You have encountered a numeric representation issue. Roughly speaking, numeric values are stored in the binary system in the computer's memory, not in the familiar decimal system.
In your example:
0.1 (decimal) = 0.00011001100110011... (binary)
Note that the binary value is a periodic fraction. For an exact representation the "0011" would need to be repeated infinitely many times, like the 3s in 1/3=0.3333... in the decimal system. Since there are only finitely many bits available to store that value (64 bits on Windows, provided that the numeric variable is stored with the default length of 8 Bytes), the computer has no other choice but to round or truncate the infinite sequence of binary digits. But this produces a small rounding error, a numeric representation error. For the decimal number 0.1 this error is 0.4*2^-56 (on Windows), which is approx. 5.55*10^-18. Very small, but, as you know, rounding errors can accumulate easily in calculations and thus lead to surprising results, like 0.1+0.1+0.1 ne 0.3.
If you want to see how your numeric values are stored internally, use the BINARY64. or (for a shorter, but equivalent output in hexacedimal notation) HEX16. format (and not something like 32.30 or BEST32. format):
data _null_;
x=0.3;
y=0.1+0.1+0.1;
put x binary64.;
put y binary64.;
put x hex16.;
put y hex16.;
run;
Result:
0011111111010011001100110011001100110011001100110011001100110011
0011111111010011001100110011001100110011001100110011001100110100
3FD3333333333333
3FD3333333333334
There you see the difference in the least significant bit.
Indeed, as @RW9 has suggested, rounding is probably the most important strategy to avoid these errors. The appropriate rounding unit depends on the order of magnitude of the numbers to be rounded, but for most practical applications (i.e., if you're dealing with numbers from, say, 0.000001 to 1000000) I found 10^-9 (1E-9) to be suitable.
If you want to dig deeper into the technical details, you could read one of the two standard references on this subject:
Conference papers about this topic include:
I have given talks about this subject several times in one of my projects in the past and could tell you much more. So, please don't hesitate to ask further questions about it.
