Visula C# Programming Lecture 4


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Visula C# Programming Lecture 4

  1. 1. Lecture #04: Loops
  2. 2. C# Control Structures: Repetition for structure/foreach structure while structure T F T do/while structure F T F 2
  3. 3. while Statement The while statement has the following syntax: while is a reserved word If the condition is true, the statement is executed. Then the condition is evaluated again. while ( condition ) statement; The statement (or a block of statements) is executed repetitively until the condition becomes false. 3
  4. 4. while Statement (cont’d) true Product <= 1000 Product = 2 * product false int product; product = 2; while (product <= 1000) { product = 2 * product; } // beginning of the next statement 4
  5. 5. while Statement Note that if the condition of a while statement is false initially, the statement is never executed Therefore, the body of a while loop will execute zero or more times 5
  6. 6. Infinite Loops The body of a while loop must eventually make the condition false If not, it is an infinite loop, which will execute until the user interrupts the program This is a common type of logical error You should always double check to ensure that your loops will terminate normally 6
  7. 7. Example 1: Counter Controlled While Loop Control variable • The variable used as a counter to determine whether or not the loop should continue Three components • Initial value of the counter • Check whether or not the counter has reached target – When the looping should continue • Incrementing/decrementing of the counter 7
  8. 8. Example 2: Sentinel Controlled while Loops This is typical of an input-driven program Continues an arbitrary amount of times Sentinel value • Causes loop to break • Avoid collisions – When flag value = user entered value 8
  9. 9. The do Statement The do statement has the following syntax: Uses both the do and while reserved words do { statement; } while ( condition ); The statement is executed once initially, then the condition is evaluated The statement is repetitively executed until the condition becomes false 9
  10. 10. do/while Flowchart action(s) true condition false Fig. 5.13 Flowcharting the do/while repetition structure. 10
  11. 11. Comparing the while and do Loops The while loops vs. the do/while loops Using a while loop • Condition is tested • The action is performed • Loop could be skipped altogether while structure T F do/while structure Using a do/while loop • Action is performed • Then the loop condition is tested • Loop will be run at least once T F Question: write a program to get max from user and then print the numbers from 1 to max 11
  12. 12. The for Statement The for statement has the following syntax: Reserved word The initialization portion is executed once before the loop begins The statement is executed until the condition becomes false for ( initialization ; condition ; increment ) statement; The increment portion is executed at the end of each iteration 12
  13. 13. Flowchart of a for loop initialization condition true action(s) increment false for ( initialization ; condition ; increment ) action(s); 13
  14. 14. The for Statement: Example Establish initial value of control variable. Determine if final value of control variable has been reached. int counter = 1 counter <= 10 false true Console.WriteLine ( counter * 10 ); Body of loop (this may be multiple statements) counter++ Increment the control variable. for (int counter = 1; counter <= 10; counter++) Console.WriteLine (counter * 10); // beginning of the next statement 14
  15. 15. The for Statement A for loop is equivalent to the following while loop: initialization; while ( condition ) { statement; increment; } 15
  16. 16. The for Statement It is well suited for executing a specific number of times that can be determined in advance Increment/Decrement • When incrementing – In most cases < or <= is used • When decrementing – In most cases > or >= is used 16
  17. 17. The flexibility of the for Statement Each expression in the header of a for loop is optional If the initialization is left out, no initialization is performed If the condition is left out, it is always considered to be true, and therefore creates an infinite loop If the increment is left out, no increment operation is performed Both semi-colons are always required in the for loop header for ( ; ; ) { // do something } 17
  18. 18. A Problem to Think About How to print this? xxxxxxxx xxxxxxx xxxxxx xxxxx xxxx xxx xx x What about this? xxxxxxxxxxxxxxx xxxxxxxxxxxxx xxxxxxxxxxx xxxxxxxxx xxxxxxx xxxxx xxx x 18
  19. 19. Statements break and continue Used to alter the flow of control The break statement • Used to exit a loop early The continue statement • Used to skip the rest of the statements in a loop and restart at the first statement in the loop Programs can be completed without their usage; use with caution. 19