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Loops are one of the fundamental programming

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concept, and is present in almost all the

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programming languages. They help us in

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executing one or more statements up to a

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desired number of times. It is a sequence of

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instructions or programming statements, which

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may be referred to as code blocks, that is

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continually repeated until a certain

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condition is reached. The condition may be

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based on some predetermined number or times

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of execution, or some expected result is being

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produced, or some expected behavior is being

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achieved or reached. So let's start learning the loops.

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The simplest form is one which has

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no associated condition. To have such a loop,

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we will write loop followed by parentheses.

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This means that, whatever we include inside

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the parentheses, it will be executed infinite times.

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Let me add a print statement to the

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body of the loop, let us execute now.

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You may note that, the print statement is

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executing infinite times, and it keeps on

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printing the print statement. Ctrl + C is

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used to stop an infinite loop,

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so let me stop it.

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Generally speaking, you may not

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frequently need this type of loop inside your code.

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Such kind of loops are rarely

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being used by programmers, even if they intend

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to use, they will typically have some

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statement inside the body of the loops,

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which quits from the body of the

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loop, once a certain condition is being reached.

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Now, let us look at some more useful

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kinds of loops. There are mainly two kinds of

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loops apart from the one that we have just

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seen, they are known as while loops and for

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loops. We will first look at the details of

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the while loop. A while loop is used to

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repeat a specific block of code in unknown

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number of times, until a condition is met.

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For example, if we want to ask the user to guess

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a number between 1 and 20, we don't know

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how many times the user may enter a number

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before he can guess the number correctly.

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He may do it in the first attempt, or in the

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second attempt, or he may take more than two

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attempts, the user will keep on guessing

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until he accurately guessed the number.

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Let us implement this guessing number using the

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while loops. I will start by having a

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variable of my_number, and we'll

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assign it a value of 5.

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Next, I will define a boolean variable and initialize

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it from a value of false.

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I will set this variable to true, whenever the

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user correctly guess the number, and at that

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point I will quit from the loop. I will ask

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the user to guess my number.

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Next, I will write a while loop. A while loop

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is indicated by the keyword of while followed

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by a suitable condition to check for

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terminating the loop. As long as the

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condition remains true, the loop body will

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execute, and as soon as the condition becomes

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false, so the loop body will terminate.

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In this case, I will terminate the loop when the

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guess becomes true.

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At the start of the while loop, this condition

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is true, because guess has a value of false,

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and false is not equal to true. So therefore,

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the condition evaluates to a value of true.

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Inside the loop, I will take the user input

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and we'll convert it to u8 type.

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If the user input is equal to my_number,

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so, I will change the boolean

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variable guess to that of true, and if that

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is not the case, so I will print a message to

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the user to try again.

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Now, what will happen that, the loop will be

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executed many times, each time the condition

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at the start of the loop will be checked.

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The loop will continue to run the code segment, as

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long as the condition remains true. When the

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condition becomes false, the loop will be

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terminated, so let us execute this.

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Ok so I will enter 15, it says try again.

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Let me enter 13, and it will say try again.

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And now I will enter the correct number, which

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is my guess, so I will enter 5. And now it

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says, that the guess is correct. And at this

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time the loop has also terminated, because the

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condition at the start of the while loop

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evaluates to false.

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Let us see some more examples.

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I want the user to enter a number,

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and I will tell the user the next number,

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following his number, which is divisible by both

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2 and 5.  Let us implement a simple program.

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First I will display a useful

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message to the user, so that he knows what the

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program is expecting as an input.

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[No Audio]

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Next I will take the user input.

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I will declare a bool variable, which will be

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used to test the condition of the while loop,

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initially, it will be set to false value.

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I will next add the while loop, along with a

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bool variable for checking when to exit from the loop.

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Inside the body of the loop, I will increment

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the value of the number by one.

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Another way of writing the same statement is,

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to write number += 1 which

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essentially has the same meaning. I will next

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check if the number is, divisible_by_2 _5.

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The mod will be used for checking this

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condition, if the remainder of the number

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after dividing by 2 and 5 are 0, so

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this means that the number is divisible by both 2 and 5.

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Inside the if statement, I will add a suitable message.

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Next, I will also update the boolean variable value.

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This will enable termination from the while

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loop, when next time the condition is

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being checked. Okay we are done with the code,

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so let's execute.

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I will provide the input of, let's say 12, and

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it gives me the next number, which is

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divisible_by_2 _5. Please note

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that, we can further refine this code and get

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rid of the extra variable, that is divisible_by_2 _5

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In fact we may note that,

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the if condition is basically

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controlling the execution of the while loop.

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So instead of checking this with the if

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statement, we can check it at the top, in

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front of the while loop. I will cut this

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condition from here, and we'll paste it in

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front of the while loop.

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I will delete the variable divisible_by_2 _5,

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because we will not need it anymore.

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[No Audio]

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Finally, I will bring the print

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statement outside the body of the while loop,

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because I do not want to execute it many times,

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rather I would like to execute it when

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we have found the desired number.

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[No Audio]

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Now you may note that, our code is more compact

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and short. In summary, when the execution of

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the loop is dependent on some condition

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inside the loop, then it may be sometimes added

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to the condition of the loop itself.

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Let us execute to check, if everything has been

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coded correctly or not.

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I will use the input of let's say 11, and

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it gave me the correct result. There is

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however a small computation error in this code.

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If I run again this code, then it will

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return to me the same number which is not,

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which is not a number, after my number is

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indicated by the description of the program,

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and indicated in the print statement also.

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To import this program to always check for the

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number after my number, I will add 1 to the

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number when while loop starts.

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[No Audio]

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Now it will give me the correct answer.

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Please note that, in the previous code, this

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computation error was not there. Because at

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the start of the loop, before checking the

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condition, we updated the variable number and

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adds 1 to its value. A key point to notice

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that, we should pay special attention to the

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variable values at the start and ending parts

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of the loop. That is when the loop starts and

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when the loop terminates.

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Majority of the times the

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error in our code happens at these

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point, and we have to be very careful about these.

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Okay, one final note before we end this tutorial,

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we have pointed out that the

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while loop is generally being used in

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situations, where we do not know in advance

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the number of times we want to execute a code block.

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This however, does not mean that, we do

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not, that we cannot use it in situations

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where we know the actual number of times we

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want to execute a code block.

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However, for that, we should typically use the for loops,

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which will be covered in the next tutorial.

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Okay, that brings us to the end of this

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tutorial. In the upcoming tutorial, we will be

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looking at a for loop with some interesting examples.

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Do come back for covering that and

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until next tutorial. Happy Rust programming.

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[No Audio]