Why does this program work in Python 2.7.6 but not in Python 3?

Question

I'm doing the problem described here:

I understand the code has this error in python 3:

File "main.py", line 3, in <module>
    Test.assert_equals(seven (1603), (7, 2))
  File "/home/codewarrior/solution.py", line 7, in seven
    newM = int(s[:(len(s)-1)]);
ValueError: invalid literal for int() with base 10: '-'

Where/Why is their a difference in evaluation?

def seven(m):
    count=0;
    while(m/100 != 0):
        s = str(m);
        lastdigit = int(s[len(s)-1]);
        if(len(s)>1):
            newM = int(s[:(len(s)-1)]);
        count+=1;
        m = newM - 2*lastdigit;

    return (m,count);

check `m`. A division probably made that a float when it was an integer in python 2 — Jean-François Fabre, Nov 30 '18 at 16:37
Well for starters `7/2` is `3` in 2.7 (integer division) and `3.5` in 3 (float division) — Cory Kramer, Nov 30 '18 at 16:38
`while(m/100 != 0):` should be `while(m//100 != 0):` to be faithful to python 2 integer division — Jean-François Fabre, Nov 30 '18 at 16:39
a bit far fetched duplicate if you ask me... OP could avoid to divide in the first place — Jean-François Fabre, Nov 30 '18 at 16:44
BTW, that code looks a bit unpythonic, with all those unnecessary semicolons. It also has 2 sets of redundant parentheses. — PM 2Ring, Nov 30 '18 at 16:44
@Jean-FrançoisFabre Fair points. And that's why I didn't hammer it straight away. I closed it because I don't think we need more than 2 answers here. But feel free to reopen it if you disagree. — PM 2Ring, Nov 30 '18 at 16:47
@PM2Ring not going to reopen, no. question isn't good enough for that. — Jean-François Fabre, Nov 30 '18 at 18:09

score 1 · Answer 1 · answered Nov 30 '18 at 16:40

1

Because the while loop does not have the same stop condition. In Python2 m/100 returns the truncated result as an integer, while in Python3 it returns the exact result as a float

answered Nov 30 '18 at 16:40

Louis Saglio

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Jean-François Fabre · Accepted Answer · 2018-11-30T18:10:15.670

this has different behaviour in python 2 or 3

while(m/100 != 0):

In python 2, if m < 100 the result is 0 and the loop stops.

In python 3, it doesn't.

Quickfix is to always use integer division

while m//100 != 0:  # or just: while m//100:

this will work the same (and properly as you expect) on any python version

At this point you could save a division and test absolute value of m instead of dividing and dropping the result:

while abs(m) >= 100:

Why does this program work in Python 2.7.6 but not in Python 3?

2 Answers2