Question

Possible Duplicate:
Why does 'return self' return None?

I’ve been trying to solve Problem 55 on Project Euler (http://projecteuler.net/problem=55)
and now that I think I have the answer, I Experience a problem. I don’t want a solution to the Problem 55, just on what I’ve done wrong.

Here’s my code: (I don’t think you need all of it)

t=0
lychrel=0
called=0

def iteratepal(n):
    global t
    global called
    called+=1
    b = int(''.join(reversed(str(n))))
    #print ("n =",n,"\nb =",b,"\nb+n =",b+n,"\n")

    if ispal(b+n) or ispal(n):
        t=0
        return False

    if t<50:
        t+=1
        iteratepal(b+n)
    else:                          # Here's the prob
        t=0                        # this block is executed (because it prints "yea")
        print("yea")               # but it doesn't return True!
        return True                # you can try it yourself (even in the interpreter)

def ispal(n):
    if n == int(''.join(reversed(str(n)))):
        return True
    return False

print(iteratepal(196))

for i in range(0,200):
    if iteratepal(i)==True:
        lychrel+=1
        print(i,"is Lychrel!")
    else:
        print(i,"is not a Lychrel!")
print(lychrel)

Thanks for any help, I’m really confused with this.

Answer 1

You call the function recursively when t < 50, but don’t do anything with the return value:

if t<50:
    t+=1
    iteratepal(b+n)
else:                          
    t=0                        
    print("yea")               
    return True

The else: branch is never executed then, so None is returned instead. You probably want to return the result of the recursive call:

if t<50:
    t+=1
    return iteratepal(b+n)
else:                          
    t=0                        
    print("yea")               
    return True

Some further tips:

• There is no need to test for ==True in an if statement, the following will work just fine:

  if iteratepal(i):
  

• You can return the test in def ispal(n) is itself a boolean result, just return that without testing:

  def ispal(n):
      return n == int(''.join(reversed(str(n))))