Error in simulation verification

The Quartus native simulator can use a couple of types of text files for inputs, but they might not be much easier than a waveform file to create manually. You might be better off using a test bench and a third-party simulator or ModelSim-Altera Edition. The test bench can both specify the input stimulus and check the results. (The Quartus native simulator can't use a test bench.)

If you want to continue with the Quartus native simulator, there might be an easier way to create the "lots of values to set for 'a' or 'b'". To test all possible combinations of a and b, you can group them and give them a count sequence. Select a and b, right click, and select "Grouping --> Group". Select the resulting group, right click, and select "Value --> Count Value".

If a and b are individual signals (not buses), another method without a group is to overwrite a and b individually with clock values of different periods that together produce all possible combinations of a and b values. If they are buses, overwrite a and b separately with count values, but use different count periods for a and b so that a and b together produce all combinations.