Costly function inside nested Modules [closed]

Question

The function initDataTable simply reset all data to Black first, then set the bottom right quarter to Blue.

dataTable = Table[1, 10, 10];

someCostlyFunction[] := Module[
   {a},
   
   For[i = 1, i < 1000, i++, a += i];
   a
   ];

changeColor[coord_] := Module[
   {},
   
   someCostlyFunction[]; (*If comment this line, the result is correct*)
   RGBColor[0, 0, 1]
   ];

initDataTable[] := Module[
   {},
   
   (*first reset all data to zero*)
   For[i = 1, i <= 10, i++,
    For[j = 1, j <= 10, j++,
      dataTable[[i]][[j]] = RGBColor[0, 0, 0];
     ]
    ];
   
   (*change some data based on some logics*)
   For[i = 6, i <= 10, i++,
    For[j = 6, j <= 10, j++,
       dataTable[[i]][[j]] = changeColor[{i, j}];
      ];
    ];
   ];

initDataTable[]
ArrayPlot@dataTable

The output are all black, the function call changeColor looks failed. If I comment out the code someCostlyFunction within changeColor, the data table initialization is correct.

My Mathematica project become bigger and bigger, so I need to use nested Module calls, hope someone could help me solve this problem.

Answer 1

This is called "side effect". In order to correct, it is sufficient to localize i

someCostlyFunction[] := 
  Module[{a = 0, i}, For[i = 1, i < 1000, i++, a += i];
   a];

Generally, the procedural style of the C-language that you are using is provoking such side effects. Try that your functions do not have them at all

dataTable=initDataTable[]