How to calculate a column of sums or test inequalities?

Question

Given

34-2
34-5
34-3

I'm looking to obtain

34-2=28
34-5=29
34-3=31

Or, this would also be helpful: given:

34-2=5
34-5<=34
34-3=31

I'm looking to obtain

!   34-2=5
    34-5<=34
    34-3=31

Where! Is some indication that inequality or equality is false. I'm looking for something that processes text files.

In the second case, it would be real easy if the equality lines used double equals signs, such as "34-2==5" instead of just a single, since a single equals sign is used for assignment in most programming languages.
I can change the signs to anything, so where it's =, I could put ==.
Also, I assumed a *nix environment since you asked about that in a separate question very recently, but it would be good if you could clarify if you want a *nix solution specifically, or some other platform or the most general solution (Perl is quite general, but for other people answering it could be good to know).

Daniel Andersson · Accepted Answer · 2012-05-11 15:35:24Z

perl -nle 'print $_."=".eval($_)' filewithexpressions

-n loops over the lines.
-l strips newlines from $_, but separates output lines with newlines for you.
-e defines an expression to be run.
$_ is a placeholder for "current line".
. concatenates strings.
eval() will take a string and evaluate it as code, so e.g. eval('5+8') returns 13.

With dual equals signs in the infile as noted in my comment to the original question:

perl -ne 'print eval($_)?" ":"!",$_' filewithequalities

As before in general.

-l is not needed/wanted since the newlines take care of themselves in this case.
The ?: construct is a ternary operator. If the preceding expression is true, return a blank space, otherwise return an exclamation mark.

jrennie · Answer 2 · 2012-05-11 15:23:53Z

Here's a python script to handle the first part of your question:

import sys
for line in sys.stdin:
    print "%s=%s" % (line.rstrip(), eval(line))

Put the above code in test.py, put the input in test.dat, and run like so:

python test.py < test.txt

I'm assuming a unix-like OS. Note that it can be dangerous to pass input to any script that runs eval---don't pass unknown data to this script.

2 Answers