Python2: Recursion doesn't work when I put print statement in function but works otherwise

Question

So I am doing minimum coin change problem recursively in python. I wanted to visualize how many recursive calls it's making. So I put a "print 'recursion'" in my function. But now it's continuously print recursion and not reaching output and program is not even stopping. Here is the program.

def min_coins(coins, change):
    print 'recursion' #program works without this line
    mincoins = change
    if change in coins:
        return 1
    else:
        for i in coins:
            if i<=change:
                numcoins = 1 + min_coins(coins, change-i)
                if(numcoins < mincoins):
                    mincoins = numcoins
    return mincoins

c = [1,5,25]
d = 50

print min_coins(c, d)

Works for me, can't reproduce. And this should be the case - the print can't affect that code. Perhaps you are not waiting long enough to see the final output, or it is being lost in a sea of "recusion" lines? — mhawke
– mhawke, Commented Feb 1, 2016 at 10:40
If you think about the problem you are trying to solve you should be able to come up with a much better solution that only needs a fraction of the recursive calls. — Duncan
– Duncan, Commented Feb 1, 2016 at 10:54
@Duncan Yes now I am working on dynamic program. But wanted to make a recursive program first. But even this recursive program is incorrent as for coins = [5,25] change = 56 it is showing 4 coins. — ailhahc
– ailhahc, Commented Feb 1, 2016 at 11:01
Recursive is fine (my first attempt is 2395 recursive calls for your original problem). Hint: remember all the results you've calculated so far and then a new value is either a single coin or the minimum of all the ways of combining two smaller amounts. — Duncan
– Duncan, Commented Feb 1, 2016 at 11:06

Karoly Horvath · Accepted Answer · 2016-02-01 10:42:12Z

6

Consoles are quite slow, and you weren't patient enough:

python your_program.py | wc -l
684886

There are better ways, use a global counter variable or return a pair of (mincoins, counter).

answered Feb 1, 2016 at 10:42

Karoly Horvath

96.7k11 gold badges123 silver badges181 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

ailhahc Over a year ago

Okay thanks for clarifying. But when I ran the code without print statement the output came in less than 2 seconds. But when I added print it's running for much more time. Why is it so?

Duncan Over a year ago

@ailhahc because Python can do 684886 recursive calls in under 2 seconds but not when it has to write the output to the console as well. print is slow!

Ogaday Over a year ago

It might be worth printing a dynamic variable when debugging, perhaps printing coins and change at each recursion to see the state of your program.

Karoly Horvath Over a year ago

@Duncan: nope. On my computer, print only doubles the execution time (when output is redirected). As I said, consoles are notoriously slow.

Collectives™ on Stack Overflow

Python2: Recursion doesn't work when I put print statement in function but works otherwise

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related