Floating point comparison `a != 0.7` [duplicate]

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In binary, 0.7 is:

b0.1011001100110011001100110011001100110011001100110011001100110...

However, 0.7 is a double-precision literal, whose value is 0.7 rounded to the closest representable double-precision value, which is:

b0.10110011001100110011001100110011001100110011001100110

In decimal, that’s exactly:

 0.6999999999999999555910790149937383830547332763671875

When you write float a = 0.7, that double value is rounded again to single-precision, and a gets the binary value:

b0.101100110011001100110011

which is exactly

 0.699999988079071044921875

in decimal.

When you do the comparison (a < 0.7), you are comparing this single-precision value (converted to double, which does not round, because all single-precision values are representable in double precision) to the original double-precision value. Because

 0.699999988079071044921875 < 0.6999999999999999555910790149937383830547332763671875

the comparison correctly returns true, and your program prints "C".

Please note that none of this is any different in C++, appearances of the code in question to the contrary. There are certain (numerically unsafe) compiler optimizations that can change the behavior, but those are not unique to C or C++.

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