How these python for loop and slicing working?
Question:
-
How this statement will work?
j for i in range(5)
Example:
x1=(j for i in range(5))
will yield five 4’s when iterated. Why it will not yield 01234 like when we replace j with i.
-
How this statement working?
a = [0, 1, 2, 3]
for a[-1] in a:
print(a[-1])
0
1
2
2
Answers:
Your first example is a generator expression (the parentheses are required, so the first version you show doesn’t actually make sense). The generator expression repeatedly yields j, which is not defined in the code you show. But from your description, it already has the value 4 in the environment you were testing in, so you see that value repeatedly. You never use the i value, which is what’s getting the values from the range.
As for your other loop, for a[-1] in a keeps rebinding the last value in the list, and then printing it.
A for loop does an assignment to the target you give it between for and in. Usually you use a local variable name (like i or j), but you can use a more complicated assignment target, like self.index or, as in this case, a[-1]. It’s very strange to be rewriting a value of your list as you iterate over it, but it’s not forbidden.
You never get 3 printed out, because each of the previous assignments as you iterated overwrote it in the list. The last iteration doesn’t change the value that gets printed, since you’re assigning a[-1] to itself.
Your first question raises error when iterating, as j is not defined. If you are getting any response from that, you probably have defined a value for j above your generator, which sounds to be 4 in your code.
About your second question:
when you have something like this:
for i in a:
...
Actually you are are doing this in each cycle: i=a[0], i=a[1], ....
When you write a[-1] in a, it is kind of equal to: a[-1]=a[0], a[-1]=a[1], ... and you are changing last element of a each time. So at the end, the last element of your list would be 2.
-
How this statement will work?
j for i in range(5)Example:
x1=(j for i in range(5))will yield five 4’s when iterated. Why it will not yield 01234 like when we replace j with i.
-
How this statement working?
a = [0, 1, 2, 3] for a[-1] in a: print(a[-1]) 0 1 2 2
Your first example is a generator expression (the parentheses are required, so the first version you show doesn’t actually make sense). The generator expression repeatedly yields j, which is not defined in the code you show. But from your description, it already has the value 4 in the environment you were testing in, so you see that value repeatedly. You never use the i value, which is what’s getting the values from the range.
As for your other loop, for a[-1] in a keeps rebinding the last value in the list, and then printing it.
A for loop does an assignment to the target you give it between for and in. Usually you use a local variable name (like i or j), but you can use a more complicated assignment target, like self.index or, as in this case, a[-1]. It’s very strange to be rewriting a value of your list as you iterate over it, but it’s not forbidden.
You never get 3 printed out, because each of the previous assignments as you iterated overwrote it in the list. The last iteration doesn’t change the value that gets printed, since you’re assigning a[-1] to itself.
Your first question raises error when iterating, as j is not defined. If you are getting any response from that, you probably have defined a value for j above your generator, which sounds to be 4 in your code.
About your second question:
when you have something like this:
for i in a:
...
Actually you are are doing this in each cycle: i=a[0], i=a[1], ....
When you write a[-1] in a, it is kind of equal to: a[-1]=a[0], a[-1]=a[1], ... and you are changing last element of a each time. So at the end, the last element of your list would be 2.