%% is turned into a single % in a call to NSLog, or -[NSString stringWithFormat:], or the printf-family of functions.
Note that %%format will become %format, even if %format usually prints an argument. For example, the code
NSLog(@"%%a will print a float in a machine-readable format, so that *scanf can read it back in from a string with no loss of precision.", 1.0f);
prints:
%a will print a float in a machine-readable format, so that *scanf can read
it back in from a string with no loss of precision.
not:
%0x1p+0 will print a float in a machine-readable format, so that *scanf can read it back in from a string with no loss of precision.