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05.12.2008 at 02:45AM PDT, ID: 23394007

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9.1

Floating Point Precision

Zone: C++ Programming Language

Tags: Borland, C++ builder, 6.0

I am using C++ builder 6.0

I have 2 forms which are part of my program.

When the user presses button1 on form 1 the secord form is called using the code below:

form2 = new Tform2(this);
form2->ShowModal();
delete form2;

I have alot of variables that are declared as floats on the second form. I carry out alot of calculations so sometimes precision is lost.
float precision = 0.001;

if (fabs(percentage) < precision)
{
percentage = 0.00;
}

I have to cater for the fact that sometimes the value can be zero so I use the above code to give the value zero instead of -1.014E which I was sometimes getting.

I then use the code below

string convertToString(const float value)
{
stringstream ss500;
ss500 << fixed << setprecision(2) << value;
return ss500.str();
}

to ensure all values have 2 decimal places.

Most of the time everything works fine but I have noticed that sometimes when I close then open the form again values that should be 0.00 are sometimes 12345.56777777 (example) or shown as -0.00 instead of 0.00 is there anyway of ensuring this does not happen as it looks very messy on the screen.

Any help greatly appreciated

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Question Stats

Zone: Programming

Question Asked By: lisa_mc

Solution Provided By: Infinity08

Participating Experts: 2

Solution Grade: A

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05.12.2008 at 02:51AM PDT, ID: 21545571

Rank: Genius

Infinity08:

>> sometimes 12345.56777777 (example)

Can you show actual values that you get ?

>> or shown as -0.00

if the value was set like this :

percentage = 0.00;

that shouldn't happen. I assume there's some more code to this that you haven't shown.

In any case, if you want precise values and precise calculations, you shouldn't use floating point values. Rather go with fixed point values.

05.12.2008 at 03:58AM PDT, ID: 21545810

Rank: Sage

evilrix:

As an aside, but not a direct answer to your question...

If you're not used to doing floating point arithmetic in C/C++ you can get some nasty surprises. The binary representation of a floating point number is not precise and you can get unexpected results. You need to be aware of this as it means you have to be very careful when, for example, comparing 2 floating point numbers for equality. I'd recommend you read the following, this link to a Q&A about floating point numbers in C/C++...

http://www.codeguru.com/forum/printthread.php?t=323835

-Rx.

05.12.2008 at 05:38AM PDT, ID: 21546301

lisa_mc:

one of the values I was getting was 713148764706369830600000000.00 where it should have been 0.00

05.12.2008 at 05:40AM PDT, ID: 21546321

Rank: Genius

Infinity08:

>> 713148764706369830600000000.00

How did you obtain that value ? What was the original data, and what were the calculations done on it ?

05.12.2008 at 06:13AM PDT, ID: 21546617

lisa_mc:

Original data is read in from a file into an array of structs and that value was then converted into a float value. The value was then if matched read into another array of type float. This value was then divided by 100 and reassigned to itself ie value = value/100 .

Then the fabs and stringstream as described earlier was done.

05.12.2008 at 06:16AM PDT, ID: 21546647

Rank: Genius

Infinity08:

>> Original data

What was the original data ?

>> read in from a file

How was it read (code ?) ?

>> and that value was then converted into a float value.

Converted from what ? Converted how ? (code ?)

>> The value was then if matched

Matched how ? (code ?)

05.12.2008 at 06:41AM PDT, ID: 21546883

lisa_mc:

Read in expense from file

getline(infile2e1, array2e[i].expensee2, '\n') // to new line charater as this is last in record

Converts string to float

exp2e[i] = (float)atof(array2e[i].expensee2.c_str());

for(int i=0; i<linecount2b; i++) // number of records in meters file
{
for(int j=0; j<linecount2e; j++) // number of records in EXPT file
{
if(meterall2b[i] == expenseall2e[j]) // if a machine numbers matches another machine numer in the expense file
{

exp_count2e[i] = exp2e[j];
flt_count2e[i] = flt2e[j];
refill_count2e[i] = refill2e[j];

}
// if no match is found
// exp_count2e[i] , flt_count2e[i] and refill_count2e == 0
}
}

for(int i=0; i<linecount2b; i++) // divides by 100 to give value in pounds
{

exp_count2e[i] = (exp_count2e[i]/100);

}

Then fabs and stringstream is carried out

05.12.2008 at 06:51AM PDT, ID: 21546959

Rank: Genius

Infinity08:

>> if(meterall2b[i] == expenseall2e[j])

Are you comparing floats here ? If so, that's not gonna work reliably !

>> exp2e[i] = (float)atof(array2e[i].expensee2.c_str());

What's the data read from the file ?
What's the type of exp2e[i] ?
Did you verify that the value in it is correct ?

>> exp_count2e[i] = exp2e[j];

What's the type of exp_count2e[i] ?

>> Then fabs and stringstream is carried out

And what's the value that it showed as output ?

05.12.2008 at 07:02AM PDT, ID: 21547028

lisa_mc:

meterall and expenseall are strings

the value read from file will be 3000 or 0 or 1000 (value is shown as pence)

exp2e is a float

I dont know the value being read into it it could be any number as it is a pence value ie could be 0 could be 150 or so on. I did test to make sure that the file was being read in correctly

exp_count2e is also a float

The value shown could be 0.00 then when I close the form and reload it - it gave me the value

713148764706369830600000000.00 this doesnt happen everytime just at random

05.12.2008 at 07:06AM PDT, ID: 21547068

Rank: Genius

Infinity08:

>> the value read from file will be 3000 or 0 or 1000 (value is shown as pence)

Then why read it as a float ?

>> I dont know the value being read into it it could be any number as it is a pence value ie could be 0 could be 150 or so on.

Well, can you check which of those values caused an output of 713148764706369830600000000.00 ? Can you also check at what point in the code the value became wrong ?

>> then when I close the form and reload it - it gave me the value

Are you sure that closing the form doesn't invalidate the data ? ie. that the next time you show it, it shows a random value ?

05.12.2008 at 07:21AM PDT, ID: 21547221

lisa_mc:

I could read it as an int then try that

Im not sure if it is the form reloading because when I close the form I free all the memory used

Ill try this the other way ill get bk to you

05.12.2008 at 07:43AM PDT, ID: 21547402

Rank: Genius

Infinity08:

>> because when I close the form I free all the memory used

Freeing the memory would invalidate it :) You can't use any data from a freed memory block, because you can't rely on its contents any more - they might have changed.

05.14.2008 at 01:48AM PDT, ID: 21562161

lisa_mc:

Sorry didnt get near a pc yesterday

I think it is something to do with opening and closing the forms because the first time the form is loaded everything is fine.

In my program all my information comes from text files so everytime the user selects a different menu option I have to read in the text file manipulate the data and display it to the user. I cant read in the file once as the user can modify the information through certain menu options so I have to overwrite any changes.

Im the above example I read my files into array of structs using arrays of pointers

eg

double *array1;
array1 = new double[50];
then when I close the form I use
delete [] array1;

The reason why I use pointers is because I constantly need to update the information and using just arrays didnt work when I reloaded the form.

Even if there was a more practical way to read the information in and manupilate it without having to change code 2 much - I would def consider that - sorry C++ is not my main language im learning as I go so there probably are better ways of doing this.

Also I know the arrays are gettin incorrect information when I reload the form is there anything more I can do 2 make sure the data is wiped when I reload the form

05.14.2008 at 02:55AM PDT, ID: 21562463

Rank: Genius

Infinity08:

As I said : if you delete the memory, then you invalidate the data ... in other words : if you want to keep the data, don't delete it :)

Accepted Solution

05.14.2008 at 02:56AM PDT, ID: 21562466

lisa_mc:

Hiya

I figured it out in the code below if meterall2b[i] == expenseall2e[j] then each array was assigned different values.
If a match wasnt found then I assumed that the arrays would just be zero oh how wrong I was

I have changed the loop now so that if a match isnt found then value = 0.

Thanks for pointing me in the right direction infinity08 thats a major problem sorted

                       
         
           
           
                            for(int i=0; i<linecount2b; i++)    // number of records in meters file
                {
                        for(int j=0; j<linecount2e; j++)  // number of records in EXPT file
                        {
                                if(meterall2b[i] == expenseall2e[j]) // if a machine numbers matches another machine numer in the expense file
                                {                           
                              
                                        exp_count2e[i] = exp2e[j];
                                        flt_count2e[i] = flt2e[j];
                                        refill_count2e[i] = refill2e[j];
 
                                }
                                        // if no match is found 
                                        // exp_count2e[i] , flt_count2e[i] and refill_count2e == 0
                        }
                }

           
         

Open in New Window

05.14.2008 at 02:57AM PDT, ID: 21562472

Rank: Genius

Infinity08:

>> I assumed that the arrays would just be zero oh how wrong I was

No ... You can't depend on that. It will be whatever happened to be in memory at that time.

05.14.2008 at 02:59AM PDT, ID: 21562479

lisa_mc:

I know that now i have learned my lesson thanks :-)

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