Iterating over arrays in Python 3

Question

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

User · Answer

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

User · Answer

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 gt 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

User · Answer

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

User · Answer

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

[python] Iterating over arrays in Python 3

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