Question

I have a file that has two columns of floating point values. I also have a C program that takes a floating point value as input and returns another floating point value as output.

What I’d like to do is the following: for each row in the original, execute the C program with the value in the first column as input, and then print out the first column (unchanged) followed by the second column minus the result of the C program.

As an example, suppose c_program returns the square of the input and behaves like this:

$ c_program 4
16
$

and suppose data_file looks like this:

1 10
2 11
3 12
4 13

What I’d like to return as output, in this case, is

1 9
2 7
3 3
4 -3

To write this in really sketchy pseudocode, I want to do something like this:

awk ‘{print $1, $2 – `c_program $1`}’ data_file

But of course, I can’t just pass $1, the awk variable, into a call to c_program. What’s the right way to do this, and preferably, how could I do it while still maintaining the “awk one-liner”? (I don’t want to pull out a sledgehammer and write a full-fledged C program to do this.)

Answer 1

you just do everything in awk

awk '{cmd="c_program "$1; cmd|getline l;print $1,$2-l}' file