Hey I've been trying to figure out the error in this code Im supposed to ask the user for a positive integer then pint out the first emirps 5 on each line... I'm just flat out stuck at this point ..thanks
#include <iostream>
using namespace std;
int isPrime(int value); //Prototyle for "prime number function"
int reverse (int value2); //Prototype for "emirp function"
int main()
{
//Ask the user for a positive number
cout << "Please enter a positive number: ";
int n;
cin >> n;
//Reject negative value input
if ( n < 1)
{
cout << "INVALID NUMBER \n";
}
else
//Calculate all emirps up to 'n'.
for (int test = 0; test < n; test++)
{
int number = 2;
if (isPrime(number))
{
cout << "\n" << reverse(number) << "\t\t\t";
}
}
return 0;
}
int isPrime(int value)
{
//If value is prime, the remainder (count) will be zero twice--for 1 and itself.
int divisor = 1;
int count = 0;
int prime = 0;
if (value % divisor == 0)
{
count++;
++divisor;
}
if ((count = 2))
{
int prime = value; //store prime value in new variable
}
return prime;
}
int reverse(int value2)
{
//reverse the number
value2*=10;
value2 = value2 %10;
value2/=10;
//same procedure as prime function
int divisor2 = 1;
int count2 = 0;
int emirp = 0;
if (value2 % divisor2 == 0)
{//if
count2++;
++divisor2;
}
if ((count2 = 2))
{
int emirp = value2;
}
return emirp;
system ("pause");
Please take some time to describe your problem properly. My psychic powers tell me that the user enters a number and the program then will print all prime number up to this number with the digits reversed. (Some punster chose to call a prime number with reversed digits an Emirp.)
Hey I've been trying to figure out the error in this code ...
Hey, there's not one single error here. The code is strewn with errors!
#include <iostream>
using namespace std;
int isPrime(int value);
int reverse (int value2);
int main()
{
cout << "Please enter a positive number: ";
int n;
cin >> n;
if ( n < 1)
{
cout << "INVALID NUMBER \n";
}
else
//Calculate all emirps up to 'n'.
for (int test = 0; test < n; test++) ## No need to test 0 or 1 for primality
{
int number = 2;
if (isPrime(number)) ## You always test whether 2 is a prime here
{
cout << "\n" << reverse(number) << "\t\t\t";
}
}
return 0;
}
int isPrime(int value)
{
//If value is prime, the remainder (count) will be zero twice--for 1 and itself.
int divisor = 1;
int count = 0;
int prime = 0; ## (A)
if (value % divisor == 0)
{
count++; ## This statelment will be executed at most once
## You should count divisors in a loop
++divisor; ## Here, divisor is incremented, but never used again
## Also, if this were inside a loop, you should increment
## the divisor unconsitionally. The condition affects
## only the count.
}
if ((count = 2)) ## This will set count to 2 and always evaluate to true
{
int prime = value; ## This variable will shadow the "prime" variable
## decralered at (A). This variable will cease to exist
## as soon as the block closes, i.e. immedietely.
## You just want "prime = 1", without the "int", which
## declares a new variable.
}
return prime; ## This prime is tze one declared at (A) and will be 0.
}
int reverse(int value2)
{
value2*=10;
value2 = value2 %10; ## The remainder of a division by 10 of a number that
## you have just multiplied by 10 is 0, rendering pretty
## much the rest of the function useless ...
value2/=10;
int divisor2 = 1; ## ... which doesn't hurt, because the rest of the
## function was useless to start with. Reversing the
## digits of a number isn't at all like finding a prime.
int count2 = 0;
int emirp = 0;
if (value2 % divisor2 == 0)
{
count2++;
++divisor2;
}
if ((count2 = 2))
{
int emirp = value2;
}
return emirp;
system ("pause"); ## First, this statement comes directly after a terurn,
## so it will never be reached. Second, if anywhere, it
## should go into the main routine. You don't want the
## user to press a key before printing each number.
}
Please, learn:
how to step through a prigram with a debugger to learn how te variables change and what a program actually does;
how loops and scope blocks (in curly braces) work;
when to declare new variables and whan to use existing variables; (You'll want the latter more often than you think);
to organise your program better, it will help you to spot logical errors.
As to your problems at hand: There are plenty of resources for testing for prime numbers and reversing the digits of a number on SO, which shouldn't be hard to find.
Related
I'm trying to write a c++ program which gets an integer n (n>=1 && n<=100000) from the user and puts the sum of its digits into b. The output needed is the b-th prime number coming after n. I'm an absolute beginner in programming so I don't know what's wrong with the for loop or any other code that it doesn't show the correct output. For example the 3rd prime number after 12 (1+2=3) is 19 but the loop counts the prime numbers from 2 instead of 12, so it prints 7 as result.
#include <iostream>
using namespace std;
bool isPrime(int n)
{
if(n <= 1)
return false;
for(int i = 2; i <= (n/2); i++)
if(n % i == 0)
return false;
return true;
}
int main()
{
long int n;
int b = 0;
cin>>n;
while(n >= 1 && n <= 100000){
b += n % 10;
n /= 10;
}
for(int i = n, counter = b; counter <= 10; i++)
if(isPrime(i)){
counter++;
if(i > n)
cout<<counter<<"th prime number after n is : "<<i<<endl;
}
return 0;
}
So one of the possible solutions to my question, according to #Bob__ answer (and converting it to the code style I've used in the initial code) is as follows:
#include <iostream>
using namespace std;
bool isPrime(long int number)
{
if(number <= 1)
return false;
for(int i = 2; i <= (number / 2); i++)
if(number % i == 0)
return false;
return true;
}
int sumOfDigits(long int number)
{
int sum = 0;
while(number >= 1 && number <= 100000)
{
sum += number % 10;
number /= 10;
}
return sum;
}
long int bthPrimeAfter(int counter, long int number)
{
while(counter)
{
++number;
if(isPrime(number))
--counter;
}
return number;
}
int main()
{
long int number;
cin>>number;
int const counter = sumOfDigits(number);
cout<<bthPrimeAfter(counter, number)<<"\n";
return 0;
}
As dratenik said in their comment:
You have destroyed the value in n to produce b in the while loop. When the for loop comes around, n keeps being zero.
That's a key point to understand, sometimes we need to make a copy of a variable. One way to do that is passing it to a function by value. The function argument will be a local copy which can be changed without affecting the original one.
As an example, the main function could be written like the following:
#include <iostream>
bool is_prime(long int number);
// ^^^^^^^^ So is `n` in the OP's `main`
int sum_of_digits(long int number);
// ^^^^^^^^^^^^^^^ This is a local copy.
long int nth_prime_after(int counter, long int number);
int main()
{
long int number;
// The input validation (check if it's a number and if it's in the valid range,
// deal with errors) is left to the reader as an exercise.
std::cin >> number;
int const counter = sum_of_digits(number);
std::cout << nth_prime_after(counter, number) << '\n';
return 0;
}
The definition of sum_of_digits is straightforward.
int sum_of_digits(long int number)
{
int sum = 0;
while ( number ) // Stops when number is zero. The condition n <= 100000
{ // belongs to input validation, like n >= 0.
sum += number % 10;
number /= 10; // <- This changes only the local copy.
}
return sum;
}
About the last part (finding the nth prime after the chosen number), I'm not sure to understand what the asker is trying to do, but even if n had the correct value, for(int i = n, counter = b; counter <= 10; i++) would be just wrong. For starters, there's no reason for the condition count <= 10 or at least none that I can think of.
I'd write something like this:
long int nth_prime_after(int counter, long int number)
{
while ( counter )
{
++number;
if ( is_prime(number) )
{
--counter; // The primes aren't printed here, not even the nth.
}
}
return number; // Just return it, the printing is another function's
} // responsabilty.
A lot more could be said about the is_prime function and the overall (lack of) efficiency of this algorithm, but IMHO, it's beyond the scope of this answer.
I have to write a program to check if the entered number has these qualifications:
A number that is prime it self, the reverse of that number is also prime, and the number's digits are prime numbers too (Like this number: 7523).
If the needs meet, it has to show "yes" when you enter and run the program otherwise "no".
I know both codes for prime and reverse numbers but I don't know how to merge them.
This is the code:
#include <iostream>
#include <conio.h>
using namespace std;
void prime_check(int x) {
int a, i, flag = 1;
cin >> a;
for (i = 2; i <= a / 2 && flag == 1; i++) {
if (a % i == 0)
flag = 0;
}
if (flag == 1)
cout << "prime";
else
break;
}
int main() {
int a, r, sum = 0;
cin >> a;
while (a != 0) {
r = a % 10;
sum = (sum * 10) + r;
a = a / 10;
}
}
The program has to check each digit of the number entered to see if it is prime or not in every step, then show "yes", but it doesn't work.
Welcome to the site.
I don't know how to merge them.
void prime_check(int n) { /*code*/ }
I'd understand that you don't know how to use this.
It's very easy!
int main()
{
int i = 0;
prime_check(i);
}
If you are confused about how the program executes, you could use a debugger to see where it goes. But since using a debugger can be a bit hard at first, I would suggest to add debug prints to see how the program executes.
This line of code prints the file and line number automatically.
std::cout << __FILE__ << ":" << __LINE__ << "\n";
I'd suggest to add it at the start of every function you wish to understand.
One step further is to make it into a macro, just so that it's easy to use.
#define DEBUGPRINT std::cout << __FILE__ << ":" << __LINE__ << "\n";
Check a working example here:
http://www.cpp.sh/2hpam
Note that it says <stdin>::14 instead of the filename because it's running on a webpage.
I have done some changes to your code, and added comments everywhere I've made changes. Check it out:
#include <iostream>
#include <conio.h>
using namespace std;
bool prime_check(int x) { // I have changed the datatype of this function to bool, because I want to store if all the digits are prime or not
int i, flag = 1; // Removed the variable a, because the function is already taking x as input
for (i = 2; i <= x / 2 && flag == 1; i++) {
if (x % i == 0)
flag = 0;
}
return flag == 1;
}
int main() {
int a, r, sum = 0, original; // added original variable, to store the number added
bool eachDigit = true; // added to keep track of each digit
cin >> a;
original = a;
while (a != 0) {
r = a % 10;
eachDigit = prime_check(r); // Here Each digit of entered number is checked for prime
sum = (sum * 10) + r;
a = a / 10;
}
if (eachDigit && prime_check(original) && prime_check(sum)) // At the end checking if all the digits, entered number and the revered number are prime
cout << "yes";
else
cout<< "no";
}
For optimization, you can check if the entered number is prime or not before starting that loop, and also you can break the loop right away if one of the digits of the entered number is not prime, Like this:
#include <iostream>
#include <conio.h>
using namespace std;
bool prime_check(int x) { // I have changed the datatype of this function to bool, because I want to store if all the digits are prime or not
int i, flag = 1; // Removed the variable a, because the function is already taking x as input
for (i = 2; i <= x / 2 && flag == 1; i++) {
if (x % i == 0)
flag = 0;
}
return flag == 1;
}
int main() {
int a, r, sum = 0;
bool eachDigit = true, entered; // added to keep track of each digit
cin >> a;
entered = prime_check(a);
while (a != 0 && entered && eachDigit) {
r = a % 10;
eachDigit = prime_check(r); // Here Each digit of entered number is checked for prime
sum = (sum * 10) + r;
a = a / 10;
}
if (eachDigit && entered && prime_check(sum)) // At the end checking if all the digits, entered number and the revered number are prime
cout << "yes";
else
cout<< "no";
}
Suppose you have an int variable num which you want to check for your conditions, you can achieve your target by the following:
int rev_num = 0;
bool flag = true; // Assuming 'num' satisfies your conditions, until proven otherwise
if (prime_check(num) == false) {
flag = false;
}
else while (num != 0) {
int digit = num % 10;
rev_num = rev_num * 10 + digit;
// Assuming your prime_check function returns 'true' and 'false'
if (prime_check(digit) == false) {
flag = false;
break;
}
num /= 10;
}
if (prime_check(rev_num) == false) {
flag = false;
}
if (flag) {
cout << "Number satisfies all conditions\n";
}
else {
cout << "Number does not satisfy all conditions\n";
}
The problem is that each of your functions is doing three things, 1) inputting the number, 2) testing the number and 3) outputting the result. To combine these functions you need to have two functions that are only testing the number. Then you can use both functions on the same number, instead of inputting two different numbers and printing two different results. You will need to use function parameters, to pass the input number to the two functions, and function return values to return the result of the test. The inputting of the number and the outputting of the result go in main. Here's an outline
// returns true if the number is a prime, false otherwise
bool prime_check(int a)
{
...
}
// returns true if the number is a reverse prime, false otherwise
bool reverse_prime_check(int a)
{
...
}
int main()
{
int a;
cin >> a;
if (prime_check(a) && reverse_prime_check(a))
cout << "prime\n";
else
cout << "not prime\n";
}
I'll leave you to write the functions themselves, and there's nothing here to do the digit checks either. I'll leave you do to that.
I have a program like this: given a sequence of integers, find the biggest prime and its positon.
Example:
input:
9 // how many numbers
19 7 81 33 17 4 19 21 13
output:
19 // the biggest prime
1 7 // and its positon
So first I get the input, store it in an array, make a copy of that array and sort it (because I use a varible to keep track of the higest prime, and insane thing will happen if that was unsorted) work with every number of that array to check if it is prime, loop through it again to have the positon and print the result.
But the time is too slow, can I improve it?
My code:
#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
int main()
{
int n;
cin >> n;
int numbersNotSorted[n];
int maxNum{0};
for (int i = 0; i < n; i++)
{
cin >> numbersNotSorted[i];
}
int numbersSorted[n];
for (int i = 0; i < n; i++)
{
numbersSorted[i] = numbersNotSorted[i];
}
sort(numbersSorted, numbersSorted + n);
for (int number = 0; number < n; number++)
{
int countNum{0};
for (int i = 2; i <= sqrt(numbersSorted[number]); i++)
{
if (numbersSorted[number] % i == 0)
countNum++;
}
if (countNum == 0)
{
maxNum = numbersSorted[number];
}
}
cout << maxNum << '\n';
for (int i = 0; i < n; i++)
{
if (numbersNotSorted[i] == maxNum)
cout << i + 1 << ' ';
}
}
If you need the biggest prime, sorting the array brings you no benefit, you'll need to check all the values stored in the array anyway.
Even if you implemented a fast sorting algorithm, the best averages you can hope for are O(N + k), so just sorting the array is actually more costly than looking for the largest prime in an unsorted array.
The process is pretty straight forward, check if the next value is larger than the current largest prime, and if so check if it's also prime, store the positions and/or value if it is, if not, check the next value, repeat until the end of the array.
θ(N) time compexity will be the best optimization possible given the conditions.
Start with a basic "for each number entered" loop:
#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
int main() {
int n;
int newNumber;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> newNumber;
}
}
If the new number is smaller than the current largest prime, then it can be ignored.
int main() {
int n;
int newNumber;
int highestPrime;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> newNumber;
if(newNumber >= highestPrime) {
}
}
}
If the new number is equal to the highest prime, then you just need to store its position somewhere. I'm lazy, so:
int main() {
int n;
int newNumber;
int highestPrime;
int maxPositions = 1234;
int positionList[maxPositions];
int nextPosition;
int currentPosition = 0;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> newNumber;
currentPosition++;
if(newNumber >= highestPrime) {
if(newNumber == highestPrime) {
if(nextPosition+1 >= maxPositions) {
// List of positions is too small (should've used malloc/realloc instead of being lazy)!
} else {
positionList[nextPosition++] = currentPosition;
}
}
}
}
}
If the new number is larger than the current largest prime, then you need to figure out if it is a prime number, and if it is you need to reset the list and store its position, etc:
int main() {
int n;
int newNumber;
int highestPrime = 0;
int maxPositions = 1234;
int positionList[maxPositions];
int nextPosition;
int currentPosition = 0;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> newNumber;
currentPosition++;
if(newNumber >= highestPrime) {
if(newNumber == highestPrime) {
if(nextPosition+1 >= maxPositions) {
// List of positions is too small (should've used malloc/realloc instead of being lazy)!
} else {
positionList[nextPosition++] = currentPosition;
}
} else { // newNumber > highestPrime
if(isPrime(newNumber)) {
nextPosition = 0; // Reset the list
highestPrime = newNumber;
positionList[nextPosition++] = currentPosition;
}
}
}
}
}
You'll also want something to display the results:
if(highestPrime > 0) {
for(nextPosition= 0; nextPosition < currentPosition; nextPosition++) {
cout << positionList[nextPosition];
}
}
Now; the only thing you're missing is an isPrime(int n) function. The fastest way to do that is to pre-calculate a "is/isn't prime" bitfield. It might look something like:
bool isPrime(int n) {
if(n & 1 != 0) {
n >>= 1;
if( primeNumberBitfield[n / 32] & (1 << (n % 32)) != 0) {
return true;
}
}
return false;
}
The problem here is that (for positive values in a 32-bit signed integer) you'll need 1 billion bits (or 128 MiB).
To avoid that you can use a much smaller bitfield for numbers up to sqrt(1 << 31) (which is only about 4 KiB); then if the number is too large for the bitfield you can use the bitfield to find prime numbers and check (with modulo) if they divide the original number evenly.
Note that Sieve of Eratosthenes ( https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes ) is an efficient way to generate that smaller bitfield (but is not efficient to use for a sparse population of larger numbers).
If you do it right, you'll probably create the illusion that it's instantaneous because almost all of the work will be done while a human is slowly typing the numbers in (and not left until after all of the numbers have been entered). For a very fast typist you'll have ~2 milliseconds between numbers, and (after the last number is entered) humans can't notice delays smaller than about 10 milliseconds.
But the time is too slow, can I improve it?
Below loop suffers from:
Why check smallest values first? Makes more sense to check largest values first to find the largest prime. Exit the for (... number..) loop early once a prime is found. This takes advantage of the work done by sort().
Once a candidate value is not a prime, quit testing for prime-ness.
.
// (1) Start for other end rather than as below
for (int number = 0; number < n; number++) {
int countNum {0};
for (int i = 2; i <= sqrt(numbersSorted[number]); i++) {
if (numbersSorted[number] % i == 0)
// (2) No point in continuing prime testing, Value is composite.
countNum++;
}
if (countNum == 0) {
maxNum = numbersSorted[number];
}
}
Corrections left for OP to implement.
Advanced: Prime testing is a deep subject and many optimizations (trivial and complex) exist that are better than OP's approach. Yet I suspect the above 2 improvement will suffice for OP.
Brittleness: Code does not well handle the case of no primes in the list or n <= 0.
i <= sqrt(numbersSorted[number]) is prone to FP issues leading to an incorrect results. Recommend i <= numbersSorted[number]/i).
Sorting is O(n * log n). Prime testing, as done here, is O(n * sqrt(n[i])). Sorting does not increase O() of the overall code when the square root of the max value is less than log of n. Sorting is worth doing if the result of the sort is used well.
Code fails if the largest value was 1 as prime test incorrectly identifies 1 as a prime.
Code fails if numbersSorted[number] < 0 due to sqrt().
Simply full-range int prime test:
bool isprime(int num) {
if (num % 2 == 0) return num == 2;
for (int divisor = 3; divisor <= num / divisor; divisor += 2) {
if (num % divisor == 0) return false;
}
return num > 1;
}
If you want to find the prime, don't go for sorting. You'll have to check for all the numbers present in the array then.
You can try this approach to do the same thing, but all within a lesser amount of time:
Step-1: Create a global function for detecting a prime number. Here's how you can approach this-
bool prime(int n)
{
int i, p=1;
for(i=2;i<=sqrt(n);i++) //note that I've iterated till the square root of n, to cut down on the computational time
{
if(n%i==0)
{
p=0;
break;
}
}
if(p==0)
return false;
else
return true;
}
Step-2: Now your main function starts. You take input from the user:
int main()
{
int n, i, MAX;
cout<<"Enter the number of elements: ";
cin>>n;
int arr[n];
cout<<"Enter the array elements: ";
for(i=0;i<n;i++)
cin>>arr[i];
Step-3: Note that I've declared a counter variable MAX. I initialize this variable as the first element of the array: MAX=arr[0];
Step-4: Now the loop for iterating the array. What I did was, I iterated through the array and at each element, I checked if the value is greater than or equal to the previous MAX. This will ensure, that the program does not check the values which are less than MAX, thus eliminating a part of the array and cutting down the time. I then nested another if statement, to check if the value is a prime or not. If both of these are satisfied, I set the value of MAX to the current value of the array:
for(i=0;i<n;i++)
{
if(arr[i]>=MAX) //this will check if the number is greater than the previous MAX number or not
{
if(prime(arr[i])) //if the previous condition satisfies, then only this block of code will run and check if it's a prime or not
MAX=arr[i];
}
}
What happens is this- The value of MAX changes to the max prime number of the array after every single loop.
Step-5: Then, after finally traversing the array, when the program finally comes out of the loop, MAX will have the largest prime number of the array stored in it. Print this value of MAX. Now for getting the positions where MAX happens, just iterate over the whole loop and check for the values that match MAX and print their positions:
for(i=0;i<n;i++)
{
if(arr[i]==MAX)
cout<<i+1<<" ";
}
I ran this code in Dev C++ 5.11 and the compilation time was 0.72s.
Question: How to find, for a given integer n, the first prime number that is larger than n?
My own work so far
I've managed to write a program that checks whether or not a given integer is a prime or not:
#include <iostream>
#include <cmath>
using namespace std;
bool is_prime (int n)
{
int i;
double square_root_n = sqrt(n) ;
for (i = 2; i <= square_root_n ; i++)
{
if (n % i == 0){
return false;
break;
}
}
return true;
}
int main(int argc, char** argv)
{
int i;
while (true)
{
cout << "Input the number and press ENTER: \n";
cout << "To exit input 0 and press ENTER: \n";
cin >> i;
if (i == 0)
{
break;
}
if (is_prime(i))
cout << i << " is prime" << endl;
else
cout << i << " isn't prime'" << endl;
}
return 0;
}
I'm struggling, however, on how to proceed on from this point.
You have a function is_prime(n), and a number n, and you want to return the smallest number q such that is_prime(q)==true and n <= q:
int q = n;
while (!is_prime(q)) {
q++;
}
// here you can be sure that
// 1. q is prime
// 2. q >= n -- unless there was an overflow
If you want to be a bit more efficient, you can check explicitly for the even case, and the increment by 2 each time.
It's a concrete example of a general theme: if you have a test function and a method for generating elements, you can generate the elements that pass the test:
x = initial_value
while (something) {
if (test(x)) {
// found!
// If you only want the first such x, you can break
break;
}
x = generate(x)
}
(note that this is not a valid C++ code, it's pseudocode)
int i;
**int k_koren_od_n = (int)(sqrt(n) + 0.5)**
for (i = 2; i <= k_koren_od_n ; i++){
To get around casting issues, you might want to add this fix.
I am not sure whether I should ask here or programmers but I have been trying to work out why this program wont work and although I have found some bugs, it still returns "x is not a prime number", even when it is.
#include <iostream>
using namespace std;
bool primetest(int a) {
int i;
//Halve the user input to find where to stop dividing to (it will remove decimal point as it is an integer)
int b = a / 2;
//Loop through, for each division to test if it has a factor (it starts at 2, as 1 will always divide)
for (i = 2; i < b; i++) {
//If the user input has no remainder then it cannot be a prime and the loop can stop (break)
if (a % i == 0) {
return(0);
break;
}
//Other wise if the user input does have a remainder and is the last of the loop, return true (it is a prime)
else if ((a % i != 0) && (i == a -1)) {
return (1);
break;
}
}
}
int main(void) {
int user;
cout << "Enter a number to test if it is a prime or not: ";
cin >> user;
if (primetest(user)) {
cout << user << " is a prime number.";
}
else {
cout << user<< " is not a prime number.";
}
cout << "\n\nPress enter to exit...";
getchar();
getchar();
return 0;
}
Sorry if this is too localised (in which case could you suggest where I should ask such specific questions?)
I should add that I am VERY new to C++ (and programming in general)
This was simply intended to be a test of functions and controls.
i can never be equal to a - 1 - you're only going up to b - 1. b being a/2, that's never going to cause a match.
That means your loop ending condition that would return 1 is never true.
In the case of a prime number, you run off the end of the loop. That causes undefined behaviour, since you don't have a return statement there. Clang gave a warning, without any special flags:
example.cpp:22:1: warning: control may reach end of non-void function
[-Wreturn-type]
}
^
1 warning generated.
If your compiler didn't warn you, you need to turn on some more warning flags. For example, adding -Wall gives a warning when using GCC:
example.cpp: In function ‘bool primetest(int)’:
example.cpp:22: warning: control reaches end of non-void function
Overall, your prime-checking loop is much more complicated than it needs to be. Assuming you only care about values of a greater than or equal to 2:
bool primetest(int a)
{
int b = sqrt(a); // only need to test up to the square root of the input
for (int i = 2; i <= b; i++)
{
if (a % i == 0)
return false;
}
// if the loop completed, a is prime
return true;
}
If you want to handle all int values, you can just add an if (a < 2) return false; at the beginning.
Your logic is incorrect. You are using this expression (i == a -1)) which can never be true as Carl said.
For example:-
If a = 11
b = a/2 = 5 (Fractional part truncated)
So you are running loop till i<5. So i can never be equal to a-1 as max value of i in this case will be 4 and value of a-1 will be 10
You can do this by just checking till square root. But below is some modification to your code to make it work.
#include <iostream>
using namespace std;
bool primetest(int a) {
int i;
//Halve the user input to find where to stop dividing to (it will remove decimal point as it is an integer)
int b = a / 2;
//Loop through, for each division to test if it has a factor (it starts at 2, as 1 will always divide)
for (i = 2; i <= b; i++) {
//If the user input has no remainder then it cannot be a prime and the loop can stop (break)
if (a % i == 0) {
return(0);
}
}
//this return invokes only when it doesn't has factor
return 1;
}
int main(void) {
int user;
cout << "Enter a number to test if it is a prime or not: ";
cin >> user;
if (primetest(user)) {
cout << user << " is a prime number.";
}
else {
cout << user<< " is not a prime number.";
}
return 0;
}
check this out:
//Prime Numbers generation in C++
//Using for loops and conditional structures
#include <iostream>
using namespace std;
int main()
{
int a = 2; //start from 2
long long int b = 1000; //ends at 1000
for (int i = a; i <= b; i++)
{
for (int j = 2; j <= i; j++)
{
if (!(i%j)&&(i!=j)) //Condition for not prime
{
break;
}
if (j==i) //condition for Prime Numbers
{
cout << i << endl;
}
}
}
}
main()
{
int i,j,x,box;
for (i=10;i<=99;i++)
{
box=0;
x=i/2;
for (j=2;j<=x;j++)
if (i%j==0) box++;
if (box==0) cout<<i<<" is a prime number";
else cout<<i<<" is a composite number";
cout<<"\n";
getch();
}
}
Here is the complete solution for the Finding Prime numbers till any user entered number.
#include <iostream.h>
#include <conio.h>
using namespace std;
main()
{
int num, i, countFactors;
int a;
cout << "Enter number " << endl;
cin >> a;
for (num = 1; num <= a; num++)
{
countFactors = 0;
for (i = 2; i <= num; i++)
{
//if a factor exists from 2 up to the number, count Factors
if (num % i == 0)
{
countFactors++;
}
}
//a prime number has only itself as a factor
if (countFactors == 1)
{
cout << num << ", ";
}
}
getch();
}
One way is to use a Sieving algorithm, such as the sieve of Eratosthenes. This is a very fast method that works exceptionally well.
bool isPrime(int number){
if(number == 2 || number == 3 | number == 5 || number == 7) return true;
return ((number % 2) && (number % 3) && (number % 5) && (number % 7));
}