Python - Simple input program is hanging at input statement

Question

I'm writing a simple program that takes 5 numbers, puts them in a list, divides each number by 2 and prints the output list.

list1 = input("Type 5 numbers: ").split()
for eachElement in list1:
    list1.append(str(int(eachElement)//2))
print("final numbers are "," ".join(list1[5:]))

PROBLEM: The program hangs after the first input line. In the terminal, it takes the 5 numbers but never goes onto the next line.

Type 5 numbers: 56 67 84 45 78

What can be the problem? I have used input with split in many other programs, but it hangs sometimes and works most of the time.

Why are you adding elements to the same list1 you are iterating over? The for loop becomes an infinite loop — Devesh Kumar Singh
– Devesh Kumar Singh, Commented Jun 18, 2020 at 15:08

L3viathan · Accepted Answer · 2020-06-18 15:07:46Z

2

You're iterating over your list and appending to it at the same time, meaning your list grows into infinity.

Observe what happens when you print something inside the loop body:

list1 = input("Type 5 numbers: ").split()
for eachElement in list1:
    val = str(int(eachElement)//2)
    print("Appending", val)
    list1.append(val)
print("final numbers are "," ".join(list1[5:]))

This prints:

Type 5 numbers: 1 2 3 4 5
Appending 0
Appending 1
Appending 1
Appending 2
Appending 2
Appending 0
Appending 0
Appending 0
...

You can fix this by putting the new numbers in a different list, first:

list1 = input("Type 5 numbers: ").split()
list2 = []
for eachElement in list1:
    val = str(int(eachElement)//2)
    print("Appending", val)
    list2.append(val)
list1.extend(list2)
print("final numbers are "," ".join(list1[5:]))

answered Jun 18, 2020 at 15:07

L3viathan

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1 Comment

adityaishwar1994 Over a year ago

Damn. Such a lame mistake. Thanks! I never saw that. Thanks a lot!

John Gordon · Accepted Answer · 2020-06-18 15:08:03Z

1

for eachElement in list1:
    list1.append(str(int(eachElement)//2))

The loop body adds more elements to list1, so the for element in ... loop will never end.

answered Jun 18, 2020 at 15:08

John Gordon

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1 Comment

adityaishwar1994 Over a year ago

Thank you John! It was a lame mistake on my part.

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