C# XNA Space Invaders issue -

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i'm making space invaders clone using c# in xna 4.0 , i've run couple of issues. first when shoot of invaders on right-hand side column of array top one, invader moves off-screen until next column reaches predetermined limit; entire array moves down. want still detect remaining invader. i'm pretty sure issue following section of code, i'm not sure problem is.

    (int rows = 4; rows > 0; rows--) // detects right-most invader         (int cols = 10; cols > 0; cols--)         {             if (invaderarray[rows, cols] != null)             {                 rightinvader = invaderarray[rows, cols];                 break;             }         }

the second issue if destroy 1 row of invaders 'nullreferenceexception unhandled' notification on piece of code: 

    if (rightinvader.getxpos() > 800) // right edge limit     {         invaderdir = -1;          (int rows = 0; rows < 5; rows++)             (int cols = 0; cols < 11; cols++)             {                 if (invaderarray[rows, cols] != null)                  {                     invaderarray[rows, cols].movevertical(8);                 }             }     }

again, not sure problem is. following code detecting remaining invader:

    // detecting remaining invader     bool invaderfound = false;      (int rows = 0; rows < 5; rows++)         (int cols = 0; cols < 11; cols++)         {             if (invaderarray[rows, cols] != null)             {                 invaderfound = true;                 break;             }         }

any either issue appreciated.

so might suffering off 1 errors. looking @ code, looks use lot of 'magic' numbers everywhere. first step remove 4s, 10s, 5s, , 11s, , create public constants:

static readonly int numrows = 5; static readonly int numcolumns = 11;

now, first section of code never tests 0th row or column, leading of badness you're seeing. loops written as:

for (int rows = numrows - 1; rows >= 0; rows--)     (int cols = numcols - 1; cols >= 0; cols--)

currently, loop never testing col or row 0.

edit: missed 'break' problem. easy way solve is:

bool found = false; (int rows = numrows - 1; rows >= 0 && !found; rows--)     (int cols = numcols - 1; cols >= 0 && !found; cols--)

then set found true instead of breaking. way place nested loops inside function, , return when find want.

for example, found function written:

for (int rows = 0; rows < numrows ; rows++)     (int cols = 0; cols < numcols; cols++)     {         if (invaderarray[rows, cols] != null)         {             return true;         }     } } return false;

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