Rounding but not approximation [Code help]

[Fernando Jose Velasco Borea]

Hello all:) I've been working on several systems and I want to implement a function that would require a rounding process but not approximation, so far I've been using normalize double but I can't reach my goal with this function. Any ideas?

Example:

Turning 5.19 into 5.1 (Not rounding to the nearest value)

With the NormalizeDouble I do get my goal of transforming 5.19408523245 (this is a random number) into a 1 decimal value,but instead of making it 5.1 it rounds up and then turning the actual value to 5.2

Regards,

Fernando.

[Anthony Garot]

Turning 5.19 into 5.1 (Not rounding to the nearest value)

void OnStart()
{
    double numberTest = 5.19;
        
    double truncated = ((long) (numberTest * 10) / 10.);
    PrintFormat("%f", truncated);
}

[William Roeder]

Anthony Garot:

Don't write one time use code. Write generalized functions.
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Fernando Jose Velasco Borea: With the NormalizeDouble

See links provide. Never use NormalizeDouble.

[Anthony Garot]

whroeder1:
Don't write one time use code. Write generalized functions.

Knowing how to truncate a number is more informative than being handed a general function.

[nicholish en]

The recommended method is to round down. 

double roundToStepDown(double number, double step)
{
   return step * floor(number / step);
}

So in this case:

double num = roundToStepDown(5.19, 0.1);

Also, the easiest way is to use the already existing CDouble library. It has all the methods you need for working with floating point numbers. 

https://www.mql5.com/en/code/19727

[kypa]

nicholi shen:

double roundToStepDown(double number, double step)
{
   return step * floor(number / step);
}

double roundToStepDown(double number, double step)
{
   if(number>=0) return step * floor(number / step);
   else return step * ceil(number / step);
}

[Anthony Garot]

kypa:

Good catch. It appears the floor() function rounds towards negative infinity, and isn't suited for negative numbers.

[kypa]

It should round up to the bigger digit. If it rounds up to a much bigger digit the problem might be double/float precision, not really the function.

I don't really know though, I don't understand these quite well.

[Anthony Garot]

whroeder1:
Don't write one time use code. Write generalized functions.

The thing about @whroeder1 is that he pushes me to not be lazy.

Here's my solution as a generalized function.

// Truncates positive or negative numbers to the specified precision
double MathTruncate(const double number, const uint digits)
{
    double multiplier = MathPow(10,digits);
    return (long) (number * multiplier) / multiplier;
}

Per https://en.wikipedia.org/wiki/Truncation#Truncation_and_floor_function