python recursive function to compute sum of the elements of a list

Question

I’m having difficulty in an exercise where I have to show the sum of all the elements of a list recursively.

The code I arrived has only the basis of the recursion, the recursion itself did not do, because I did not understand how it could be applied to some list.

I’ll leave the code here for you to analyze:

def soma_lista(lista):
if len(lista) == 1:
    return lista[0]
else:
    for i in range (len(lista)):
        soma = lista[i] + soma_lista(lista[i + 1])
    return soma

NOTE: as I said, I don’t know how to apply the recursion part; so I tried something random. Ignore the Else code block.

Answer 1

A recursion, in this case, would play the role of the repeat loop, so you don’t need to use both. Your recursive stop condition is correct: if the list has only one element, return it. No else, instead of the loop, you should return only the first element added with the return of the function to the rest of the list:

return lista[0] + soma_lista(lista[1:])

Where lista[1:] returns the list from the first element. That is, the sum of a list will be equal to the first element added with the result of the sum of the rest of the list and thus recursion is made:

def soma_lista(lista):
    if len(lista) == 1:
        return lista[0]
    else:
        return lista[0] + soma_lista(lista[1:])

See working on Ideone.

Answer 2

Recursion is the technique where a method or function calls itself to arrive at the result.

I implemented this in pythonic form:

def soma_lista(lista=list()):
    '''
    Calcula a somatória dos elementos de uma lista.

    :param lista: lista de elementos inteiros
    :return: int
    >>> soma_lista([1,2,3])
    6
    >>> soma_lista([1,2,3,4])
    10
    >>> soma_lista([1,2,3,4,5,6])
    21
    '''
    try:
        return lista.pop() + soma_lista(lista)
    except IndexError:
        return 0

if __name__ == '__main__':
    import doctest
    doctest.testmod()
    print()

And this one in a conventional way:

def soma_lista(lista=list()):
    '''

    :param lista:
    :return:
    >>> soma_lista([1,2,3])
    6
    >>> soma_lista([1,2,3,4])
    10
    >>> soma_lista([1,2,3,4,5,6])
    21
    '''
    if len(lista) >= 1:
        return lista.pop() + soma_lista(lista)
    else:
        return 0

if __name__ == '__main__':
    import doctest
    doctest.testmod()
    print()

Answer 3

A way that I consider to be well didactic to show the "mysteries" of recursion in this case:

def recl(l, i=0):
    try:
        elmnt = l[i]
        i+=1
        return elmnt + recl(l,i)     
    except:
        return 0

print (recl([1,2]))   
3

Run the code in repl.it.