Is there anything wrong with syntax of "for" loop in my list comprehension?

Question

If this is correct

v = [22, 13, 45, 50, 98, 69, 43, 44, 1]
[ (x+1 if x >=45 else x+5)  for x in v ]

then what's wrong with this code?:

def isPrime(num):
is_prime = False
if num > 1:
    for i in range(2, num):
        if (num % i) == 0:
            is_prime = False
            break
    else:
        is_prime = True
else:
    is_prime = False
return is_prime


listTablesOnlyPrimes = [[ (i*num if isPrime(i*num)==True else pass) for i in range(1, 11)] for num in range(1, 11)]
for listTable in listTablesOnlyPrimes:
    print(listTable)

I want to know what's the SyntaxError in message by interpreter:

listTablesOnlyPrimes = [[ (i*num if isPrime(i*num)==True else pass) for i in range(1, 11)] for num in range(1, 11)]
                                                                  ^
SyntaxError: invalid syntax

The ternary `if` operator requires an `else`. – Klaus D. Sep 11 '20 at 18:45 — Klaus D., Sep 11 '20 at 18:45

Kenny Ostrom · Answer 1 · 2020-09-12T00:10:48.120

1

This uses a conditional expression, of the form https://docs.python.org/3/reference/expressions.html#conditional-expressions

(x+1 if x >=45 else x+5)

But this does not fit that syntax because there's no else:

(i*num if isPrime(i*num)==True)

I believe that was intended to be an if condition as part of the list comprehension syntax, but that comes at the end, as described here https://docs.python.org/3/reference/expressions.html#displays-for-lists-sets-and-dictionaries

addendum:

not part of the question, but since you asked, here's a way to quickly add some unit testing to your code. Just put this right after your isPrime function

if not isPrime(11):
    raise ValueError("isPrime isn't right")

You do need a few more checks, but that is enough for now. I don't want to go into the whole unit test library.

edited Sep 12 '20 at 00:10

answered Sep 11 '20 at 18:46

Kenny Ostrom

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I've actually tried that one as well but still ain't working. If you can share your working code script I will really be grateful.Thanks for replying sir. – Coder Sep 11 '20 at 20:21
That was the syntax error. Your isPrime function always returns false, but at least it's not a syntax error, which was the question. And the other numbers are all a product of two integers, so they have a pretty good chance to be not prime. A few of them actually should be prime like 1*7 for example, but useful trivia -- 1 itself is not prime. – Kenny Ostrom Sep 12 '20 at 00:02
There are plenty of other questions on how to test if a number is prime. You can look at those. – Kenny Ostrom Sep 12 '20 at 00:17

deadshot · Answer 2 · 2020-09-11T18:43:00.293

0

Problem is with if condition inside the nested list

listTablesOnlyPrimes = [[ (i*num) for i in range(1, 11) if isPrime(i*num)==True] for num in range(1, 11)]

Note:

Your code always return empty lists I don't understand why are you doing this i * num never become prime if i > 1

edited Sep 11 '20 at 18:43

answered Sep 11 '20 at 18:36

deadshot

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Is there anything wrong with syntax of "for" loop in my list comprehension?

2 Answers2