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Copy file name to clipboardexpand all lines: Content/Chapter-7-1-complex-loops/do-while-loop/do-while-loop.md
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### Testing in the Judge System
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Test your solution here: [https://judge.softuni.org/Contests/Practice/Index/514#7](https://judge.softuni.org/Contests/Practice/Index/514#7).
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## Example: Summing Up Digits
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Sum up the digits of a **positive** integer **n**. For example, if **n = 5634**, the result will be: 5 + 6 + 3 + 4 = **18**.
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### Video: Sum of Digits
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Watch this video lesson to learn how to sum the digits of given integer: https://youtu.be/sbzlzdoEbFc.
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### Hints and Guidelines
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We can use the following idea to solve the problem:
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* We create the variable **`n`**, to which we assign a value equal to the number entered by the user.
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* We create a second variable – **`sum`**, which initial value is 0. We will use it for the calculation and storage of the result.
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* For a loop condition, we will use **`n > 0`** because after each calculation of the result in the body of the loop, we will remove the last digit of **`n`**.
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* In the body of the loop:
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* We assign a new value of **`sum`** that is the result of the sum of the current value of **`sum`** with the last digit of **`n`**.
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* We assign a new value to **`n`**, which is the result of removing the last digit of **`n`**.
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* Outside the body of the loop, we print the final value of the sum.
Let's practice the `do-while` loop with the following exercise:
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Sum up the digits of a **positive** integer **n**. Examples:
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- If **n = 5634**, the result will be: 5 + 6 + 3 + 4 = **18**.
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- If **n = 920**, the result will be: 9 + 2 + 0 = **11**.
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## Video: Sum of Digits
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Watch this video lesson to learn how to sum the digits of given integer: https://youtu.be/sbzlzdoEbFc.
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## Hints and Guidelines
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We can use the following **idea to solve the problem**: extract many times the last digit from the input number and sum the extracted digits until the input number reaches 0. Example:
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- sum = 0
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- n = 563**4** 🡒 extract 4; sum += 4; n = 563
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- n = 56**3** 🡒 extract 3; sum += 3; n = 56
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- n = 5**6** 🡒 extract 6; sum += 6; n = 5
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- n = **5** 🡒 extract 5; sum += 5; n = 0 🡒 end
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In more detail the above idea looks like this:
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* We create the variable **`n`**, to which we assign a value equal to the number entered by the user.
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* We create a second variable – **`sum`**, which initial value is 0. We will use it for the calculation and storage of the result.
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* As a loop condition, we will use **`n > 0`** because after each calculation of the result in the body of the loop, we will remove the last digit of **`n`**.
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* In the body of the loop:
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* We assign a new value of **`sum`** that is the result of the sum of the current value of **`sum`** with the last digit of **`n`**.
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* We assign a new value to **`n`**, which is the result of removing the last digit of **`n`**.
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* Outside the body of the loop, we print the final value of the sum.
Copy file name to clipboardexpand all lines: Content/Chapter-7-1-complex-loops/greatest-common-divisor/greatest-common-divisor.md
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## Video: Greatest Common Divisor \(GCD\)
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Watch the video lesson to learn about the Euclidean algorithm for calculating the GCD of given two integers: https://youtu.be/1-SEOWupvrA.
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Watch the video lesson to learn about the Euclidean algorithm for calculating the GCD of given two integers: [https://youtu.be/1-SEOWupvrA](https://youtu.be/1-SEOWupvrA).
We already know that the infinite loop performs a certain code infinitely, but what if we want at some point under a given condition to go out of the loop? The **`break`** operator comes in handy in this situation.
<td>The <b><code>break</code></b> operator stops the execution of a loop at the time it is called and continues from the first line after the end of the loop. This means that the current iteration of the loop will not be completed, accordingly, the rest of the code in the body of the loop will not be executed.</td>
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</tr></table>
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## Prime Numbers – Example
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The next problem we are going to solve is to **check for a prime number**. Before proceeding to it, let's recall what prime numbers are.
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**Definition**: An integer is **prime** if it is divisible only by itself and by 1. By definition, the prime numbers are positive and greater than 1. The smallest prime number is 2.
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We can assume that an integer **n** is a prime number if **n > 1** and **n** is not divisible by a number between **2** and **n-1**.
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The first few prime numbers are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, …
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Unlike these, **composite numbers** are such numbers, the composition of which is the result of the multiplication of prime numbers.
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Here are some examples of composite numbers:
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***10** = 2 * 5
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***42** = 2 * 3 * 7
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***143** = 13 * 11
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**Algorithm to check** if an integer is **prime**: check if **n > 1** and **n** is divisible by **2**, **3**, …, **n-1** without reminder.
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* If it is divisible by any of the numbers, it is **composite**.
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* If it is not divisible by any of the numbers, then it is **prime**.
<td>We can optimize the algorithm instead of checking it to <code><strong>n-1</strong></code>, to check divisors to <code><strong>√n</strong></code>. Think what is the reason for that.</td>
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## Video: Prime Numbers
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Watch this video lesson to learn how to find the prime numbers in the range [1...n]: https://youtu.be/4lWOaPWKf0I.
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## Example: Check for Prime Number. Break Operator
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Check if a number **n** is prime. We will do this by checking if **n** is divisible by the numbers between 2 and √n.
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Here is an algorithm for checking a prime number, step by step:
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* We create the variable **`n`** to which we assign an integer taken from the console input.
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* We create an **`isPrime`** bool variable with an initial value **`true`**. We assume that a number is prime until proven otherwise.
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* We create a **`for`** loop in which we set an initial value 2 for the loop variable, for condition **the current value `<= √n`**. The loop step is 1.
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* In **the body of the loop**, we check if **`n`**, divided by **the current value**, has a remainder. If there is **no reminder** from the division, then we change **`isPrime`** to **`false`** and we exit the loop through the **`break`** operator.
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* Depending on the value of **`isPrime`**, we print whether the number is prime (**`true`**) or composite (**`false`**).
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### Implementation of the Prime Checking Algorithm
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Here is a sample implementation of the algorithm described:
What remains is to add a **condition that checks if the input number is greater than 1**, because by definition numbers such as 0, 1, -1 and -2 are not prime.
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### Testing in the Judge System
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Test your solution here: [https://judge.softuni.org/Contests/Practice/Index/514#9](https://judge.softuni.org/Contests/Practice/Index/514#9).
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## Example: Infinite Loop with Break
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Write a program that checks if a particular number **n** is even, and if it is – to print it on the screen. An even number is one that can be divided by 2 without remainder. If an invalid number is entered, the system should ask the user to enter it again and display notification that the input number is not even.
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Here is an idea how we can solve the problem:
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* We create a variable **`n`** to which we assign an initial value of **0**.
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* We create an infinite **`while`** loop and as condition we will set **`true`**.
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* In **the body of the loop**:
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* We take an integer value from the console input and assign it to **`n`**.
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* If **the number is even**, we exit the loop by **`break`**.
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***Otherwise**, we display a message stating that **the number is not even**. The iterations continue until an even number is entered.
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* Finally, after the loop, print the even number on the screen.
Note: Although the code above is correct, it will not work if the user enters **text** instead of numbers, such as "**Invalid number**". Then parsing the text to a number will break and the program will display **an error message (exception)**. How to deal with this problem and how to capture and process exceptions using **the `try-catch` construction** will be learned later.
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### Testing in the Judge System
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Test your solution here: [https://judge.softuni.org/Contests/Practice/Index/514#10](https://judge.softuni.org/Contests/Practice/Index/514#10).
The next example will be to write a program that **enters an even number** from the console. If an odd number is entered, the program should enter a number again, until an even number is entered.
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We shall use an **infinite loop with `break`** to solve this problem, because we don't know how many times the loop body will be repeated.
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We shall check if a particular number **n** is even, and if it is, we will print it on the screen. An even number is one that can be divided by 2 without remainder. If an invalid number is entered, we will ask the user to enter a number again and will display a notification that the input number is not even.
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## Hints and Guidelines
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Here is an idea how we can implement the above described logic:
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* We create a variable **`n`** to which we assign an initial value of **0**.
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* We create an infinite **`while`** loop and as condition we will set **`true`**.
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* In **the body of the loop**:
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* We take an integer value from the console input and assign it to **`n`**.
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* If **the number is even**, we exit the loop by **`break`**.
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***Otherwise**, we display a message stating that **the number is not even**. The iterations continue until an even number is entered.
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* Finally, after the loop, print the even number on the screen.
Note: Although the code above is correct, it will not work if the user enters **text** instead of numbers, such as "**Invalid number**". Then parsing the text to a number will break and the program will display **an error message (exception)**. How to deal with this problem and how to capture and process exceptions using **the `try-catch` construction** will be learned later.
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## Testing in the Judge System
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Test your solution here: [https://judge.softuni.org/Contests/Practice/Index/514#10](https://judge.softuni.org/Contests/Practice/Index/514#10).
The next problem we are going to solve is to **check whether given number is prime**. An integer is prime if it cannot be decomposed to a product of other numbers. For example 2, 5 and 19 are primes, while 9, 12 and 35 are composite.
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## Video: Prime Number Checking
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Watch this video lesson to learn how to check if a number is prime: https://youtu.be/4lWOaPWKf0I.
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#Hints and Guidelines
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Before proceeding to the hints about solving the "prime checking" problem, let's recall in bigger detail what prime numbers are.
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**Definition**: an integer is **prime** if it is **divisible only by itself and by 1**. By definition, the prime numbers are positive and greater than 1. The smallest prime number is 2.
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We can assume that an integer **n** is a prime number if **n > 1** and **n** is not divisible by a number between **2** and **n-1**.
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The first few prime numbers are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, …
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Unlike these, **composite numbers** are such numbers, the composition of which is the result of the multiplication of prime numbers.
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Here are some examples of composite numbers:
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***10** = 2 * 5
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***42** = 2 * 3 * 7
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***143** = 13 * 11
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**Algorithm to check** if an integer is **prime**: check if **n > 1** and **n** is divisible by **2**, **3**, …, **n-1** without reminder.
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* If it is divisible by any of the numbers, it is **composite**.
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* If it is not divisible by any of the numbers, then it is **prime**.
<td>We can optimize the algorithm instead of checking it to <code><strong>n-1</strong></code>, to check divisors to <code><strong>√n</strong></code>. Think what is the reason for that.</td>
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</tr></table>
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## Example: Check for Prime Number. Break Operator
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Check if a number **n** is prime. We will do this by checking if **n** is divisible by the numbers between 2 and √n.
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Here is an algorithm for checking a prime number, step by step:
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* We create the variable **`n`** to which we assign an integer taken from the console input.
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* We create an **`isPrime`** bool variable with an initial value **`true`**. We assume that a number is prime until proven otherwise.
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* We create a **`for`** loop in which we set an initial value 2 for the loop variable, for condition **the current value `<= √n`**. The loop step is 1.
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* In **the body of the loop**, we check if **`n`**, divided by **the current value**, has a remainder. If there is **no reminder** from the division, then we change **`isPrime`** to **`false`** and we exit the loop through the **`break`** operator.
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* Depending on the value of **`isPrime`**, we print whether the number is prime (**`true`**) or composite (**`false`**).
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### Implementation of the Prime Checking Algorithm
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Here is a sample implementation of the algorithm described:
What remains is to add a **condition that checks if the input number is greater than 1**, because by definition numbers such as 0, 1, -1 and -2 are not prime.
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### Testing in the Judge System
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Test your solution here: [https://judge.softuni.org/Contests/Practice/Index/514#9](https://judge.softuni.org/Contests/Practice/Index/514#9).
Copy file name to clipboardexpand all lines: Content/Chapter-7-1-complex-loops/while-loop/while-loop.md
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Test your solution here: [https://judge.softuni.org/Contests/Practice/Index/514\#4](https://judge.softuni.org/Contests/Practice/Index/514#4).
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## Example: Number in Range \[1 … 100\]
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Enter an integer in the range \[**1 … 100**\]. If the entered number is invalid, enter it again. In this case, an invalid number will be any number that **is not** within the specified range.
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To solve the problem, we can use the following algorithm:
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* We create a `num` variable to which we assign the integer value obtained from the console input.
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* For a loop condition, we put an expression that is `true` if the number of the input **is not** in the range specified in the problem's description.
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* In **the body of the loop**: we print a message "**Invalid number!**" on the console, then we assign a new value to `num` from the console input.
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* Once we have validated the entered number, we print the value of the number outside the body of the loop.
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Here's a sample implementation of the algorithm using a `while`** loop**:
Enter an integer in the range \[**1 … 100**\]. If the entered number is **invalid**, enter it **again**. In this case, an invalid number will be any number that **is not** within the specified range.
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## Video: Numbers in the Range \[1..100\]
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Watch this video lesson to learn how to enter a number in the range \[1...100\]: [https://youtu.be/8W-CIbF4cdA](https://youtu.be/8W-CIbF4cdA).
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## Hints and Guidelines
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To solve the problem, we can use the following algorithm:
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* We create a `num` variable to which we assign the integer value obtained from the console input.
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* For a loop condition, we put an expression that is `true` if the number of the input **is not** in the range specified in the problem's description.
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* In **the body of the loop**: we print a message "**Invalid number!**" on the console, then we assign a new value to `num` from the console input.
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* Once we have validated the entered number, we print the value of the number outside the body of the loop.
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Here's a sample implementation of the algorithm using a `while`** loop**:
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