Having a problem with dividing long doubles...

**Danni111111** · 02-07-2014

Hi,

I'm afraid I'm a novice in c programming, and no matter what I do I cannot get a division to work:

Code:

        //END RANGE INPUT
        long double End;
        printf("\nPlease enter the start of the range (Lower Bound):\n\n");
        scanf("%Lf",&End);
        //START RANGE INPUT
        long double Start;
        printf("\nPlease enter the end of the range (Upper Bound):\n\n");
        scanf("%Lf",&Start);
        //PLOT INTERVAL INPUT
        long double Interval;
        printf("\nPlease enter the interval between each plot:\n\n");
        scanf("%Lf",&Interval);
        //RANGE N INPUT
        int N;
        printf("\nPlease enter a value for N:\n\n");
        scanf("%d",&N);
        //RANGE SIZE CREATOR
        long double SizeL;
        SizeL = ((Start-End)/Interval);
        printf("%Lf",Start);
        printf("%Lf",End);
        printf("%Lf",Interval);
        printf("%Lf",SizeL);

No matter what I input for the values of 'Start', 'End' and 'Interval', the value of 'SizeL' always seems to be -2.

I've got no idea how to fix this, but any and all help would be greatly appreciated!

**grumpy** · 02-07-2014

Turn up the warnings on your compiler in the first instance. long double (and the scanf())/printf() %LF format specifier) is not supported by all compilers (particularly older compilers). If you're using a compiler that doesn't support those features (or supports them in a manner that is broken) then you'll need to get a compiler/library that does.

**Danni111111** · 02-07-2014

I'm pretty sure it isn't a problem with the %Lf not being supported- all the other variables printf with correct values, without any issues

.

Edit: It seems when the long doubles are inputted on their own, they can then be printed without difficulty. But when any long double which has a value from an equation using other long doubles, it always gives -2 as the printf...

**rcgldr** · 02-07-2014

It would help to step through the code with a source level debugger to actually confirm if the variables are getting proper values. If scanf and printf with %L treat it as a reference to a double instead of a long double, then printf would correctly display the values input by scanf, even though the long double variables had regular double variables stored in them. Once you did math with the messed up variables, you would get an error, although I'm not sure why it would always show -2 as a result (could be related to the range of values input).

You could also change the variables to be regular doubles and test that with %Lf in scanf and printf, if that works, then %Lf is apparently the format specifier for doubles instead of long doubles. If there isn't an alternative format specifier for long doubles, you may have to input and output doubles, then convert to and from long doubles in your code. You could also create custom functions to convert a string to a long double and back.

Link to a question / answer with someone else having similar problems:

c - printf and long double - Stack Overflow

**anduril462** · 02-07-2014

@Danni111111:
You don't mention which compiler you're using, the version, what architecture you're on, etc. This seems to be a fairly compiler specific issue. It works for me with the following: gcc version 4.6.3, running Ubuntu, on Intel x84-64.

Note, giving us the exact input you used may help too. It could be that just certain values don't work right.

**nonoob** · 02-07-2014

Try %llf instead of %Lf everywhere.

**grumpy** · 02-07-2014

Originally Posted by Danni111111

I'm pretty sure it isn't a problem with the %Lf not being supported- all the other variables printf with correct values, without any issues.

Being able to print an int correctly with the %d format has no relationship to being able to being able to print a float with %f, let alone the ability to print a long long double with %Lf. A C89 compiler, for example, is guaranteed to support the first two, but not the last.

Different code, within the implementation of the printf() family of functions themselves is involved.

As others have said, you need to identify your compiler, compiler version, operating system, operating system version inputs you are providing, and outputs being received.

AND you need to specify whether that code you have supplied is complete. If there is other code involved that you have deemed irrelevant, it may be a malfunction in that code which is compromising behaviour.

**Hodor** · 02-07-2014

When I'm faced with a problem like this I like to rule stuff out. I would have written a simple test case such as

Code:

#include <stdio.h>
 
int main(void)
{
    long double SizeL, Start, End, Interval;
    
    Start = 100.0;
    End = 200.0;
    Interval = 3.0;
    
    SizeL = ((Start-End)/Interval);
    
    printf("%Lf\n",SizeL);
    
    return 0;
}

And see if that produces the expected output. If it does, then you've narrowed the bug down to how you're getting the numbers. If it doesn't then you've isolated the bug.

**hk_mp5kpdw** · 02-10-2014

Kinda confusing asking for start of range and storing value into End and then asking for end of range and storing into Start... but, whatever.

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