Iterating over arrays in Python 3

33
votes

I haven't been coding for awhile and trying to get back into Python. I'm trying to write a simple program that sums an array by adding each array element value to a sum. This is what I have:

def sumAnArray(ar):
    theSum = 0
    for i in ar:
        theSum = theSum + ar[i]
    print(theSum)
    return theSum

I get the following error:

line 13, theSum = theSum + ar[i]
IndexError: list index out of range

I found that what I'm trying to do is apparently as simple as this:

sum(ar)

But clearly I'm not iterating through the array properly anyway, and I figure it's something I will need to learn properly for other purposes. Thanks!

4
pythonarrayspython-3.xsum
i is the value of the item you're looping over in the array... so if you had 3 items [10, 11, 12] you're trying on the first iteration of accessing ar[10] which won't work... You could just use the builtin sum, eg: sum(ar) ? - Jon Clements♦
@JonClements you will notice that I mentioned sum(ar) in my original question as an easier way to do it. I am asking how to properly iterate over array elements. - q-compute
(sorry - somehow glanced over that bit) you can just change your loop to be theSum += ar... - Jon Clements♦
If you want to use the INDEX of each element, rather than the value of each element in a list, you can use enumerate - dawg

4 Answers

58
votes

When you loop in an array like you did, your for variable(in this example i) is current element of your array.

For example if your ar is [1,5,10], the i value in each iteration is 1, 5, and 10. And because your array length is 3, the maximum index you can use is 2. so when i = 5 you get IndexError. You should change your code into something like this:

for i in ar:
    theSum = theSum + i

Or if you want to use indexes, you should create a range from 0 ro array length - 1.

for i in range(len(ar)):
    theSum = theSum + ar[i]
4
votes

The for loop iterates over the elements of the array, not its indexes. Suppose you have a list ar = [2, 4, 6]:

When you iterate over it with for i in ar: the values of i will be 2, 4 and 6. So, when you try to access ar[i] for the first value, it might work (as the last position of the list is 2, a[2] equals 6), but not for the latter values, as a[4] does not exist.

If you intend to use indexes anyhow, try using for index, value in enumerate(ar):, then theSum = theSum + ar[index] should work just fine.

1
votes

You can use 

    nditer

Here I calculated no. of positive and negative coefficients in a logistic regression:

b=sentiment_model.coef_
pos_coef=0
neg_coef=0
for i in np.nditer(b):
    if i>0:
    pos_coef=pos_coef+1
    else:
    neg_coef=neg_coef+1
print("no. of positive coefficients is : {}".format(pos_coef))
print("no. of negative coefficients is : {}".format(neg_coef))

Output:

no. of positive coefficients is : 85035
no. of negative coefficients is : 36199
0
votes

While iterating over a list or array with this method:

ar = [10, 11, 12]
for i in ar:
    theSum = theSum + ar[i]

You are actually getting the values of list or array sequentially in i variable. If you print the variable i inside the for loop. You will get following output:

10
11
12

However, in your code you are confusing i variable with index value of array. Therefore, while doing ar[i] will mean ar[10] for the first iteration. Which is of course index out of range throwing IndexError

Edit You can read this for better understanding of different methods of iterating over array or list in Python