Floating Point Exception C++ Why and what is it?

I’m building a program for the Euler projects question 3, and while that might not really matter as a result I’m current trying to make this code take a number and test if it is prime or not. Now then before I get to troubleshoot the function it gives me the error “floating point exception” right after inputting the number. Here’s the code:

int main()
{
    int input;
    cout << "Enter number: " << endl;
    cin>> input;
    int i = input/2;
    int c;
    for (i>0; i--;) {
        c= input%i;
        if (c==0 || i == 1)
            cout << "not prime" << endl;
        else
            cout << "prime" << endl;
    }
    return 0;
}

so essentially why is it giving me a floating point exception and what does that even mean?

5 Answers

A “floating point number” is how computers usually represent numbers that are not integers — basically, a number with a decimal point. In C++ you declare them with float instead of int. A floating point exception is an error that occurs when you try to do something impossible with a floating point number, such as divide by zero.

for (i>0; i--;)

is probably wrong and should be

for (; i>0; i--)

instead. Note where I put the semicolons. The condition goes in the middle, not at the start.

Lots of reasons for a floating point exception. Looking at your code your for loop seems to be a bit “incorrect”. Looks like a possible division by zero.

for (i>0; i--;){
c= input%i;

Thats division by zero at some point since you are decrementing i.

Since this page is the number 1 result for the google search “c++ floating point exception”, I want to add another thing that can cause such a problem: use of undefined variables.

Problem is in the for loop in the code snippet:
for (i > 0; i–;)

Here, your intention seems to be entering the loop if (i > 0) and decrement the value of i by one after the completion of for loop.

Does it work like that? lets see.

Look at the for() loop syntax:

**for ( initialization; condition check; increment/decrement ) {  
    statements;  
}**

Initialization gets executed only once in the beginning of the loop. Pay close attention to “;” in your code snippet and map it with for loop syntax.

Initialization : i > 0 : Gets executed only once. Doesn’t have any impact in your code.

Condition check : i — : post decrement.

              Here, i is used for condition check and then it is decremented. 
              Decremented value will be used in statements within for loop. 
              This condition check is working as increment/decrement too in your code. 

Lets stop here and see floating point exception.

what is it? One easy example is Divide by 0. Same is happening with your code.

When i reaches 1 in condition check, condition check validates to be true.
Because of post decrement i will be 0 when it enters for loop.

Modulo operation at line #9 results in divide by zero operation.  

With this background you should be able to fix the problem in for loop.

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