What do I do about a compiler error " variable 'font' must be const" I am trying to run the test program from Adafruit, with their libraries, for an Arduino controlling a Nokia5110 display and get this error message:

Arduino: 1.5.8 (Linux), Board: "Arduino Pro or Pro Mini, ATmega328 (5V, 16 MHz)"

    /home/Harry/Arduino/libraries/Adafruit_GFX/glcdfont.c:9:23: error: variable 'font' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
     static unsigned char  font[] PROGMEM = {
                           ^
    Error compiling.


Please can anyone explain this? I can happily give more details, but I am not yet sure how much is needed and the files are rather too long to include them all at this stage.
The problem is you're using an old version of the Adafruit GFX Library, which was written for a previous version of the Arduino IDE. You just need to install the latest version of the library and your problem will be solved:


Download https://github.com/adafruit/Adafruit-GFX-Library/archive/master.zip
Unzip the downloaded file.
Rename the unzipped folder Adafruit_GFX
Replace /home/Harry/Arduino/libraries/Adafruit_GFX with the folder.


If you run into any other problems after doing that I recommend trying the latest version of the Arduino IDE. The version you're using is very outdated.