What Every Computer Scientist Should Know About Floating-Point Arithmetic
In this case, the code
printf "%0.018f\n", 0.345;
printf "%0.018f\n", (3.49 / 2) - 1.4;
outputs (on my computer, it's platform dependent)
You are getting different numbers because you are dealing with different numbers. If you need absolute precision, there are ways to handle it, but they are generally not worth the effort it takes to implement them.
#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.