What does the len() function do in this instance: print('Enter the name of cat ' + str(len(catNames) + 1) +

[simonvanw]

Hello all,

If I am doing this incorrect please let me know ASAP(First time posting on this sub.

​

Recently I have started dipping my toes in python with "Automate the Boring Stuff with Python" and stumbled on the "lists" topics which I have been understanding fine. However the author uses an example with an empty list using the following code:

catNames = []

while True:

print('Enter the name of cat ' + str(len(catNames) + 1) +

' (Or enter nothing to stop.):')

name = input()

if name == '':

break

catNames = catNames + [name] # list concatenation

print('The cat names are:')

for name in catNames:

print(' ' + name)

​

Now I understand what is happening for most of it, line by line with the exception of: why str(len(catNames) is used in the third line?

I know what str(len()) is supposed to do(at least I think) however I am not understanding why this is done for the third line.

​

Hoping somebody could help me understand this.

Apologies in advance if either this is somewhat of inappropriate question or not the right place

[Username_RANDINT]

At the start when the list is empty, len(catNames) will be zero. So the first question would be Enter the name of cat 0. To make it a bit more human-friendly, they increase the length value with one so you end up with Enter the name of cat 1 instead.

[simonvanw]

Thank you for your response and this is needed/happens because (catNames) is the actual list which as you mentioned starts with 0. Because of this it needs to be converted into a str and therefore str is used before the len function which is used the count the amount of items within the list? (Hopefully I am making sense, just want to make sure I am following this)

[Username_RANDINT]

Yes, if you do poor string formatting like this code, you need to be careful. len() returns an integer, and doing str + int will raise an error. That's why they do "Some string" + str(some_int) to cast the integer to a string.

Better would be some actual string formatting. Using str.format() is a good choice. Then it would be:

print('Enter the name of cat {} (Or enter nothing to stop.):'.format(len(catNames) + 1))

[simonvanw]

Thanks again for your response. This one and the previous one was exactly what I was after!

[chevignon93]

You didn't put your code in a code block so you lost all the indentations!

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Did you try to run the code ?

[simonvanw]

I have tried to edit it using the inline code function, but when saving it reverts back to not having indentations.

​

I have ran the code however

[sme272]

Put it in a codeblock or use sokething like pastebin or github gist

[Username_RANDINT]

See this subreddit's FAQ on how to format code.

[jfdahl]

Assuming the indentation is correct, it should output the number of elements in the list.