Python Loop Double Printing

Question

Using the code below, I am able to get the correct answer, however it is repeated twice.

For instance, I would only want a result of [1.2038, 1.206], but the code below prints [1.2038, 1.206, 1.2038, 1.206]. Anybody knows what is wrong with my code? Any help would be greatly appreciated. Thanks!

spot = [1.2020, 1.2040]
forwardSwap = [0.0018, 0.0020]
forwardOutright = []

for i in range(len(spot)):
    for i in range(len(forwardSwap)):
        forwardOutright.append(spot[i] + forwardSwap[i])

print forwardOutright

Possible duplicate of [Add SUM of values of two LISTS into new LIST](https://stackoverflow.com/questions/14050824/add-sum-of-values-of-two-lists-into-new-list) — yinnonsanders, Oct 30 '17 at 13:36
Will spot and forwardSwap always have the same number of elements? — Daniel Gale, Oct 30 '17 at 13:40

score 21 · Answer 1 · edited Oct 30 '17 at 13:34

21

You should zip instead of a nested loop to iterate both lists concurrently:

forwardOutright = [x+y for x, y in zip(spot, forwardSwap)]

edited Oct 30 '17 at 13:34

akash karothiya

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answered Oct 30 '17 at 13:30

Moses Koledoye

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score 3 · Answer 2 · answered Oct 30 '17 at 13:33

3

As per the given code in your question, both of your loops are using a variable named as i.

for i in range(len(spot)):
    for i in range(len(forwardSwap)):

answered Oct 30 '17 at 13:33

Nandeep Mali

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score 3 · Answer 3 · answered Oct 30 '17 at 13:34

3

This should work

list(map(lambda i: sum(i), zip(spot, forwardSwap)))

answered Oct 30 '17 at 13:34

daemon24

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score 3 · Answer 4 · answered Oct 30 '17 at 13:37

The problem you have is caused by the fact that the inner loop is executed completely every time the outer loop is executed. i.e for every item in spot you are going through every item in forwardSwap and appending a new value to forwardOutright. Instead you want a one-to-one matching between these two lists, so you could use:

for i,j in zip(spot, forwardSwap): forwardOutright.append(i+j)

You should also avoid shadowing your variables, i.e using i in both loops, instead you could use i and j for example. Otherwise this will cause your program to execute in unexpected ways.

score 1 · Answer 5 · answered Oct 30 '17 at 13:34

Because you have nested for loops, the outer loop is run as many times as there are elements of spot and the inner loop is run as many times as the product of the lengths. Instead you should use zip:

for s, fs in zip(spot, forwardSwap):
    forwardOutright.append(s + fs)

Or you can use a list comprehension:

forwardOutright = [s + fs for s, fs in zip(spot, forwardSwap)]

score 1 · Accepted Answer · answered Oct 30 '17 at 13:35

1

You just need to loop over once:

spot = [1.2020, 1.2040]
forwardSwap = [0.0018, 0.0020]
forwardOutright = []
for i in range(len(spot)):
    forwardOutright.append(forwardSwap[i]+spot[i])

Outout:

[1.2038, 1.206]

answered Oct 30 '17 at 13:35

Ajax1234

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This only works if both arrays have the same length of items. What if fowardswap only had 1 element? – Daniel Gale Oct 30 '17 at 13:37

score 0 · Answer 7 · answered Oct 30 '17 at 13:35

0

You have two nested loops that run twice each.

Therefore you execute the code inside the second loop (2x2) 4 times.

answered Oct 30 '17 at 13:35

rwms

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Python Loop Double Printing

7 Answers7