Using dynamic val in vim substitution

gistfile1.txt

BACKGROUND

Was using a file with a bunch of named data points [1].  Didn't 
want to edit the name of each data point to repurpose it for my 
prototyping efforts but needed a command mode substitution to 
iterate over a visual block and do the replacement but with a 
dynamic expression (line number is adequate.)

Found [2] and then figured out I'd need something like printf() 
for use in eval() so stumbled upon [3].

SOLUTION 

:'<,'>s/"name":".*"/\=eval("printf('%s','\"name\":\"foo') . printf('%s', line('.')) . '\"'")/ 

REFERENCES

[1] : http://bost.ocks.org/mike/sankey/energy.json
[2] : http://vim.wikia.com/wiki/Using_an_expression_in_substitute_command
[3] : http://vim.1045645.n5.nabble.com/what-is-vim-s-printf-as-it-is-in-c-td4973892.html