modelsim: Cannot open `include file "Constants.v".

was trying to use ModelSim to test my Quartus project, but was getting this error when i did "compile" -> "compile..." from ModelSim's menu.

vlog -reportprogress 300 -work work C:/lab4/ALU.v
# Model Technology ModelSim ALTERA vlog 10.1b Compiler 2012.04 Apr 27 2012
# -- Compiling module ALU
# ** Error: C:/lab4/IDEX_regs.v(56): Cannot open `include file "Constants.v".
# ** Error: C:/lab4/ALU.v(77): (vlog-2730) Undefined variable: 'constant 1'.
# ** Error: C:/lab4/ALU.v(77): (vlog-2730) Undefined variable: 'constant 2'.

verilog lets you use header files to share constants between modules.  the problem was that modelsim couldn't find where my header file / constants file was.   my constants file was in my lab4 folder with the rest of my project's files.  it was probably looking in the lab4/simulation/modelsim folder, which was the current modelsim folder.  how to help modelsim find your header files:

• in ModelSim, click "Compile" -> "Compile Options..."
• click on "Verilog & SystemVerilog" tab
• click "Include Directory..."
• move up a couple directories to get to your main project folder, or whatever folder your header files are in.
• click OK, OK

that should've been good enough, but it wasn't in my case.  i got this message:

# Modified C:\Program Files (x86)\altera\modelsim12.1\modelsim_ase\win32aloem/modelsim.ini

but then when i clicked Compile -> Compile Options again,  i saw that my changes were lost.

so, i used notepad to open the file: C:\Program Files (x86)\altera\modelsim12.1\modelsim_ase\win32aloem/modelsim.ini

one of the lines of the file said this:

OptionFile = C:/lab2/simulation/modelsim/vlog.opt

that's lab2!  it seems modelsim was permanently configured to pull options from one of my old projects.  so i changed it to say lab4 and saved.

at this point, you should be able to save changes made in Compiler Options (Compile -> Compile Options).

i used notepad to open the settings file in my lab4 folder:  C:/lab4/simulation/modelsim/vlog.opt

the contents should simply be:
+incdir+C:/lab4

hope this helped you!