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Can someone explain me how to choose the precision of a float with a C function?

Examples:

theFatFunction(0.666666666, 3) returns 0.667

theFatFunction(0.111111111, 3) returns 0.111

AtilioA
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Zat42
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    possible duplicate of [Rounding Number to 2 Decimal Places in C](http://stackoverflow.com/questions/1343890/rounding-number-to-2-decimal-places-in-c) – MK. Feb 09 '12 at 15:11

6 Answers6

15

You can't do that, since precision is determined by the data type (i.e. float or double or long double). If you want to round it for printing purposes, you can use the proper format specifiers in printf(), i.e. printf("%0.3f\n", 0.666666666).

Dan Fego
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You can't. Precision depends entirely on the data type. You've got float and double and that's it.

Joe
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Floats have a static, fixed precision. You can't change it. What you can sometimes do, is round the number.

See this page, and consider to scale yourself by powers of 10. Note that not all numbers are exactly representable as floats, either.

unwind
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It might be roughly the following steps:

  1. Add 0.666666666 with 0.0005 (we get 0.667166666)
  2. Multiply by 1000 (we get 667.166666)
  3. Shift the number to an int (we get 667)
  4. Shift it back to float (we get 667.0)
  5. Divide by 1000 (we get 0.667)

Thank you.

Krishnananda. K. Hegde
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Most systems follow IEEE-754 floating point standard which defines several floating point types.

On these systems, usually float is the IEEE-754 binary32 single precision type: it has 24-bit of precision. double is the binary64 double precision type; it has 53-bit of precision. The precision in bit numbers is defined by the IEEE-754 standard and cannot be changed.

When you print values of floating point types using functions of the fprintf family (e.g., printf), the precision is defined as the maximum number of significant digits and is by default set to 6 digits. You can change the default precision with a . followed by a decimal number in the conversion specification. For example:

printf("%.10f\n", 4.0 * atan(1.0));  // prints 3.1415926536

whereas

printf("%f\n", 4.0 * atan(1.0));     // prints 3.141593
ouah
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0

Precision is determined by the data type (i.e. float or double or long double).

If you want to round it for printing purposes, you can use the proper format specifiers in printf(), i.e. 

printf("%0.3f\n", 0.666666666)  //will print 0.667 in c

Now if you want to round it for calculating purposes you have to first multiply the float by 10^number of digits then typecast to int , do the calculation and then again typecast to float and divide by same power of 10

float f=0.66666; 
f *= 1000;  // 666.660
int i = (int)f; // 666
i = 2*i;  //  1332
f = i;   //  1332
f /= 1000;  // 1.332
printf("%f",f);  //1.332000
abe312
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