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Chapter 9 Repetition Structure (Loop)

Chapter 9 Repetition Structure (Loop). Prepared by: Lec . Ghader Kurdi. Introduction. A Repetition structure represents certain parts of the code that will be executed repeatedly, or not at all based on the current state (condition ). In C++, there are 3 forms of implementing repetition:

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Chapter 9 Repetition Structure (Loop)

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  1. Chapter 9Repetition Structure(Loop) Prepared by: Lec. GhaderKurdi

  2. Introduction • A Repetition structure represents certain parts of the code that will be executed repeatedly, or not at all based on the current state (condition). • In C++, there are 3 forms of implementing repetition: • While Loop While a loop is a group of instructions that the computer executes repeatedly until a terminating condition is satisfied. • Do-while Loop • For Loop

  3. While Loop

  4. While Loop (cont.) • Initialization of the control variable which will be used to test the condition is applied only once. • If the conditionis true, the statement and the increment are executed, then the whole thing is done again. • The statement and the increment are executed repeatedly until the conditionbecomes false. • If the condition starts out false, the while-body will never be executed. • Increment is the statement that makes change on the condition, otherwise, the loop will continue running (Infinite loop).

  5. Example void main() { intcount = 1; while (count <=2) { cout << "Welcome to C++!"; count++; } } Outputs:

  6. Example void main() { int count = 0; while (count < 2) { cout << "Welcome to C++!"; } } Outputs :

  7. Example void main() { int count = 0; while (count < 2) { cout << "Welcome to C++!"; count++; } } Outputs:

  8. Example void main() { intcount = 2; while (count < 2) { cout << "Welcome to C++!"; } } Outputs:

  9. Do-While Loop do { …any number of statements… // action } while (condition) ;

  10. Do-While Loop (cont.) • The do-while statement performs the test after executing the body. • As long as the test is true, the body of the program will be executed again. • In all cases, the body will be executed at least once, even if the condition was wrong because testing happened at the end.

  11. Example void main() { int count = 1; do { cout << "Welcome to C++!"; count++; } while (count <= 2); } Outputs:

  12. Example void main() { int count = 1; do { cout << "Welcome to C++!"; count++; } while (count > 2); } Outputs:

  13. Example #include <iostream.h> void main () { do { cout<< "\nYou can't stop me!"; }while (1); // infinite do-while } Outputs:

  14. Example #include <iostream.h> void main () { do { cout<< "You can't stop me!\n"; }while (0); // infinite do-while } Outputs:

  15. Do-While Loop (cont.) The difference between while and do-while • The testing happens before executing the body in the while structure. • So if the condition is false from the beginning, the body will not be executed at all.

  16. For Loop for ( initialize; condition; increment ) { statements ; }

  17. For Loop • For structure is composed of 4 parts: Initialize: initializes the loop control variable, and done only once. Condition: that is the condition that determines whether the loop should continue executing or not. Statements: the body of the loop. The increment: is a statement that changes the value of the initialization either by adding, subtracting, or any math operation. But usually it is either adding or subtracting. • It should be noticed that there is no semicolon after the increment. • This "for structure" structure is better to be used when the number of iterations is known.

  18. Example void main() { inti; for (i = 0; i < 2; i++) { cout << "Welcome to C++!"; } } Outputs:

  19. Example #include <iostream> intmain () { for (int n=10; n>0; n--) { cout<< n << ", "; } cout<< «Done!\n"; } Outputs:

  20. Example #include <iostream> intmain () { intn = 10; while (n>0) { cout<< n << ", "; --n; } cout<< "Done!\n"; } Outputs:

  21. Exercises • Request the user to type positive numbers until either a zero or a negative is typed, and then show the user how many positives were typed in. • Write a program that prints the cubes of the numbers from 1 to 10. Here’s the output from the program:   1     1           2     8   3     27   4     64   5     125   6     216   7     343   8     512   9     729   10   1000

  22. 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> intmain() { intcount = 1; while (count <= 5) { cout<< count << "\n"; count++; } return 0; } #include <iostream> intmain() { intcount ; for (count = 0; count < 5; count++) { cout<< count << "\n"; } return 0; }

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