CODEWITHSUNDEEP
pythonloops
William Ray Lawler
William Ray Lawler

Reputation: 11

How do you add adjacent numbers in a list? I can't get the last variable

Let x = [3, 8, -2, 6, 9, -4, 7, 1, -5, 8]

Use a for loop to add adjacent elements in x.
Store each of these results in a vector, sa.
Display both the vector x and the vector sa. For example, the first 3 numbers of sa will be: 

sa = [11, 9, 12, …]  = [(3+8), (3+8+(-2)), (8+(-2)+6), …]

I have something like this... 

x = [3, 8, -2, 6, 9, -4, 7, 1, -5, 8]
sa = []

for i in range (0, len(x)-1):
    if i<1:
        sa.append(x[0] + x[1])
    elif i >= 1 & i< len(x):
        sa.append(x[i-1] + x[i] + x[i+1])
    if i == 0:
        sa.append(x[i] + x[i+1])
print("Here is sa", sa)

but I can't get the last variable of -5 +8 to appear please help what I end up getting is Here is sa, [11, 11, 9, 12, 13, 11, 12, 4, 3, 4] but I also need the last value it should be (-5+8)= 3 so the total final answer should include the three like

[11, 11, 9, 12, 13, 11, 12, 4, 3, 4, 3]

or even 

[11, 9, 12, 13, 11, 12, 4, 3, 4, 3]

Upvotes: 1

Views: 1935

Answers (4)

wwii
wwii

Reputation: 23783

>>> x = [3, 8, -2, 6, 9, -4, 7, 1, -5, 8]

Create a list with the sum of the first two elements,

>>> q = [sum(x[:2])]

Iterate over the list three at a time and append the sums.

>>> for thing in zip(x,x[1:],x[2:]):
...     q.append(sum(thing))

Append the sum of the last two items.

>>> q.append(sum(x[-2:]))

>>> q
[11, 9, 12, 13, 11, 12, 4, 3, 4, 3]

Upvotes: 0

Yi Zhao
Yi Zhao

Reputation: 346

Your code has some mistakes:

x = [3, 8, -2, 6, 9, -4, 7, 1, -5, 8]
sa = []

# the last element of the loop will be x[len(x)-2], it is -5 in this case
# -5 is the last element this loop process, it adds up 1, -5 and 8
# this loop will not take 8 as the central number, so the results miss the last value
for i in range (0, len(x)-1):
    if i<1:
        sa.append(x[0] + x[1])
    # in python, it is `and` not &, & is bitwise and.
    elif i >= 1 & i< len(x):
        sa.append(x[i-1] + x[i] + x[i+1])
    if i == 0: # this is meaningless, i == 0 is covered in the first if branch
        sa.append(x[i] + x[i+1])
print("Here is sa", sa)

Fix your code:

x = [3, 8, -2, 6, 9, -4, 7, 1, -5, 8]
sa = []

# fix one: loop to the last element
for i in range (0, len(x)):
    if i<1:
        sa.append(x[0] + x[1])
    # fix two: do not include the last element in this elif branch
    elif i >= 1 and i < len(x) - 1:
        sa.append(x[i-1] + x[i] + x[i+1])
    # fix three: process the last element.
    else:
        sa.append(x[i - 1] + x[i])
print("Here is sa", sa)

output:

Here is sa [11, 9, 12, 13, 11, 12, 4, 3, 4, 3]

Upvotes: 0

Shakeel
Shakeel

Reputation: 2035

Just in case if you are looking for non comprehensive way, here it is.

for i in range(len(x)):
    if i == 0:
        sa.append(x[i] + x[i+1])
    elif 1 <= i < len(x)-1:
        sa.append(x[i-1] + x[i] + x[i+1])
    else:
        sa.append(x[i] + x[i-1])

print("Here is sa", sa)

Output:

Here is sa [11, 9, 12, 13, 11, 12, 4, 3, 4, 3]

Upvotes: 1

Nick
Nick

Reputation: 147216

You can write this as a list comprehension, noting the ith element in sa is the sum of the x values from x[i-1] to x[i+1]. Now those indexes may overrun the boundaries of x, so we need to use max and min to keep them in range:

x = [3, 8, -2, 6, 9, -4, 7, 1, -5, 8]

sa = [sum(x[max(i-1, 0):min(i+2, len(x))]) for i in range(len(x))]
print(sa)

Output:

[11, 9, 12, 13, 11, 12, 4, 3, 4, 3]

Upvotes: 2

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