Replace arithmetic expression by their result

Question

I am just trying to replace several arithmetic expressions inside a script file, which I cannot seem to find a simple solution for on the web. Adapting this answer did not do the trick.

Suppose I have the following, very simple file test.py:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
u1 = [13 - 1];
u2 = [ 9 - 1];
print(u2)

I already tried (from the bash):

awk '{while(match($0, /([0-9]+)\-([0-9]+)/, m)) sub(/([0-9]+)\-([0-9]+)/, m[1] - m[2]) } 1' test.py

which does not seem to do anything. The following command

perl -pe 's/([\d+)\-(\d+])/$1-$2/ge' test.py

gives

#!/usr/bin/env python3
# 0*0 coding: utf08 0*0
u1 = [13 0 1];
u2 = [ 9 0 1];
print0u20

so not really what I want, not even valid code.

Inside Vim, I selected the text in visual (block) mode and piped it into bc or bc -l using !, which gives

(standard_in) 1: syntax error

though it used to work if the whole line just was 13 - 1, instead of u1 = [13 - 1];

Does anyone have a suggestion of a working solution which I could just apply on the whole file, which would replace any occurrence of [n - 1], [nn - 1] and [ n - 1] by the corresponding arithmetic result, retaining the square brackets?

Answer 1

Here's one solution: vi[s<C-r>=eval(@")<cr><esc>. You can probably xnoremap the bit after s. How?

• vi[ select expression
• s delete it, go to Insert mode
• <C-r>=…<CR> insert result of the expression …
  • eval(@") evaluate the text in the default register, which is what we just deleted when we pressed s
• <esc> exit Insert mode

Note that this evaluates the expression like Vimscript, so beware of subtle differences and possible security holes.

---

Note that the :! filter is linewise. To use it here, I would run

:substitute/\[/&\r/ | substitute/]/\r&
k!!bc<CR>va[J

to split the brackets across lines, run the expression on its own line through bc, then rejoin all the brackets. (With tpope's surround plugin, the first line is just cS[[.