مقدمة في الحلقات التكرارية

• This lecture covers how to repeat tasks a specific number of times or as long as a condition is met using loops.

Learning Objectives

• Differentiate between loops that iterate a specific number of times and those that depend on a condition.
• Implement a for loop to repeat an instruction or set of instructions a fixed number of times.
• Explain the components of a for loop: counter, continuation expression, and value change expression.
• Use the for loop with an incrementing or decrementing counter.
• Differentiate between defining a counter inside or outside a for loop.
• Terminate a loop before reaching its natural end.
• Explain how nested for loops work.
• Implement Do While loops.
• Implement While loops.
• Explain the difference between Do While and While loops.
• Design an application using the Console Application type.
• Use input and output statements in the Console Application environment.

Introduction to Loops

• One of the main advantages of computers is their ability to perform tasks quickly and accurately, no matter how many tasks there are.
• It can also repeat execution of a specific task any number of times with the same accuracy.
• For example, to design an application to input the grades of a student in 10 subjects, and the program calculates the sum and grade for the student, there is no problem so far, but suppose you have 1000 students, will you repeat writing the previous program several times according to the number of students? Of course not.
• Programming languages, including C#.NET, provide what are called loops, where a specific task is executed repeatedly instead of writing the steps of this task several times.
• C#.NET uses two types of loops

Types of Loops

• Counter loop for executing the required commands a specific number of times.
• Conditional loop to execute commands based on the continued existence of a specific condition.

For Loop

• Used to execute a command or more a specific number of times. The idea of ​​the loop depends on the presence of a variable that acts as a counter, so it is called a counter. We specify a certain value ​​for this variable to start with.

For Loop Syntax

• The general form of the For statement is as follows: for (int Counter = StartVal; Continue expression; Update_Expression) The command or set of commands to be repeated
• Counter: The integer variable that represents the loop counter, and can be named anything else.
• StartVal: The value at which the counter starts, and can be the value itself or a variable containing this value.
• Here's how to make the counter start at value 1: for (counter=1; ....
• Continue Expression: A comparison expression that compares the value of the counter to a specific value, and is tested with each iteration of the loop, and the loop continues to iterate as long as the result of this expression is True.
• Examples: for (counter=1; counter <=10;.... for (counter=1; counter != 10;....
• Update Expression: An expression that changes the value of the counter with each iteration of the loop, such as incrementing or decrementing it by a certain value.
• Examples: for (counter=1; counter <=10; ++counter) for (counter=1; counter != 20; counter +=5)
• What happens when execution reaches the loop?
  • The counter starts at the specified value, and the continuation expression is tested, If the result is True, it enters the loop and executes the commands enclosed between { } , and if it is False, it does not enter the loop and continues executing the program after it.
  • If the loop is entered for the first time, and after completing the execution of the required commands, the value change expression is executed.
  • Then the continuation expression is tested, If the result is True, the commands are executed a second time, and the value change expression is executed, and so the loop continues to execute as long as the comparison expression is True until the expression produces False, the loop ends and exits from it, and complete executing the program after { which is the ending brace.
• Exercise: Design a program that displays the message box ten times, and each time the number for the numbers 1 to 10 appears in the box.
• Start a new project, then put a command button (Button) on the form window, then open the programming window, and in the event of the command button, write the following code: for (int x = 1; x<=10;++x) { MessageBox.Show(x.ToString()); }
• Executing the previous loop as follows:
  • When the loop is reached, the initial value (1) is stored in the variable (x), which works here as a counter.
  • The value of the variable (x) is compared with the value (10) where if the value of the counter (x) is greater than the value (10), the loop is not entered and the program execution moves to the first sentence after it.
  • If the value of the counter (x) is less than or equal to the value (10), the commands required to be repeated between { } are executed, and since the value of the variable (x) in the first cycle is (1), the command to display the message is executed and the value (1) appears in it.
• Then the continuation expression is tested, so if the result is True, the commands are executed for the third time, and the value change expression is executed, and so the loop continues to execute as long as the result of the comparison is True until the expression produces False, then the loop is terminated and exited from it, and the program execution is completed after the { which is the closing brace.
• The value of the variable (x) is increased by (۱).
• The program returns to the beginning of the loop, and the value of the variable (x) is compared with the value (۱۰) again, so it will find that it is (۲), so the loop is executed again.
• Thus, the repetition continues and the counter has the following values ​​in the following times (٣, ٤, ٥, ٦ .........) as long as the variable has not reached the value (۱۰) and the loop commands are executed, and when the value of the variable reaches the value (10), the result of the continue expression will be Flase and the repetition will stop.
• Run the program by pressing the F5 key and then click on the button on the window to see the output.

Declaring a Counter Inside or Outside the Loop

• Look at the following two examples of a loop, the example on the left has the counter defined inside the loop, while the example on the right has the variable defined outside the loop, but it was used as a counter normally. What's the difference? for (int x = 1; x <= 10; ++x) int x; for (x = 1; x <= 10; ++x)
• When the variable is defined within the loop, it becomes like a local variable that is only used for the loop.
• You cannot use it outside the curly braces { }.
• If the variable is defined before the loop as in the example on the right, then this variable can be used as a counter for the loop, and can be used after exiting the loop as a normal variable.
• When using it after exiting the loop, it retains its last value ​​in the loop until it is changed by a statement as usual.
• Exercise: Suppose that in the previous exercise we want to display even numbers from 1 to 10 in the message squares.
• Here, the counter will start with the first even number, which is (۲), and its value will increase by (۲) in each loop cycle, and we make the value change expression increase the counter by (۲) as follows: for (int x = 2; x<= 10; x+=2) { MessageBox.Show(x.ToString()); }
• In this example, the command will only be repeated five times, because the counter starts at the value 2, then becomes 4, then 6, then 8, then 10.

Decrementing For Loop

• In the previous examples, the loop counter started at a small value and increased, but the nature of the application may require starting with a large value, gradually decreasing to a smaller value, and here we will make the expression to change the value of the counter decrease the value of the counter.
• Exercise: Required to display messages with the values ​​of the odd numbers from ten to one.
• Here the counter will start with the first odd value ​​in the required range, which is (۹), and the value change expression is used to decrease the counter by (۲) with each cycle of the loop. for (int x = 9; x>= 1; x-=2) { MessageBox.Show(x.ToString()); }
• Exercise: Required to write a program to calculate the sum of even numbers from (1) to (10).
• Create a new project, and put a Label, (Button).
• Modify the properties of the tools to make the shape of the window and the tools as shown in the following figure.
• Double-click the Calculate button to open the Click event procedure, and write the following code: int sum =0; for (int i=2; i<= 10; i+=2) { sum += i; } label1.Text = sum.ToString();
• In the previous code, note the following:
  • An integer variable named sum was declared, which will be used to accumulate each number.
  • The variable sum, we put an initial value (0) in it to ensure that it is completely empty, and always remember that any variable that will work as a storage for collection must have the value (0) in it initially.
  • The loop starts with the first even value ​​for the counter, which is (۲), and the loop increases by (۲), and the loop stops when the value of the counter reaches (۱٠).
  • The sum = i expression is used to add the value of the variable (i) to the variable (sum), then store the result in the variable (sum), because as we mentioned it is a collection field and each new value ​​is added to its contents, this is the opposite of the expression sum = i. Here the variable (Sum) will not retain the value in it, but it will be replaced with the value of the variable (i).
• Show in the following figure the change in the value of the variable sum with each cycle of the loop
  • variable i = 2, sum + i = 0 + 2 then sum = 2
  • variable i = 4, sum + i = 2 + 4 then sum = 6
  • variable i = 6, sum + i = 6 + 6 then sum = 12
  • variable i = 8, sum + i = 12 + 8 then sum = 20
  • variable i = 10, sum + i = 20 + 10 then sum = 30

Observations

• The value of the variable i at the beginning of the loop is 2, and the value of the variable sum at the beginning is (۰), so with the first cycle the value of the expression sum + i is (۲).
• In the second cycle i becomes 4, and the arithmetic expression is executed by adding the previous value (sum) which is (۲) to the value of the counter (i) which is (٤) to become the current value ​​of sum is (٦). Thus, the execution of the loop is repeated until it reaches the value (sum) to (۳۰) and the resulting of summing the value of i up to ((۱۰) and the last value ​​of the variable sum is (۲۰).
• The ToString function was used to convert the value that located in sum from numeric value to textual value and can be displayed through the Labell control.
• Implement the program by pressing the F5 to see the final result.
• The loop remains in rotation and executes what is inside it from the beginning until the result of a comparison expression is false and it is exited from it. Sometimes the application requires ending the loop before the result of the comparison expression is false as a result for example of something happened or a certain condition becomes valid.
• You can implement that by using the statement (break), often used with the conditional statement if to examine if a certain event occurred or a condition is validated and the break statement is processed at that time.
• Code example for(int x = 1; x<= 10; ++x) { MessageBox.Show(x.ToString()); if (x == 7) break; } MessageBox.Show ("welcome");
• In this example, the message square will display several times with the value of x, and it should display 10 times, but when the counter "x" reach the value (7) it will exit from the loop as a result of the existence of if statement that validate the condition and then break statement is processed, after exiting from the loop,the first statement will be performed right behind the "{", so the "welcome" message will show.
• We still can use variables with float data to construct for loops.
• You can use a counter of type (float) and use an expression to change the value of a decimal value, as shown in the following example: for (float x = 1f; x <= 2f; x+=.1f)
• In this loop, the counter starts at the value (1), and with each cycle the counter increases by (۰.۱), note the placement of the letter f after the number because, as we mentioned, decimal numbers must have this letter placed after them when using them.

Nested For Loops

• By that is meant creating a for loop inside another for loop for (int i = 1; i <= 10; .........) { for (int j = 1; j<= 5; .........) { //Set of internal loop commands } //Set of external loop commands }
• Following points can be made from the above code structure:
  • External loop with counter (i) will cycle its set of commands (10) times, with respect to the internal loop with counter (j) will cycle its set of commands (5) times.
  • Internal loop is exist inside external loop commands, so internal loop will be cycle (10) times.
  • So all commands found in Group (1) will be performed 50 times (i.e inner loop commands(5) * outer loop(10) = 50).
  • All commands found in Group (2) will be performed 10 times only because found in outer or external scope.

Example

• Put Button on window
• Write codes with same event of Button "click"

Illustration

for (int i=1; i <= 3; i++) { for (int j = 1; j<= 5; j++) { MessageBox.Show ("welcome"); } }

• Following the implementation of mentioned example, 15 messages of "welcome" shows because inner loop will expose 5 messages and outer loop will tell inner loop to repeat itself 3 times, thus, it result : (3*5 = 15).
• Exercise: implement "for" loop inside "for" loop that outer loop "i" starts 1 to 3 and inner loop starts 1 to 5, with message that output values of counters with it so the user can see.
• New project and Button, set properties and name variables
• Double click on button with code and click event
• Write the following codes in click scope for (int i = 1; i<= 3 ; i++) { for (int k = 1; k<= 5 ;k++) { MessageBox.Show("I = " + i + "K= " + k); } }

Do While

• Relays on the existence of a condition to stop or start the loop. stops at wrong input condition
• General form of Loop do { Set of command to repeat } While (Condition)
• If condition equals TRUE it will relay command loop . so, once it reaches FALSE it will terminates operation.
• Condition is compare form/term so if results are true continue cycle command otherwise stop with operations/process .

Example

• Showing Message values with values from 1 though 10. int i=1; do MessageBox.Show(i.ToString()); i=i+1; }while (i <= 10);
• Variable starts " i " with one and it's displayed with values.
• i is incremental command.
• Value examines either it is equesl or not equal 10, or its keep cycle unit reach or stop condition.
• Notice, command is always display first cycle but does not validate input until enter into commands.
• The loop will give you an output even the first entry is not validated, because it is entered before examined