << x << ") = " << log(x) << endl; The Ohio State University

  39. for Loops • for loops and while loops are interchangeable. • A for loop is a pre-test loop • Whether to use a while or a for loop is a question of style and readability. • Use for loops to count from a to b; • Use while loops to iterate until some condition is satisfied. The Ohio State University

  40. for Loop Example4 for (int k = 1; k <= 8; k++) { cout << "log(" << k << ") = " << log(double(k)) << endl; } cout << k << endl; // SYNTAX ERROR // Variable k can be declared before the for-loop int k(0); for (k = 1; k <= 8; k++) { cout << "log(" << k << ") = " << log(double(k)) << endl; } cout << k << endl; // What is the value of k? The Ohio State University

  41. Using Nested Loops • A loop inside of another loop • Extremely useful and very common for (int i = 1; i <= 5; i++) { cout << “i is now “ << i << endl; //inner (nested) loop for (int j = 1; j <= 4; j++) { cout << “j is now “ << j << endl; } } //What is the output? The Ohio State University

  42. The Ohio State University

  43. Nested for loops: square.cpp // print a square of x's ... int length(0); cout << "Enter square edge length: "; cin >> length; for (int row = 1; row <= length; row++) { // print length x's for (int col = 1; col <= length; col++) { cout << "x"; } cout << endl; // print newline to finish row } ... The Ohio State University

  44. Nested for loops: square.cpp ... for (int row = 1; row <= length; row++) { // print length x's for (int col = 1; col <= length; col++) { cout << "x"; } cout << endl; // print newline to finish row } ... > square.exe Enter square edge length: 6 xxxxxx xxxxxx xxxxxx xxxxxx xxxxxx xxxxxx The Ohio State University

  45. Nested for loops: diagonal.cpp // print a diagonal of x's ... int length(0); cout << "Enter diagonal length: "; cin >> length; for (int row = 1; row <= length; row++) { // print (row-1) spaces for (int col = 1; col <= row-1; col++) { cout << " "; } cout << "x" << endl; // print x on diagonal } ... The Ohio State University

  46. Nested for loops: diagonal.cpp ... for (int row = 1; row <= length; row++) { // print (row-1) spaces for (int col = 1; col <= row-1; col++) { cout << " "; } cout << "x" << endl; // print x on diagonal } ... > diagonal.exe Enter diagonal length: 6 x x x x x x The Ohio State University

  47. squareError.cpp // version of square.cpp with errors ... int length(0); cout << "Enter square edge length: "; cin >> length; for (int row = 1; row <= length; row++); { // print length x's for (int col = 1; col <= length; col++); { cout << "x"; } cout << endl; // print newline to finish row } ... The Ohio State University

  48. do-while loops • A do-while loop checks the condition at the end of the loop; • Example: char c(‘n’); do { ... //rest of the program cout<< “Do you wish to continue: “; cin>> c; } while (c == ‘y’ || c == ‘Y’); • See text for more details. The Ohio State University

  49. Conclusion on Loops • Loops are generally used for repetition of a section of code. • There are three basic types: • while, for, and do-while • while and for are pretest (entrance controlled) • do-while is posttest (exit controlled) The Ohio State University

  50. Common Programming Errors (1) • Use == when comparing for equivalence in while, for, and do-while statements! (Same as using == in if-statements.) • Precision problem: double x, y; . . . “while (x != y)” may always be true even though mathematically x should equal y. The Ohio State University

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