Testing a value in a range

Pishky
Pishky used Ask the Experts™
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Hi there gurus.

I am writing a program that converts numerical values into colors

For example

if value is between 0.0 and 0.01 color value is RGB(255,255,0)
                            0.011 and 0.02 color value is RGB(255,255,1)
                                        ...
                                        ...
                                        ...
                            0.091 and 0.1  color value is RGB(255,255,255)
There will be up to 1000 of these ranges
Any idea how to code this.

Regards

Peter
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Commented:
Many options here:

1) if...else if...else if...else...
2) Table lookup.  If this, write a quick/dirty program to generate your data tables rather then hand-code them.
3) Algorithm.  The above ranges, I assume, are for example only.  But your real situation most likely follows a pattern that can be modeled with an algorithm and implemented in code.

Commented:
If there are only a few ranges, you can just chain if-else statements.  If there are a lot, then store the RGB values in an array, and convert the ranges to integers (e.g. irange = static_cast<int>(range*100).  This second approach pays the cost of a float->int conversion, but it's much cleaner and easier to maintain, assuming your ranges are all hundredths apart.

Gary

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Commented:
Thank you for the above but I am not sure that this is answering my question. The suggestions above imply a single mapping from one value to another. The problem I have is to map a range of values to a single value.

Regards

Peter
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Commented:
No, they don't.  Note the multiplication by 100 in my cast:

If r is in the range [0.01, 0.02) (using the standard meaning of [ for inclusive and ) for exclusive), then r*100 will be between 1.0 and 2.0, so that static_cast<int>(r) will be 1.  That's how you get the mapping of an entire range to a single int.

Also, your ranges were (0.01, 0.02] (i.e. exclusive at the low end and inclusive at the high end).  This could be an issue, but more likely it won't matter because most of these decimals can't be represented exactly in binary anyway.

Gary

Commented:
Well

Why not simply use the integer version of the colour.

All the colours are represented by a number in the range [0, 0xFFFFFF]. If your number is in the range [0, 1) just multiply by 2^24. Then if you need to convert it to rgb it's simple.

Value = (long) (YourNumber * 0x1000000);
Red = (Value 0xFF0000) >> 16;
Green = (Value 0xFF00) >> 8;
Blue = (Value 0xFF);

If you want to convert RGB to your number then...

Value = (Red << 16) + (Green << 8) + Blue;
YourNumber = (float) Value / 0x1000000;

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