[Qgis-developer] iteration over raster

[Andruit at gmx.de]

Hello,
I would like to write a qgis python plugin, which modifies cell values based on the values of their surrounding neighbors.

But how can I iterate over a raster layer in order to read the value of each raster cell (and its surrounding neighbors)?
Is there any method querying raster cell values

for i in range (1, numberofrows)
  for j in range (1, numberofcolumns)
     
       value=???  #read cell value of cell[i j]


Thank you very much in advance. 
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