bwrr bwrr - 3 months ago 8
Python Question

Why does this for loop only work if it iterates at the zeroth index?

I have a list:

oldlist = ['do gs\n','ca ts\n','monk eys\n','fro gs\n','\n','lio ns\n','tige rs\n','she ep\n']


I'm trying to extract only the first portion of the list ['do gs\n', 'ca ts\n', 'monk eys\n', 'fro gs\n'] before the newline '\n'.

This is my code and it outputs the correct solution:

oldlist = ['do gs\n','ca ts\n','monk eys\n','fro gs\n','\n','lio ns\n','tige rs\n','she ep\n']
print("Before Loop: ", oldlist)
newlist = []
for iter in oldlist:
if '\n' in iter[0]:
break
else:
newlist.append(iter)

print("After Loop: ", newlist)


However the line which begins from

if '\n' in iter[0]:


is the one which I do not understand how it works. Why does this loop only work when iter[0]? Why is it that if I just iterate over iter it outputs only a blank list?

OUTPUT if "iter[0]" used:

Before Loop: ['do gs\n', 'ca ts\n', 'monk eys\n', 'fro gs\n', '\n', 'lio ns\n', 'tige rs\n', 'she ep\n']
After Loop: ['do gs\n', 'ca ts\n', 'monk eys\n', 'fro gs\n']


OUTPUT if "iter" used :

Before Loop: ['do gs\n', 'ca ts\n', 'monk eys\n', 'fro gs\n', '\n', 'lio ns\n', 'tige rs\n', 'she ep\n']
After Loop: []


Also if I do this:

for iter in oldlist:
print(iter[0])


It spits out only d from the first string do gs\n

So in summary, what is going on behind the scenes that is allowing iter[0] to iterate through the oldlist to give the correct solution in newlist?

Answer

The in operator when used on strings checks if a given character exists at all in the string. So \n in "do gs\n" evaluates to true, because there is a newline character in this string. However, if you check for \n in "do gs\n"[0] it only checks if the first character is the newline.

It would be more clear if you simply did if iter == '\n' since that's basically what you're doing.

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