 # R Program to check prime number

February 4, 2023, Learn eTutorial
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## What are prime numbers and composite numbers?

The Prime number is an integer with no other positive divisors other than 1 and that number itself. All other numbers except the prime numbers are Composite numbers. For example, 2, 3, 5, 7, 11, 13, etc are prime numbers because they have only 2 divisors, and the number '4', '6', which have more than two divisors are composite numbers. Note:  The number '1' is not a prime number as it has only one divisor.

## How to check whether the given number is a prime number or not?

Given below are the steps for checking a number is a prime.

1. Accept a number from the user.
2. Find all factors of that number. The integers that could divide a number completely will be a factor of that number.
3. If the number of factors is more than 2, then the input number is not prime otherwise it is a prime number.

## How to write an R program for checking the number is prime?

In this R program, we accept the user's value into num by providing an appropriate message to the user using '`prompt`'. We can use the `readline()` function to take input from the user (terminal). Here the `prompt` argument can choose to display an appropriate message for the user. Check the given number is greater than 1 because prime numbers must be greater than one. Check if num is exactly divisible by any number from 2 to num – 1 and if find a factor in that range then set isPrime = 0 otherwise isPrime = 1. Print the message as the number is a prime number if the isPrime == 1 otherwise the number is not prime.

## ALGORITHM

STEP 1: Accept user input using `readline()` into variable num

STEP 2: Set isPrime = 0 at first

STEP 3: Check if the num is greater than 1, if yes do steps 4,5

STEP 4: Set isPrime = 1

STEP 5: Start a `for loop` from 2 to num – 1 and check if num is exactly divisible by any number,

STEP 5.1: If we find a factor in that range then reset isPrime = 0 and `break` from loop

STEP 6: If num is 2 then set isPrime =1

STEP 7: Check if isPrime == 1 then `print` the number is a prime number, else `print` the number is not a prime number

## R Source Code

```                                          ```num     = as.integer(readline(prompt = "Enter a number: "))
# num  = 15
isPrime = 0
if (num > 1) {
isPrime = 1
for (i in 2: (num - 1)) {
if ((num %% i) == 0) {
isPrime = 0
break
}
}
}
if (num == 2) isPrime = 1
if (isPrime == 1) {
print(paste(num, "is a prime number"))
} else {
print(paste(num, "is not a prime number"))
}```
```

## OUTPUT

```Enter a number: 15
 "15 is not a prime number"

Enter a number: 13
 "13 is a prime number"```