how to increment the iterator from inside for loop in python 3?

Question

for i in range (0, 81):
    output = send command
    while True:
        last_byte = last_byte - offset
    if last_byte > offset:
       output = send command
       i+
    else:
        output = send command
        i+
        break

I want to increase the iterator every time the send command is executed. Right now it only increases by one when the for loop is executed. Please advise

for i in range(0,10):
    print(i)
    i +=2
    print("increased i", i)

I ran this code and it produced out from 0 to 9. I was expecting it would increase the iterator by 2.

Answer 1

Save a copy of the iterator as a named object. Then you can skip ahead if you want to.

>>> myiter = iter(range(0, 10))
>>> for i in myiter:
    print(i)
    next(myiter, None)
...
0
2
4
6
8

Answer 2

You can't do this inside a for loop, because every time the loop is restarted it reassigns the variable i regardless of your changes on the variable.

To be able to manipulate your loop counting variable, a good way is to use a while loop and increase the throwaway variable manually.

>>> i = 0
>>> while i < 10 :
...     print(i)
...     i += 2
...     print("increased i", i)
... 
0
('increased i', 2)
2
('increased i', 4)
4
...

Additionally, if you want to increase the variable on a period rather than based on some particular condition, you can use a proper slicers to slice the iterable on which you're looping over. For instance, if you have an iterator you can use itertools.islice() if you have a list you can simply use steps while indexing (my_list[start:end:step]).

Answer 3

range() has an optional third parameter to specify the step. Use that to increment the counter by two. For example:

for i in range(0, 10, 2):
    print(i)
    print("increased i", i)

The reason that you cannot increment i like a normal variable is because when the for-loop starts to execute, a list (or a range object in Python 3+) is created, and i merely represents each value in that object incrementally.

Answer 4

@ilaunchpad Sorry I know it's too late to post this but here is what you were looking for

i=0
for J    in range(0,10):
    print(i)
    i = i + 2
print("increased i", i)

You should not use the same variable throughout in the For statement.

Output
vaibhav@vaibhav-Lenovo-G570:~$ python3 /home/vaibhav/Desktop/Python/pattern_test.py
0
2
4
6
8
10
12
14
16
18
increased i 20

Answer 5

How about this?

for i in range(10):
     if i == 3:
         i += 1
         continue
     print(i)

Just adding the continue makes the counter go up - the print result is:

0
1
2
4
5
6
7
8
9

Note that without the continue, the 4 will be printed twice. I think this answers the question.

Answer 6

I wrote something like this.

    while True:
        if l < 5:
            print "l in while", l
            l += 1
        else:
            break

Now I am having a full control but the backdrop is that I have to check all the conditions.

Answer 7

As there are no answers that allow to use the result of the next iterator, here is my proposed solution, that works with many lists and iterables by using either the enumerate() function or just the iter() function:

x = [1, True, 3, '4', 5.05, 6, 7, 8, 9, 10]
x_enum = enumerate(x)
for idx, elem in x_enum:
    if idx == 1: # Example condition to catch the element, where we want to iterate manually
        print('$: {}'.format(elem))
        idx, elem = next(x_enum)
        print('$: {}'.format(elem))
    else:
        print(elem)

will print:

1
$: True        # As you can see, we are in the if block
$: 3           # This is the second print statement, which uses the result of next()
4
5.05
6
7
8
9
10

This is also possible with a simple iterator:

x_iter = iter(x)
for elem in x_iter:
    if elem == '4': # Example condition to catch the element, where we want to iterate manually
        print('$: {}'.format(elem))
        elem = next(x_iter)
        print('$: {}'.format(elem))
    else:
        print(elem)