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# Absolute value

## Absolute value

(OP)
So this is my 2nd week learning prolog.

I wrote a function to calculate the sum of a list of numbers.
sum([], 0).
sum(Tail, Sum1),

So:
input: ?- (sum([4,3,-5],X))
output: X=2

However, the goal is get the absolute value of the sum of the list.
So the correct output should be: X=12

I've tried different things with the "abs()" like abs(sum([4,3,-5],X)) or modify the program with putting in some "abs" but didn't work
Where should I use it?

Replies continue below

### RE: Absolute value

Maybe TotalSum is abs(Head) + Sum1.

### RE: Absolute value

(OP)
ERROR: toplevel: Undefined procedure: sum/2 (DWIM could not correct goal)

### RE: Absolute value

With SWI-Prolog (and other dialects I think), you must type your code in a file and compile it to use the predicate sum at toplevel.

### RE: Absolute value

(OP)
I don't know what I did, but it works now. Thanks! All I did was re-save the program I did using notepad.

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