I have go a problem with function in SML. This function should return list index of number which will not be summed, but was taken to sum. A call of a function: index(10, [1,2,3,4,5,6,7]) Result should be 3 (10 is a sum of numbers, we seek an index from the list which gives us 10, e.g: 1+2+3=6, 1+2+3+4=10, and return previuos one)
fun index (sum : int, numbers : int list) =
if null numbers
then 0
else if hd(numbers) > sum
then 0
else 1 + index(sum, (hd(numbers)+(hd(tl numbers)))::(tl numbers))
It seems to work, but result is wrong. Function increments the result every two calling even if it should not. Can anybody tell me how to fix this?
You're almost there. While I agree with @koodawg that adding a counter and a running total is another solution for this problem, having those in your code will complicate it more than it needs to be.
First, I have a few comments about your code. You must remove the unnecessary parens. hd(numbers) is same as hd numbers and (hd(tl numbers)) is equal to hd(tl numbers). So your (hd(numbers)+(hd(tl numbers))) could be simplified to (hd numbers + hd(tl numbers)). Also, you can combine if null numbers and if hd(numbers) > sum in a single condition for code brevity since they yield the same result: 0.
I'll try to explain how code works and I hope you'll get the idea where you have to amend your code.
Using your example, index(10, [1,2,3,4,5,6,7]), your code execution will be like this:
1)
fun index(10, [1,2,3,4,5,6,7]) =
if 1 > 10
then 0
else 1 + (10, [1 + 2] append to [2,3,4,5,6,7])
new list: [3,2,3,4,5,6,7]
result: 1
2)
fun index(10, [3,2,3,4,5,6,7]) =
if 3 > 10
then 0
else 1 + (10, [3 + 2] append to [2,3,4,5,6,7])
new list: [5,2,3,4,5,6,7]
result: 1
3)
fun index(10, [5,2,3,4,5,6,7]) =
if 5 > 10
then 0
else 1 + (10, [5 + 2] append to [2,3,4,5,6,7])
new list: [7,2,3,4,5,6,7]
result: 1
4)
fun index(10, [7,2,3,4,5,6,7]) =
if 7 > 10
then 0
else 1 + (10, [7 + 2] append to [2,3,4,5,6,7])
new list: [9,2,3,4,5,6,7]
result: 1
5)
fun index(10, [9,2,3,4,5,6,7]) =
if 9 > 10
then 0
else 1 + (10, [9 + 2] append to [2,3,4,5,6,7])
new list: [11,2,3,4,5,6,7]
result: 1
6)
fun index(10, [11,2,3,4,5,6,7]) =
if 11 > 10
then 0
result: 0
To sum all results: 1 + 1 + 1 + 1 + 1 + 0 = 5 (just like what you said that your function adds 2 to the expected result)
The correct code must behave like this:
1)
fun index(10, [1,2,3,4,5,6,7]) =
if 1 > 10
then 0
else 1 + (10, [1 + 2] append to [3,4,5,6,7])
new list: [3,3,4,5,6,7]
result: 1
2)
fun index(10, [3,3,4,5,6,7]) =
if 3 > 10
then 0
else 1 + (10, [3 + 3] append to [4,5,6,7])
new list: [6,4,5,6,7]
result: 1
3)
fun index(10, [6,4,5,6,7]) =
if 6 > 10
then 0
else 1 + (10, [6 + 4] append to [5,6,7])
new list: [10,5,6,7]
result: 1
4)
fun index(10, [10,5,6,7]) =
if 10 > 10
then 0
result: 0
To sum all results: 1 + 1 + 1 + 0 = 3 which is the expected answer.
HINT: You always make sure that the new list your function is processing must be smaller than the previous list/original list.
I hope I explained clearly why your code isn't working. I didn't include the code because I know this is a homework for an online class.
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