范围如何与 python 中的 for 循环配合使用

Question

for i in range(1,4,-1):
    print(i)

Here, "i" will be initialized as "1" which lies in [1, 4), So it should print 1, but it doesn't print anything. Why?

Answer 1

Here your step value is negative

For a positive step, the contents of a range r are determined by the formula r[i] = start + step*i where i >= 0 and r[i] < stop.

For a negative step, the contents of the range are still determined by the formula r[i] = start + step*i, but the constraints are i >= 0 and r[i] > stop.

The first value would be 1 + (-1) * 0 = 1, which is less than your step value (4) and this fails the second constraint.

so if you go through the formula, its obvious nothing would get printed - Reference

Answer 2

In order to make it work you should have: range(4, 1, -1).

As said on the documentation:

For a positive step, the contents of a range r are determined by the formula r[i] = start + step*i where i >= 0 and r[i] < stop.

For a negative step, the contents of the range are still determined by the formula r[i] = start + step*i, but the constraints are i >= 0 and r[i] > stop.

Answer 3

According to this part of the documentation :

For a negative step, the contents of the range are still determined by the formula r[i] = start + step*i, but the constraints are i >= 0 and r[i] > stop.

Your first value would be 1 + (-1) * 0 = 1, which is less than your stop value of 4. So the second constraint applies, and this value isn't kept. Your range is empty.