C++ cout will not print

Question

What the title says, cout will not print anything! But main finished normally and returns a negative number, what's going on? Here is my code:

#include <iostream>
using namespace std;
unsigned long fibonacci(long unsigned int *);
int main(void)
{
    cout<<"IT WILL NOT PRINT!!!!!";
    unsigned long int fib[4000000];
    cout<<"SUM OF EVEN FIBONACCI NUMBERS: "<<fibonacci(fib)<<endl;
    return 0;
}
unsigned long fibonacci(unsigned long int *FIBO)
{
    unsigned long i;
    int sum=0,c=0, *EVEN = new int[2000000];
    FIBO[0]=1; FIBO[1]=2;
    for (i=2;i<3999999;i++){
        FIBO[i]=FIBO[i-1]+FIBO[i-2];
        if (FIBO[i]%2==0){
            EVEN[c]=FIBO[i];
            sum+=EVEN[c];
            c++;
        }
    }
    delete [] EVEN;
    return sum;
}

Welcome to the site! Please post your code into your question. Indent with four spaces to make it look like code on the site. Good luck! — Sergey Kalinichenko, Sep 11 '14 at 19:22
try putting an `endl` after your *first* `cout`. You're probably failing in `fibonacci` just before you flush stdout. — Paul Evans, Sep 11 '14 at 19:24
@PaulEvans I did but that's not the problem, as you can see it doesn't print the cout<<"IT WILL NOT PRINT!!!!!"; which is right after the main :( — SasukeItachi UchihaClan, Sep 11 '14 at 19:26
Believe it is not the same I have tried this post and nothing. Actually nothing works! I even return 293; after int main(void) and nothing, why is that? — SasukeItachi UchihaClan, Sep 11 '14 at 19:30

score 1 · Accepted Answer · answered Sep 11 '14 at 19:30

1

std::cout by default is buffered. Without an explicit flush, you won't see anything printed until the internal buffers need to be flushed. This is done for performance reasons.

You can add a specific flush, like so: std::cout<<"IT WILL NOT PRINT!!!!!" << std::endl;

That said, you're not seeing output because your program is crashing.

unsigned long int fib[4000000]; is going to require nearly 15MB of space (on a 32bit long int platform). There simply won't be enough stack space to allocate such a large block of memory within that storage duration.

For such large blocks you will need to dynamically allocate the block, or better yet:

std::vector<unsigned long int> fib(4000000);

answered Sep 11 '14 at 19:30

Chad

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still the same problem ;( – SasukeItachi UchihaClan Sep 11 '14 at 19:32
So, this doesn't print anything? `#include int main { std::cout << "Hello." }`? If it _does_, your real program is crashing (and no, you won't see the output). – Chad Sep 11 '14 at 19:33
@SasukeItachiUchihaClan: Your program crashes for me too, but if I decrease the size of the huge array "fib", it doesn't crash. – Thomas Padron-McCarthy Sep 11 '14 at 19:35
that does print, but even after using flush and endl on my program it won't do anything, actually it is not a cout problem, it won't do anythinhg, just returns a negative number. I even return -293 after int main(void){ but nothing happens – SasukeItachi UchihaClan Sep 11 '14 at 19:35
Because your program is crashing. Did you not read the answer? You can't allocate 15MB on the stack, it will crash immediately -- before it does anything else. – Chad Sep 11 '14 at 19:37
@ThomasPadron-McCarthy so it's the size , because probably c++ can't support 4 million integers on an array? That's the logical explanation – SasukeItachi UchihaClan Sep 11 '14 at 19:38
Not 4 million integers _on the stack_. You need to allocate it differently. – Thomas Padron-McCarthy Sep 11 '14 at 19:39
@Chad I read the answer but it doesn't crash, it returns a negative number – SasukeItachi UchihaClan Sep 11 '14 at 19:39
The code you wrote doesn't return anything but 0. So, if you are seeing anything else (return of a negative number) that _is_ a crash. – Chad Sep 11 '14 at 19:39
Do you have any suggestions? – SasukeItachi UchihaClan Sep 11 '14 at 19:41
My suggestion is in my answer. If you need `fib`, use a `std::vector` to hold it. – Chad Sep 11 '14 at 19:42
There are other errors now, but anyway at least I found the answer, I need to learn vectors. I guess this problem should be solved now. – SasukeItachi UchihaClan Sep 11 '14 at 19:50

rwjb · Answer 2 · 2014-09-11T20:21:21.170

Let's take a look at this problem: we want to produce a sum of even Fibonacci numbers within a predefined range.

Rearranged with minor edits for clarity:

#include <iostream>
using namespace std;
unsigned long fibonacci(unsigned long int *FIBO)
{
    unsigned long i;
    int sum=0,c=0, *EVEN = new int[2000000];
    FIBO[0]=1; FIBO[1]=2;
    for (i=2;i<3999999;i++){
        FIBO[i]=FIBO[i-1]+FIBO[i-2];
        if (FIBO[i]%2==0){
            EVEN[c]=FIBO[i];
            sum+=EVEN[c];
            c++;
        }
    }
    delete [] EVEN;
    return sum;
}
int main(void)
{
    unsigned long int fib[4000000];
    cout << "SUM OF EVEN FIBONACCI NUMBERS: " << fibonacci(fib) << endl;
    return 0;
}

First, let's remove unneeded code in the function fibonacci. We don't need to store even numbers, so we'll remove the EVEN array.

unsigned long fibonacci(unsigned long *FIBO)
{
    unsigned long i, sum=0;
    FIBO[0]=1; FIBO[1]=2;
    for (i=2;i<3999999;i++){
        FIBO[i]=FIBO[i-1]+FIBO[i-2];
        if (FIBO[i]%2==0){
            sum += FIBO[i];
        }
    }
    return sum;
}

We can get a bit more clever though- we can prove that every third Fibonacci number will be even. Starting from Fib(0)=0, this allows us to make a new function:

unsigned long sum_even_fibonacci(unsigned long n)
{
    unsigned long i, a=0, b=1, c=1, sum=0;
    for (i=0;i<n/3;i++){
        a = b + c; // Fib(3i)
        b = a + c; // Fib(3i+1)
        c = a + b; // Fib(3i+2)
        sum += a;  // Fib(3i) will always be even- add it to sum.
    }
    return sum;
}

This should produce the sum of the even Fibonacci numbers within the first n numbers of the series.

The new program, in full:

#include <iostream>
using namespace std;
unsigned long sum_even_fibonacci(unsigned long n)
{
    unsigned long i, a=0, b=1, c=1, sum=0;
    for (i=0;i<n/3;i++){
        a = b + c; // Fib(3i)
        b = a + c; // Fib(3i+1)
        c = a + b; // Fib(3i+2)
        sum += a;  // Fib(3i) will always be even- add it to sum.
    }
    return sum;
}
int main(void)
{
    cout << "SUM OF EVEN FIBONACCI NUMBERS: " << sum_even_fibonacci(4000000) << endl;
    return 0;
}

score 1 · Answer 3 · answered Sep 11 '14 at 20:17

But main finished normally and returns a negative number,

In your code main returns 0 and that is the only return path. So, the process exiting with a negative return code means that main() never finished.

Most likely, the array unsigned long int fib[4000000]; is too large for your system's default stack size, and the operating system handles this by making your process exit with that negative number.

To fix this, either make that array a lot smaller or remove it entirely; or use dynamic allocation.

Note that there is no need to even have this array (and no need for EVEN either). You're keeping the sum as you go, so you only need to keep the last 2 numbers ; not the entire history.

C++ cout will not print

3 Answers3