Python 列表和 for 循环

Question

a = ["a","b","c","d","e","f"]
for i in range(len(a)):
    print(i)

The output of this code is: 0 1 2 3 4 5

How can I modify my code so that the output will be: 1 2 3 4 5 or only 2 3 4 without changing the values of the a variable ? I just want to extract some parts of the iteration and not the whole.

Answer 1

I just want to extract 1 2 3 4 5 or 2 3 4 in my code without altering the aforementioned array. So what I did was this:

a = ["a","b","c","d","e","f"]
for i in range(len(a)):
    if i == 0:
        continue
    print(i)

Output: 1 2 3 4 5 without the 0

And if I only want to extract 2 3 4 what I did was this:

a = ["a","b","c","d","e","f"]
for i in range(len(a)):
    if i < 2:
        continue
    elif i == 5:
        continue
    print(i)

Output: 2 3 4

Answer 2

You can try using slicing:

left to right indexing works as 0 to len(a)-1
and right to left is -1 to -len(a)

Syntax is:

a[start_index:end_index]

Above can give you elements as output then you can apply index function of list to get the index values.

NOTE: This approach will work if you want to extract an intact part of a list. It won't iterate on whole list, instead only on the part you want to extract.

While giving slicing remember end_index is not included.

For extracting 1,2,3,4,5:

for i in a[1:]:
    print(a.index(i))

#or 

For extracting 2,3,4:

for i in a[2:5]:
    print(a.index(i))

Answer 3

As you mentioned you are new to this, then let's understand what's happening first then we can modify it according to our requirements. for i in range(len(a)): The range() function is used to generate a sequence of numbers. here,from (0 to len(a)-1) and here len(a)=6 so your for loop is iterating from 0 to 5. Now if we wanna modify our output say we want 2,3,4 then we can provide an 'if' statement to select any particular subset ie

for i in range(len(a)):
    if(i>1) & (i<5):
        print(i)

or, you can try slicing if u are comfortable with: for output as 2,3,4

print(a[2:5])

remember a[starting:ending+1]