Combining two numbers in prolog

Question

Kindly, could you help me in the following: I am writing a Prolog program that takes two numbers digits then combine them as one number, for example: ``` Num1: 5 Num2: 1 ``` Then the new number is 51. Assume `V1` is the first number digit and `V2` is the second number digit. I want to combine `V1` and `V2` then multiply the new number with `V3`, so my question is how I can do it? ``` calculateR(R, E, V1, V2, V3, V4):- R is V1 V2 * V3, E is R * V4. ``` Your help is appreciated.

Accepted Answer

Edit: In a comment, @false pointed out that this answer is SWI-Prolog specific.

You can achieve your desired goal by treating the numerals as atoms and concatenating them, and then converting the resultant atom into a number.

I'll use `atom_concat/3` to combine the two numerals. In this predicate, the third argument with be the combination of atoms in its first and second arguments. E.g.,

```
?- atom_concat(blingo, dingo, X).
X = blingodingo.
```

Note that, when you do this with two numerals, the result is an atom not a number. This is indicated by the single quotes enclosing the the result:

```
?- atom_concat(5, 1, X).
X = '51'.
```

But `51 \= '51'` and we cannot multiply an atom by number. We can use `atom_number/2` to convert this atom into a number:

```
?- atom_number('51', X).
X = 51.
```

That's all there is to it! Your predicate might look like this:

```
calculateR(No1, No2, Multiplier, Result) :-
    atom_concat(No1, No2, NewNoAtom),
    atom_number(NewNoAtom, NewNo),
    Result is NewNo * Multiplier.
```

Usage example:

```
?- calculateR(5, 1, 3, X).
X = 153.
```

Of course, you'll need more if you want to prompt the user for input.

I expect @Wouter Beek's answer is more efficient, since it doesn't rely on converting the numbers to and from atoms, but just uses the assumption that each numeral is a single digit to determine the resulting number based on their position. E.g., if 5 is in the 10s place and 1 is in the 1s place, then the combination of 5 and 1 will be `5 * 10 + 1 * 1`. The answer I suggest here will work with multiple digit numerals, e.g., in `calculateR(12, 345, 3, Result)`, `Result is 1234 * 3`. Depending on what you're after this may or may not be a desired result.

Combining two numbers in prolog

Solution

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