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I want to multiply only every second number on a list(0-100) but I just can't get it to work.

Ez Az
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    Possible duplicate of [How do I loop through a Python list by twos?](https://stackoverflow.com/questions/2990121/how-do-i-loop-through-a-python-list-by-twos) – asongtoruin Jun 06 '17 at 09:06
  • Please clarify. Do you want to multiply all those numbers together, do you want to multiply each by a specific number and store the results, or other? Are the numbers to multiply `1`, `3`, etc., or the 2nd, 4th, etc. items in a given list? Also, what have you tried? – Rory Daulton Jun 06 '17 at 09:14
  • I want to change to list so: 1,2,3,4,5,6,7,8 becomes 1,4,3,8,5,16 etc. – Ez Az Jun 06 '17 at 09:26

5 Answers5

0

I want to change to list so: 1,2,3,4,5,6,7,8 becomes 1,4,3,8,5,16 etc.

Though I don't understand how you multiply 6 to end up with 16, I assume this is what you need:

new_list = []

for x in range(1,100):
  if x % 2 == 0: new_list.append(x*2)
  else: new_list.append(x)

print(new_list)

If the number is divisible by 2, you multiply it with 2 and append it to a new list. If not, you just append it without multiplying.

Running this program, you get the following output:

[1, 4, 3, 8, 5, 12, 7, 16, 9, 20, 11, 24, 13, 28, 15, 32, 17, 36, 19, 40, 21, 44, 23, 48, 25, 52, 27, 56, 29, 60, 31, 64, 33, 68, 35, 72, 37, 76, 39, 80, 41, 84, 43, 88, 45, 92, 47, 96, 49, 100, 51, 104, 53, 108, 55, 112, 57, 116, 59, 120, 61, 124, 63, 128, 65, 132, 67, 136, 69, 140, 71, 144, 73, 148, 75, 152, 77, 156, 79, 160, 81, 164, 83, 168, 85, 172, 87, 176, 89, 180, 91, 184, 93, 188, 95, 192, 97, 196, 99]
trotta
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0

You can`t multiply with 0 , because the result will be always 0.

result=1
for i in range (1 , 10 ):
   if i%2==0:
     result*=i
print(result)
kkicoo7
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0
[x*10 for x in range(100) if x%2==0] # take vale to be multiply instead of 10, use x if you want to multiply with number it self

below code will only multiply even elements and keep other as it is.

def iterate_lis(get_list):
    ls = []
    for x in get_list:
        if x%2==0:
            ls.append(x*2)
        else:
            ls.append(x)
    print(ls)
    return ls

iterate_count = 5  # list will be iterate 5 times

for i in range(iterate_count):
    if i ==0:
        get_lis = iterate_lis(range(100))
    else:
        get_lis = iterate_lis(get_lis)

result for iterate_count=5 will be as follow:

>>> 
[0, 1, 4, 3, 8, 5, 12, 7, 16, 9, 20, 11, 24, 13, 28, 15, 32, 17, 36, 19, 40, 21, 44, 23, 48, 25, 52, 27, 56, 29, 60, 31, 64, 33, 68, 35, 72, 37, 76, 39, 80, 41, 84, 43, 88, 45, 92, 47, 96, 49, 100, 51, 104, 53, 108, 55, 112, 57, 116, 59, 120, 61, 124, 63, 128, 65, 132, 67, 136, 69, 140, 71, 144, 73, 148, 75, 152, 77, 156, 79, 160, 81, 164, 83, 168, 85, 172, 87, 176, 89, 180, 91, 184, 93, 188, 95, 192, 97, 196, 99]
[0, 1, 8, 3, 16, 5, 24, 7, 32, 9, 40, 11, 48, 13, 56, 15, 64, 17, 72, 19, 80, 21, 88, 23, 96, 25, 104, 27, 112, 29, 120, 31, 128, 33, 136, 35, 144, 37, 152, 39, 160, 41, 168, 43, 176, 45, 184, 47, 192, 49, 200, 51, 208, 53, 216, 55, 224, 57, 232, 59, 240, 61, 248, 63, 256, 65, 264, 67, 272, 69, 280, 71, 288, 73, 296, 75, 304, 77, 312, 79, 320, 81, 328, 83, 336, 85, 344, 87, 352, 89, 360, 91, 368, 93, 376, 95, 384, 97, 392, 99]
[0, 1, 16, 3, 32, 5, 48, 7, 64, 9, 80, 11, 96, 13, 112, 15, 128, 17, 144, 19, 160, 21, 176, 23, 192, 25, 208, 27, 224, 29, 240, 31, 256, 33, 272, 35, 288, 37, 304, 39, 320, 41, 336, 43, 352, 45, 368, 47, 384, 49, 400, 51, 416, 53, 432, 55, 448, 57, 464, 59, 480, 61, 496, 63, 512, 65, 528, 67, 544, 69, 560, 71, 576, 73, 592, 75, 608, 77, 624, 79, 640, 81, 656, 83, 672, 85, 688, 87, 704, 89, 720, 91, 736, 93, 752, 95, 768, 97, 784, 99]
[0, 1, 32, 3, 64, 5, 96, 7, 128, 9, 160, 11, 192, 13, 224, 15, 256, 17, 288, 19, 320, 21, 352, 23, 384, 25, 416, 27, 448, 29, 480, 31, 512, 33, 544, 35, 576, 37, 608, 39, 640, 41, 672, 43, 704, 45, 736, 47, 768, 49, 800, 51, 832, 53, 864, 55, 896, 57, 928, 59, 960, 61, 992, 63, 1024, 65, 1056, 67, 1088, 69, 1120, 71, 1152, 73, 1184, 75, 1216, 77, 1248, 79, 1280, 81, 1312, 83, 1344, 85, 1376, 87, 1408, 89, 1440, 91, 1472, 93, 1504, 95, 1536, 97, 1568, 99]
[0, 1, 64, 3, 128, 5, 192, 7, 256, 9, 320, 11, 384, 13, 448, 15, 512, 17, 576, 19, 640, 21, 704, 23, 768, 25, 832, 27, 896, 29, 960, 31, 1024, 33, 1088, 35, 1152, 37, 1216, 39, 1280, 41, 1344, 43, 1408, 45, 1472, 47, 1536, 49, 1600, 51, 1664, 53, 1728, 55, 1792, 57, 1856, 59, 1920, 61, 1984, 63, 2048, 65, 2112, 67, 2176, 69, 2240, 71, 2304, 73, 2368, 75, 2432, 77, 2496, 79, 2560, 81, 2624, 83, 2688, 85, 2752, 87, 2816, 89, 2880, 91, 2944, 93, 3008, 95, 3072, 97, 3136, 99]
Gahan
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  • Only problem that i want to keep the other numbers too but in their original state :/ – Ez Az Jun 06 '17 at 09:34
  • @Ez Az see now. – Gahan Jun 06 '17 at 09:39
  • I need to change the same list over and over again, but if I write: for x in new_list: if x%2==0: ls.append(x*10) else: ls.append(x) It just wont work. – Ez Az Jun 06 '17 at 10:16
  • then you need to create function which creates new list each time from returned value. wait a sec.. i'll make you one – Gahan Jun 06 '17 at 10:19
  • initially list of first 100 numbers from 0 will be passed to function `iterate_lis` and from next on it will take input of previous result. – Gahan Jun 06 '17 at 10:32
  • Okay thanks for the help but i can't put this in my code. I need to make a code for this You have 100 doors in a hallway in a row that are all initially closed. You make 100 passes by the doors. The first time through, you visit every door and toggle the door (if the door is closed, you open it; if it is open, you close it). The second time you only visit every 2nd door (door #2, #4, #6, ...). The third time, every 3rd door (door #3, #6, #9, ...), etc, until you only visit the 100th door.My plan was to always multiply with -1 so if positive then its open if its negative then its closed – Ez Az Jun 06 '17 at 10:38
  • then you should've ask this in your question with your effort. you didn't put your code. you just put a statement so I wrote this answer. however if it helped you,you can up vote or accept it as answer. if you still want me or someone to get you answer for this try elaborating problem statement, your issue, and your effort.Can't change answer from comment. this change makes answer off-topic to the question. – Gahan Jun 06 '17 at 10:43
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import numpy as np
l = range(100)
np.product(l[0::2])

This will give you every second element of your list and multiply all.

Franz
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Use range() to generate the indexes of the entries you want to change...

numbers = [1, 2, 3, 4, 5, 6, 7, 8]

for i in range(1, len(numbers), 2):
    numbers[i] *= 2

will result in numbers containing the list

[1, 4, 3, 8, 5, 12, 7, 16]
Dave Hines
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