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I have list of numbers like this (S20170146) and I want to turn them to this format(S2017-01-46).Basically, I need to add "-" between the numbers. I am using Arcmap I know how to use feild calculator. But I don't know to write python code to do this?

  • Are the segments of the string always the same length? S(4 digits)-(2 digits)-(2 digits)? If so, it would be pretty easy to write an expression to grab the right chunks and then concatenate them with dashes. – lambertj Oct 1 at 14:27
  • Yes. they are start with S. But I don't know how to write the expression to do this – Kylie Oct 1 at 14:41
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You can use slicing along with string formatting to accomplish this:

new_string = '{}-{}-{}'.format(old_string[:5], old_string[5:7], old_string[7:])

Explanation of the 3 slices:

  1. The first 5 elements of the sequence (index 0-4)
  2. 2 elements, index 5 & 6
  3. All remaining elements (index 7-end)

This page gives a decent explanation on slicing strings

  • can i run this in Field Calculator? – Kylie Oct 1 at 14:54
  • Yes. Be sure to select the Python interpreter option and replace old_string with the attribute name. – Bjorn Oct 1 at 14:57
  • I post my Field Calculator setting.I cannot run your code and I get error. – Kylie Oct 1 at 15:30
  • Try removing those spaces between the attribute name and the [] brackets to be safe. You also need to actually calculate the value to new_string: replace thing(!LAST_allpaving_!) with new_string. – Alex Oct 1 at 15:41
  • You don't need to use the codeblock. Just paste in everything to the right of the '=' from my expression, and make the changes @Alex suggests. – Bjorn Oct 1 at 17:06
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pre-logic code block might work better as:

def thing(last_allpaving):
  return '{}-{}-{}'.format(last_allpaving[:5], last_allpaving[5:7], last_allpaving[7:])

as stated in some comments, it is not necessary to use a pre-logic code block, but if you choose to do so, this should help.

In general...

When using The Field Calculator with a Python expression, the 'expression' part is said to be the value that you set your field equal to. This expression has to be something that Python can parse like a constant or a built-in pre-defined function like:

arcpy.GetInstallInfo()['SourceDir'][:40]

as shown here:

enter image description here

An Advanced Python Expression implies we want to run our own pre-processing function which is previously un-defined in order to arrive at the calculated value. For this, we need to check the Show Codeblock option box.

enter image description here

In the Pre-Logic Script Code text area, we can enter our user-defined python function

def my_new_function(field_data):
  x = field_data
  y = x[:8]
  z = y[5:]
  return z

and in the Expression text area, we are now able to call our new user-defined function using

my_new_function( !LOC_DATA! )

Care must be taken to ensure that the value returned by your user-defined function is a type that is compatible with the field that you are calcualting. (ie. I wouldn't recommend trying to put a String into an Integer field)

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