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

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.

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

[python] Iterating over arrays in Python 3

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