Understanding this simple code

Question

I'm a total beginner in programming. My question is what actually this code does. I think it takes the whole division result from 10, and then calculates mod 10, and prints it. The res here I think is not a built-in command in Python, and is just a variable used here, but its value is zero.

Here's the code in question:

res=0
num=int(input("Enter a postive integer:  "))
while num > 0:
    res=res+(num % 10)
    num = num // 10
    print(res)

Did you run the code? [This looks similar](http://stackoverflow.com/questions/14939953/sum-the-digits-of-a-number-python#14940026) — OneCricketeer, Nov 12 '16 at 20:35
It's a given code which is part of homework. I did run it, and that's what I think it does, but if that's true, how come the results can be greater than 9. — Shalom .I, Nov 12 '16 at 20:37
It is, *outside the loop*. It then gradually gets incremented. — Dimitris Fasarakis Hilliard, Nov 12 '16 at 20:39
But how? I thought it gets the original "res" value (which is zero), and then it's added by the num % 10. So, how does it get greater than 9? Sums the digits? But still I don't understand, if that's the case, is still should'nt be greaer than nine. — Shalom .I, Nov 12 '16 at 20:40
because res can have some initial value after the first loop. — Natecat, Nov 12 '16 at 20:41
You seem to not understand how loops work. Is your question correctly indented? You should see the print statement increasing the value — OneCricketeer, Nov 12 '16 at 20:42
mod 10 is only applied to `num` not `res` so it can go above 9 — Steven Summers, Nov 13 '16 at 03:32

OneCricketeer · Answer 1 · 2016-11-12T21:08:33.357

0

Try running this.

res=0
num=int(input("Enter a postive integer:  "))
while num > 0:
    print("res = {0} + {1}".format(res, num % 10))
    res = res + (num % 10)
    num = num // 10
print(res)

In words, while num > 0, the digits of num are summed from right-to-left.

res starts at 0, and increases each time over the loop.

edited Nov 12 '16 at 21:08

answered Nov 12 '16 at 20:45

OneCricketeer

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Thanks, now I understand what it does. But I don't actually understand how it does that. I mean, which commnand here means "sums of all digits"? The "num // 10" in Python gives me the whole number result which is divised by ten, and "num % 10" gives me the remainder of the division number by ten. So what exactly here tells the program to sum up the digits? – Shalom .I Nov 12 '16 at 21:05
`res` is an integer. `+` adds things. `res = res +` adds onto the existing `res` value and can be re-written as `res += ` – OneCricketeer Nov 12 '16 at 21:07

Understanding this simple code

1 Answers1