Benchmarks to evaluation of exam programs:

• [45=] Full satisfaction of question requirements:
  • [+32] correct math for "any" number of charges
  • [+2] validation, including: (a) observation point at charge location;   (b) number of charges <=0 or greater than the program can afford
  • [+10] user interface allowing to calculate the potential at multiple locations without re-entering coordinates and magnitudes of charges
  • [-0] designated named variable or symbolic constant for parameter A   (-0.5 if value is typed directly in formula)
  • [+1] structure and style are up to a 100%-worth program


• [max of 42] if multi-point interface is not implemented:
  • [+32] math
  • [+2] validation
  • [+5] user interface allowing to enter all the parameters
  • [+3] structure, style, advanced programming techniques (e.g. C++ classes)


• [33] Simple Passing-Guaranteed program: no user interface, no advanced features, hard-coded non-special set of data (necessary to demonstrate correct summation).


• [22] Basic program not qualifying to the above, but working correctly in the single charge case (fixed data, no interface)
  • [up to +10] for UI and good programming practices

• -10%   failure to write any program information on exam sheet
• -5%   failure to write program location on exam sheet
• -10%   missing title comments
• -12%   title comments with wrong authorship
• -25%   uncompilable program, if error causing uncompilability is easy to fix (otherwise, point loss is higher)
• -0.5..-3 pts   syntax and programming errors
• -5 pts   failure to organize summation loop

In the posted solutions, Sergey's passing-guaranteed program would be worth 33.5 pts (0.5 for debugging output, which is a good programming practice here, because the program is not "finished").
Shannon's program would get the full mark of 45   (-0.5 for unneccesarily long main() offset by demostration of good skills in working with files).