Org Mode Babel - Interactive code block evaluation

Question

I would like to evaluate in org mode source code blocks in C++ containing "cin" instructions but I can't find a way to have an interactive evaluation (with user input) during evaluation. Example of code :

#+BEGIN_SRC C++  :results output :export code :tangle myfile.cpp 
#include <iostream>
using namespace std;

int main()
{
int a;
cin>>a;
cout<<a+1;
}
#+END_SRC

Is it possible to have such an interactive evaluation or to simulate (by giving to the source code a fake input) ?

Answer 1

Like this:

#+begin_src C++ :results output :cmdline < in.txt
#include <iostream>
int main(int argc, char *argv[]) {
  int a;
  std::cin >> a;
  std::cout << a + 1;
  return 0;
}

#+end_src

#+RESULTS:
: 11

Create a file in.txt in the same directory as the Org file, with contents 10.

Answer 2

You can ask Emacs to get the interactive input instead using a named elisp block. Then pass the collected value to the C++ souce block using the :var c-variable=block-name syntax:

#+name: input_block
#+BEGIN_SRC elisp :export none :results none
(completing-read "a=" nil)

#+END_SRC

#+BEGIN_SRC C++  :results output :export code :tangle myfile.cpp :var input=input_block
  #include <stdlib.h>
  #include <iostream>
  using namespace std;

  int main()
  {
  int a = atoi(input);
  cout<<a+1;
  }
#+END_SRC 

Note that the outputs of source blocks are passed around as strings, so we have to convert it to an integer, hence the atoi and the extra #include.

Answer 3

Thanks @abo-abo for your help. I have followed your suggestion which is right and even enhanced it a little to edit only my org file (and to create automatically the external input file). The idea is to create an external code block (here a bash script named build_input_file) which creates the data file and to call it automatically before evaluating the other block thanks to the:var tmp=build_input_file.

#+name: build_input_file
#+BEGIN_SRC bash  :results none :exports none 
echo "10 12" > in.txt
#+END_SRC

#+name: my_function_with_cin
#+BEGIN_SRC C++  :results output :exports both  :var tmp=build_input_file :cmdline < in.txt
#include <iostream>
using namespace std;

int main()
{
 int a,b;
 cin>>a>>b;
 cout<<a<<" "<<b;
}
#+END_SRC

Answer 4

If you would like use :stdin like awk block, just add this to your Emacs config:

Updated: I ended up with an Emacs package: https://github.com/cxa/ob-C-stdin/tree/main

(defun org-babel-C-execute/filter-args (args)
  (when-let* ((params (cadr args))
              (stdin (cdr (assoc :stdin params)))
              (res (org-babel-ref-resolve stdin))
              (stdin (org-babel-temp-file "c-stdin-")))
        (with-temp-file stdin (insert res))
    (let* ((cmdline (assoc :cmdline params))
           (cmdline-val (or (cdr cmdline) "")))
      (when cmdline (setq params (delq cmdline params)))
      (setq params
            (cons (cons :cmdline (concat cmdline-val " <" stdin))
                  params))
      (setf (cadr args) params)))
  args)

(with-eval-after-load 'ob-C
  (advice-add 'org-babel-C-execute :filter-args
              #'org-babel-C-execute/filter-args))

Then you can write your org like:

#+name: data
24444 4444

#+begin_src cpp :results output :stdin data
...
#+end_src