Proper use of constants in C

  • A+

I just found a bug in my code which can be simplified to:

float c; int a,b; a=5;b=6; c=(a+b)/2; 

In my debugging c has the value 5, i.e. integer division was executed.

This leads me to the question: What is a good practice when using numbers that are not stored in a variable, i.e. constant. In my case, the denominator 2.

When I'm implementing mathematical formulas that carry various constant like this, should I always put .0 at the end to make sure its internally used as a double? Does it speed up the operating time of my program? How much extra work has to be done if a constant has to be converted from int to double?

formulas like

z = 180 * 3.1415 * x  + 5*y^2 - (10*x-y)/(y+x); 

where x,y,z are doubles.

So my question is,

Is it "clean code" to write all factors with a .0 at the end ? It does decrease readability of the code quite alot, especially if the formulas are very long. But with this convention, I would prevent errors like the one discribed at the beginning.


Constants have a type at compile time, just as variables do. If a numeric constant contains a decimal point and no type suffix then its type is double. If it does not contain a decimal point and no suffix it will be one of the integer types (which one depends on the value of the constant and the range of the relevant types). So there's no speedup or slowdown associated solely with the type of the constant.

When it comes to performing calculations, integer arithmetic tends to be faster than floating point, so as a rule don't use floating point values unless you need to.


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