stdin --> random interference --> stdout

hey wondering if you can help me

im trying to read in a file which contains 256 double floating point numbers.

im using scanf to do this.

my problem comes when i need to add random values to these numbers, which need to range between +15 and -15, at an interval of around 10-50 samples.

ive heard that 'int rand(void);' included in the library stdlib.h would be useful, but my knowledge of this function is rather limited!

any help would be gratefully appreciated. thank you