function variable = getVariableFromStateEquation(variableName, state, gas)
  stateHasSpecificVolume   = isfield(state, "v") ;
  variableIsSpecificVolume = isequal(variableName, "v") ;
  specificVolume = variableIsSpecificVolume || stateHasSpecificVolume ;
  if specificVolume
    equation.variableNames = ["p" "v" "R" "T"] ;
    equation.exponents     = [ 1   1  -1  -1 ] ;
  else
    equation.variableNames = ["p" "V" "m" "R" "T"] ;
    equation.exponents     = [ 1   1  -1  -1  -1 ] ;
  end
  allVariables = joinStructs(state, gas) ; 
  variable = getVariableFromAlgebricEquation(variableName, allVariables, equation) ;
end