Identifying the sequence in a recursive function

Question

I need to identify the sequence order in the myst(n) function when it is called to then be able to give the output of myst(4). The function is defined as follow:

def myst(n):

    if n > 1:
        myst(n - 1)

        for i in range(n):
            print(n, end='')
        print()


myst(4)

OUTPUT

22
333
4444

But i can't understand why myst(4) gives this output, hence the misunderstanding of the sequence.

What is the expected output you want to get! Mention that in the question — Devesh Kumar Singh, Apr 27 '19 at 16:03
i don't understand why it is myst(2) printed first, then myst(3), and finally myst(4). I initially thought that this function printed nothing ... before running the code that is. This question was an exercice i had, where i needed to guess the output — Charles Lamarche, Apr 27 '19 at 16:14
Because the output depends whether the print is called before or after the recursive function! Check my answer below! — Devesh Kumar Singh, Apr 29 '19 at 06:01

score 1 · Answer 1 · answered Apr 27 '19 at 16:03

Basically what is happening is that the function is recursing before it prints. So instead of printing 4444, then recursing, printing 333, etc., it recurses to the bottom-most level first, before printing the resuts. That means that the first call to the function that actually completes is the bottom-most one (the one that prints 22), then after that, the call producing the output 333 completes, until at last, the initial function call completes to print 4444.

score 1 · Answer 2 · answered Apr 27 '19 at 16:08

In your question, you are recursing before you print, the call order will be
myst(2) -> myst(3) -> myst(4)

But if you print and then recurse, the call order will change to
myst(4) -> myst(3) -> myst(2), as below.

def myst(n):

    if n > 1:

        for i in range(n):
            print(n, end='')
        print()
        myst(n - 1)


myst(4)
#4444
#333
#22

score 0 · Answer 3 · answered Apr 27 '19 at 16:05

When you call myst(x), you firstly start to calling myst(x-1) and AFTER it ends you continue to myst(x). So you call it to the myst(2) and then start to process another mysts. If you want to print it firstly, you should move recursive calling to the bottom:

def myst(n):
    if n > 1:
        for i in range(n):
            print(n, end='')
        print()
        myst(n - 1)

myst(4)

4444
333
22

Identifying the sequence in a recursive function

3 Answers3