Python 'for' function

Question

I am trying to write a code that returns every prime palindrome with three digits. Here is my code:

def digpalprim():
    for x in range (100,1000):
        if prime(x)=='prime':
            if str(x)==str(x)[::1]:
                return x

I've already defined the prime(x) function, it works well, that stage just determines whether x is prime or not. All in all the code works, except that it only gives me the first such a palindrome. I don't really understand why, shouldn't the program consider all the numbers between 100 and 1000? Please help?

Because as soon as you say return the program will leave the function and return to where ever it was called — user2097159
– user2097159, Commented Dec 31, 2014 at 18:35
return keyword exists from the function and gives you the result. So you can use print x if you just want to see the values of x else you have to append x to list and return that list at function level scope. — Tanveer Alam
– Tanveer Alam, Commented Dec 31, 2014 at 18:41

Martijn Pieters · Accepted Answer · 2014-12-31 18:35:48Z

8

Your function returns as soon as it finds the first such palindrome; return exits a function.

Collect your finds in a list and return that:

def digpalprim():
    palindromes = []
    for x in range (100,1000):
        if prime(x)=='prime':
            if str(x)==str(x)[::1]:
                palindromes.append(x)
    return palindromes

or you can make your function a generator by replacing the return with a yield statement:

def digpalprim():
    for x in range (100,1000):
        if prime(x)=='prime':
            if str(x)==str(x)[::1]:
                yield x

Now you'll have to iterate over this function or use list() to 'pull' all values out:

all_palindromes(digpalprim())

or

for palindrome in digpalprim():
    print(palindrome)

answered Dec 31, 2014 at 18:35

Martijn Pieters

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2 Comments

Bhargav Rao Over a year ago

Dude ... how can you type that much in 42 seconds? You must appear on Stan Lee's Superhumans ...

Bhargav Rao Over a year ago

But still ... I wanted to type the program and you had posted it half way through (So I just posted the explanation) ... It's too fast ...

Matt Smith · Accepted Answer · 2014-12-31 18:42:52Z

2

You are returning the function the first time you encounter one.

def digpalprim():
    palprimes = []
    for x in range (100,1000):
        if prime(x)=='prime':
            if str(x)==str(x)[::1]:
                palprimes.append(x)
    return palprimes

This creates a list at the start of the function and appends each valid palindrome prime to that list, then returns the entire list (after completing all loops) instead of returning just the first one encountered.

Just remember, if Python hits a return statement, it's going to stop function execution right there and return that value regardless of any additional loops or code you may intend to be executed.

answered Dec 31, 2014 at 18:42

Matt Smith

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Comments

JAL · Accepted Answer · 2014-12-31 18:35:52Z

1

The function returns and ends as soon as the first result is found.

You may wish to add the results to a list and then print out the list.

answered Dec 31, 2014 at 18:35

JAL

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Comments

Bhargav Rao · Accepted Answer · 2014-12-31 18:36:18Z

1

return x This statement causes the program to return to the calling function once this statement is encountered. To return all you may put it in an list. For e.g: you can have a list called values and append it to it, and finally return it at the end

answered Dec 31, 2014 at 18:36

Bhargav Rao

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Comments

VHarisop · Accepted Answer · 2014-12-31 18:52:21Z

0

For such small tasks, I prefer using list comprehensions:

palindromes = [x for x in range(100, 1000) if (prime(x) == 'prime') and (str(x) == str(x)[::1])]

Or (equivalently):

condition = lambda f: prime(f) == 'prime' and str(f) == str(f)[::1]
palindromes = [x for x in range(100, 1000) if condition(x)]

answered Dec 31, 2014 at 18:52

VHarisop

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