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I have an arithmetic expression containing Var, and I need to match it to the following format:

Step * Var + Offset

If it can be represented in this form, I need to get Step and Offset values. If not, I need to know that too.

So far I tried matching expression trees, i. e.

Expression = ["+", ["*", Var, Step], Offset] % and other variations

and using CLP(FD):

78 ?- 2*Ind+3 #= Step*Ind+Offset, Ind in inf..sup.            
Step*Ind#=_A,
2*Ind#=_B,
_B+3#=_C,
_A+Offset#=_C.

However, the first approach would require simplifying expressions (for example, Var+5-3) and handling multiple different cases, and the second does not work as I expected it to.

Am I missing something? Is there a simpler way, or is this problem too complex?

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  • 1
    Where is the expression created? Sounds like it would be easier to control its format at the point of creation. Commented Nov 5, 2023 at 12:59
  • ?- Step*Var + Offset = 5*10+20. gives Step = 5, Var = 10, Offset = 20. Prolog terms are not nested lists like your expression tree, they are nested compound terms; ?- +(*(Step,Var),Offset) = 5*10+20. Commented Nov 5, 2023 at 13:26
  • @brebs these expressions are initially created by users, and I cannot add any format restrictions, unfortunately. Commented Nov 7, 2023 at 3:51
  • @TessellatingHeckler I don't match Prolog terms, I convert expressions to the same tree format. (in my expample, Expression would be something like ["*", Var, 2]). Comparing Prolog terms would not work for expressions that don't follow the Step*Var+Offset structure for the same reason (for example ?- (2*Var+1)-1 = Step*Var+Offset. is false). Commented Nov 7, 2023 at 4:01
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