Wrong (strange outputs) of real numbers

[vmagnin]

Your compiler uses 32 bits real by default.

Try z1=0.7d0 to use double precision (64 bits). The last digit may still be false, but it is normal, real numbers are stored with a limited number of bits in your CPU and in binary. And for example 0.7 has an infinite number of digits in base 2:

0.7 = 0b0.10110011001100110011001100110011001100110011001101...

Note also there are better ways to declare real numbers in modern Fortran. See that discussion:

Best way to declare a double precision in Fortran?

Hi, there is several ways to declare a double precision real in Fortran: real*8 :: w real(8) :: x real(kind(0d0)) :: y double precision :: z My habits are to use double precision. Is it a good practice ?

And see also that tool:

A fun tool to explore or teach floating point numbers

I have found this site on Mastodon: Very interesting for teaching what is a floating point real! You can for example click on each bit and show how is represented Infinity or NaN, etc.