Nested for loops extending existing list

Question

Explain this code to me. This is a program to print integer inputs as outputs in letters. if i input 35 then output will be thirty five. i understood most of it but want to know how tens take the value from

twenty thirty forty fifty sixty seventy eighty ninety

The code is:

num = int(input("Enter the number:"))
numbers = ("zero one two three four five six seven eight nine".split())
numbers.extend("ten eleven twelve thirteen fourteen fifteen sixteen".split())
numbers.extend("seventeen eighteen nineteen".split())
numbers.extend(tens if ones == "zero" else (tens + " " + ones) 
    for tens in "twenty thirty forty fifty sixty seventy eighty ninety".split()
    for ones in numbers[0:10])

print(numbers[num])

Program is working fine no problem. But i want to know how "tens" in the code takes the value from "twenty thirty forty fifty sixty seventy eighty ninety" . Suppose if i input 34 how will tens get "thirty" from "twenty thirty forty fifty sixty seventy eighty ninety" — Krishna Nayak, Jun 26 '18 at 08:48
@khelwood just c&p'ed the code and it ran as expected without any changes whatsoever. What do you consider wrong with it? — shmee, Jun 26 '18 at 08:57

Gsk · Accepted Answer · 2018-09-04T14:25:36.167

The two main concept that you see in the last line are list comprehension and nested loops. Have a look.

for understanding better what is going on, we're going to split that line in simplier part:

TENS

for tens in "twenty thirty forty fifty sixty seventy eighty ninety".split():
    print(tens)

OUTPUT: twenty thirty forty fifty sixty seventy eighty ninety

the previous for loop correspond to this in list comprehension:

(tens for tens in "twenty thirty forty fifty sixty seventy eighty ninety".split())

without print, returns a generator. if you want to see the result:

print(list(tens for tens in "twenty thirty forty fifty sixty seventy eighty ninety".split()))

ONES

numbers = ("zero one two three four five six seven eight nine".split())
for ones in numbers[0:10]:
    print(ones)

OUTPUT: zero one two three four five six seven eight nine

the previous for loop correspond to this in list comprehension:

    (ones for ones in numbers[0:10])

MERGE FOR LOOPS

we can merge them:

for tens in "twenty thirty forty fifty sixty seventy eighty ninety".split():
    for ones in numbers[0:10]:
        print(tens + " " + ones)
OUTPUT: twenty zero
twenty one
twenty two
twenty three
twenty four
twenty five
twenty six
twenty seven
twenty eight
twenty nine
thirty zero
thirty one
thirty two
[...]

since we don't like very much twenty zero, we are going to add a clause:

for tens in "twenty thirty forty fifty sixty seventy eighty ninety".split():
    for ones in numbers[0:10]:
        if ones == "zero":
            print(tens)
        else:
            print(tens + " " + ones)
OUTPUT:
twenty
twenty one
twenty two
twenty three
twenty four
[...]

And this is the same behaviour with list comprehension:

(tens if ones == "zero" else (tens + " " + ones) for tens in "twenty thirty forty fifty sixty seventy eighty ninety".split() for ones in numbers[0:10])

which returns a list (*generator) that will be extended to your numbers list.
At last, the number that you input correspond to the index of the word in the list

Nested for loops extending existing list

1 Answers1