Question

I have a Ruby script that used string interpolation to build error messages.

p '#{vName} is not a defined variable'  => 'xxx is not a defined variable' 

Another programmer came through and attempted to externalize the string literals to a separate configuration file. Of course, he doesn’t get the substitution.

p err_string_from_config  => '#{vName} is not a defined variable' 

I’ve looked around, but couldn’t come up with anything better than converting to sprintf strings and using printf.

Does anybody know how to get the #{} substitution to work on strings that are not double quote literals within the Ruby script?

Answer 1

Actually Ruby has functionality very similar to John’s Python example:

$ irb >> greeting = 'hello %s, my name is %s!' >> interpolated = greeting % ['Mike', 'John'] => 'hello Mike, my name is John!' >> 

This is also useful if your argument is an array constant. If you must use #{} style interpolation you could use eval:

>> greeting = 'hi #{name}'    # notice name is not defined yet >> name = 'mike' >> eval ''' + greeting + ''' 

The eval approach is going to be much slower than using % style interpolation, so it’s a trade-off.