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Asked: 2026-05-11T06:59:29+00:00

I have a (somewhat) large truth table / state machine that I need to

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I have a (somewhat) large truth table / state machine that I need to implement in my code (embedded C). I anticipate the behavior specification of this state machine to change in the future, and so I’d like to keep this easily modifiable in the future.

My truth table has 4 inputs and 4 outputs. I have it all in an Excel spreadsheet, and if I could just paste that into my code with a little formatting, that would be ideal.

I was thinking I would like to access my truth table like so:

u8 newState[] = decisionTable[input1][input2][input3][input4];

And then I could access the output values with:

setOutputPin( LINE_0, newState[0] ); setOutputPin( LINE_1, newState[1] ); setOutputPin( LINE_2, newState[2] ); setOutputPin( LINE_3, newState[3] );

But in order to get that, it looks like I would have to do a fairly confusing table like so: 

static u8 decisionTable[][][][][] =  {{{{ 0, 0, 0, 0 },     { 0, 0, 0, 0 }},    {{ 0, 0, 0, 0 },     { 0, 0, 0, 0 }}},   {{{ 0, 0, 1, 1 },     { 0, 1, 1, 1 }},    {{ 0, 1, 0, 1 },     { 1, 1, 1, 1 }}}},  {{{{ 0, 1, 0, 1 },     { 1, 1, 1, 1 }},    {{ 0, 1, 0, 1 },     { 1, 1, 1, 1 }}},   {{{ 0, 1, 1, 1 },     { 0, 1, 1, 1 }},    {{ 0, 1, 0, 1 },     { 1, 1, 1, 1 }}}};

Those nested brackets can be somewhat confusing — does anyone have a better idea for how I can keep a pretty looking table in my code?

Thanks!

Edit based on HUAGHAGUAH’s answer:

Using an amalgamation of everyone’s input (thanks — I wish I could ‘accept’ 3 or 4 of these answers), I think I’m going to try it as a two dimensional array. I’ll index into my array using a small bit-shifting macro:

#define SM_INPUTS( in0, in1, in2, in3 ) ((in0 << 0) | (in1 << 1) | (in2 << 2) | (in3 << 3))

And that will let my truth table array look like this:

static u8 decisionTable[][] = { { 0, 0, 0, 0 }, { 0, 0, 0, 0 }, { 0, 0, 0, 0 }, { 0, 0, 0, 0 }, { 0, 0, 1, 1 }, { 0, 1, 1, 1 }, { 0, 1, 0, 1 }, { 1, 1, 1, 1 }, { 0, 1, 0, 1 }, { 1, 1, 1, 1 }, { 0, 1, 0, 1 }, { 1, 1, 1, 1 }, { 0, 1, 1, 1 }, { 0, 1, 1, 1 }, { 0, 1, 0, 1 }, { 1, 1, 1, 1 }};

And I can then access my truth table like so:

decisionTable[ SM_INPUTS( line1, line2, line3, line4 ) ]

I’ll give that a shot and see how it works out. I’ll also be replacing the 0’s and 1’s with more helpful #defines that express what each state means, along with /**/ comments that explain the inputs for each line of outputs. Thanks for the help, everyone!

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1 Answer

  1. No need for a multidimensional table. With a 4 bit => 4 bit mapping, you can have a single u8[16] array mapping inputs to outputs. State lookups will be much cheaper, and you can extract individual bits with some shift-and-mask ops.

    If the algorithm to populate rows is easy to codify, you could #define a macro to populate each row by index number.

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