Myujth

fractions (0.9) and (0.2) make a truncation error when representing in float or double
so if we represent 0.9 or 0.2 two times:

• the first one in float variable


• the second one in double variable


• it's mean that one of them is greater than the other, and the biggest is the double variable

so if we subtract the double from the float, it is expected that the result will be positive

why this code printed ( "Positive" )

float afloat = 0.9; 

if ( 0.9 - afloat > 0 ) cout << "Positive" << endl;
else if ( 0.9 - afloat == 0 ) cout << "Equal" << endl;
else if ( 0.9 - afloat < 0 ) cout << "Negative" << endl;

while this code printed ("Negative") ??

float afloat = 0.2; 

if ( 0.2 - afloat > 0 ) cout << "Positive" << endl;
else if ( 0.2 - afloat == 0 ) cout << "Equal" << endl;
else if ( 0.2 - afloat < 0 ) cout << "Negative" << endl;

The literal 0.9 is a double precision value approximation to 9/10, and your declaration

float afloat = 0.9

truncates this value to single precision, thereby losing information. This code

float afloat = 0.9f; 

if ( 0.9f - afloat > 0 ) cout << "Positive" << endl;
else if ( 0.9f - afloat == 0 ) cout << "Equal" << endl;
else if ( 0.9f - afloat < 0 ) cout << "Negative" << endl;

will generate the output

Equal

even if 0.9f is not exactly equal to the fraction 9/10. The sign of the quantization error will differ for different fractions, and for different floating point formats.