Stark/Cairo Tutorial 3-Array_sum

%builtins output

from starkware.cairo.common.alloc import alloc
from starkware.cairo.common.serialize import serialize_word

# Computes the sum of the memory elements at addresses:
#   arr + 0, arr + 1, ..., arr + (size - 1).
func array_sum(arr : felt*, size) -> (sum):
    if size == 0:
        return (sum=0)
    end

    # size is not zero
    let (sum_of_rest) = array_sum(arr=arr + 1, size=size - 1)
    return (sum=[arr] + sum_of_rest)
end

func main{output_ptr : felt*}():
    const ARRAY_SIZE = 3

    # Allocate an array
    let (ptr) = alloc()

    # Populate some values in the array
    assert [ptr] = 9
    assert [ptr + 1] = 16
    assert [ptr + 2] = 25

    # Call array_sum to compute the sum of the elements.
    let (sum) = array_sum(arr=ptr, size=ARRAY_SIZE)

    # Write the sum to the program output.
    serialize_word(sum)

    return ()
end

Array sum explanation is here.

Main points

• Recursion instead of loops, because memory assignments are immutable