C++ and Project Euler #3 Compilation Error

Question

#import <cmath>
#import <iostream>
using namespace std;

int main {
    int a;
    for(int x = 600851475142; x>cmath::sqrt(600851475143); x--) {
        if (600851475143%x==0) { //is it a factor of 600851475143?
            if p(x) { //is it also a prime?
                cout << x;
                return 0;
            }
        }
    }
}

bool p(int n) {
    for(int x = n; x<cmath::sqrt(n)+1; x++) {
        if (n%x==0) {
            return false;
        }
    }
    return true;
}

Here is my code for project euler #3 (http://projecteuler.net/problem=3). I am relatively new to C++.

Basically my approach was to count down from 600851475143, test if the number was a factor of 600851475143 and then see if it was prime. If it was then return the number and exit. However, when I compile my code I get the error:

error: function definition does not declare parameters
In function 'bool p(int)':
error: 'cmath' has not been declared

Any help would be appreciated

What compiler are you using? It should be complaining about `#import` too, that's not standard C++. You've got a lot of basic and strange errors in there. — Mat, Apr 20 '13 at 18:04
@Mat Yeah I come from a python background and I am self taught so most of this is just hacked together. I am using Sublime Text 2 on Mac so I am guessing G++ — picklebobdogflog, Apr 20 '13 at 18:07
If you want to learn C++ properly, you might want to start with a [good book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — Angew is no longer proud of SO, Apr 20 '13 at 18:21
@Angew Ok that sounds like a good starting point, I'll look into it. — picklebobdogflog, Apr 20 '13 at 18:23
You really need to learn to search first. There are a lot of answers already for project Euler and prime numbers. — Thomas Matthews, Apr 20 '13 at 18:25

score 2 · Accepted Answer · answered Apr 20 '13 at 18:25

2

Looking at your code, it really seems you are at-least new to C++, if not to programming itself.

1) In C++, we use #include instead of #import

2) you need to declare a function before calling it. Something like:

bool p(int n);

before main function

3) Parenthesis is missing in main function. Should be: 'int main() {'

4) You don't need cmath:: for sqrt. Write some thing like: (int) sqrt((double)600851475143) This is because sqrt does not take integer in parameter

5) 'if p(x) {' should be 'if (p(x)) {'

6) for-loop in function is always increasing.

7) it's a good idea to calculate sqrt before for-loop instead of in for-loop.

8) Welcome to programming world!

answered Apr 20 '13 at 18:25

Mraaz

71
2

Thanks, I am new to C++ and relatively new to programming in general. This helps a lot! – picklebobdogflog Apr 20 '13 at 18:27
Made these changes, Here is my code #include #include using namespace std; bool p(int n) { int limit = sqrt(n)+1; for(int x = n; x>limit; x++) { if (n%x==0) { return false; } } return true; } int main () { int a; for(long x = 0; x<600851475143; x++) { if (600851475143%x==0) { //is it a factor? if (p(x)) { cout << x; return 0; } } } } Now I compile fine but I get a floating point exception when I run my program – picklebobdogflog Apr 20 '13 at 18:31
1

in 6) I'm talking about for-loop in function 'p'. – Mraaz Apr 20 '13 at 18:32
Ok, made that x>limit, still getting the error @mraaz – picklebobdogflog Apr 20 '13 at 18:34
1

It should be x-- instead of x++ – Mraaz Apr 20 '13 at 18:37
Fixed that, still getting a floating point exception @mraaz – picklebobdogflog Apr 20 '13 at 18:39
1

I think, you need a minor correction in your logic. Think about it. Also, for both square root calculations, try casting to double and back to int, like I stated it 4) – Mraaz Apr 20 '13 at 18:50

score 0 · Answer 2 · answered Apr 20 '13 at 18:04

0

The name of the header is cmath, but sqrt is in namespace std.

Also: you probably want to compute the square root before the loop, then test against that result:

bool p(int n) { 
    int limit = std::sqrt(n)+1;
    for (int x=n; x<limit; x++)
    // ...
}

answered Apr 20 '13 at 18:04

Jerry Coffin

476,176
80
629
1,111

Ok changed it to #import using namespace std; int main { int a; for(int x = 600851475143; x>0; x--) { if (600851475143%x==0) { //is it a factor? if p(x) { cout << x; return 0; } } } } //is it a prime? bool p(int n) { int limit = std::sqrt(n)+1; for(int x = n; x – picklebobdogflog Apr 20 '13 at 18:09
@picklebobdogflog Just try `sqrt` on its own. – Matt Phillips Apr 20 '13 at 18:13
`int main {` You need params for main. – alexrider Apr 20 '13 at 18:16
Fixed a few things here and there. Now I am getting this - error: 'p' was not declared in this scope – picklebobdogflog Apr 20 '13 at 18:18
@picklebobdogflog Declare it! – alexrider Apr 20 '13 at 18:19
@alexrider p is my function. What do you mean declare it? (Sorry complete idiot programmer here) – picklebobdogflog Apr 20 '13 at 18:20
1

How can you use `import` in C++ without an error? Are you using a Microsoft bastardized C++? Or are you programming in C#? – Thomas Matthews Apr 20 '13 at 18:24
@ThomasMatthews No I am using Sublime Text 2 which I think compiles with g++. I fixed that though – picklebobdogflog Apr 20 '13 at 18:26

C++ and Project Euler #3 Compilation Error

2 Answers2