Append items to a map within a YAML file

Question

In a YAML file, is it possible to add an item to a map after the map has already been defined?

For example, if I have a file:

fruits:
  bananas:
    quantity: 4
  apples:
    quantity: 2
  grapes:
    quantity: 37

vegetables:
  lettuce:
    quantity: 2
  beets:
    quantity: 4

and then later on realize that my "fruits" map is not complete, can I do anything later on in the file (i.e. after the "vegetables" map is defined) to append another fruit to the existing map? Something where:

fruits:
  oranges:
    quantity: 4

would be appended to the existing map instead of overwriting it?

In other words, is there a valid way to append "oranges" to the existing "fruits" map elsewhere in the file?

A bit of background: I plan on using yaml-cpp with C++. I am using YAML as an input file format for a program I am writing. I will parse the file that a user creates and translate the structure into the objects in my program. I would like to be able to append items to a map because it is common for objects (in this example "fruits") to be scattered in an input file as a user realizes they are needed or used near other objects that reference them.

What language and library are you using, and in what context? — Mattie, Jul 29 '13 at 19:16

Jesse Beder · Accepted Answer · 2013-07-30T02:15:19.473

1

It isn't valid YAML to have a repeated key. E.g., the following is not valid YAML:

fruits: foo
fruits: bar

If your YAML file starts with:

fruits: whatever
bar: baz

then there is no way to modify the value associated with the key fruits by appending to the file. Different YAML parsers may interpret repeated keys in their own way, but the spec defines it as an error.

You could post-process a YAML file, if you want, e.g.:

fruits1: whatever
bar: baz
fruits2: something else

and then in your code, just merge them values associated with fruits1 and fruits2 (and fruits3, ...) however you like.

edited Jul 30 '13 at 02:15

answered Jul 29 '13 at 20:53

Jesse Beder

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Is this the case for yaml-cpp or all parsers? some of the online parsers (e.g., http://yaml-online-parser.appspot.com/) show that subsequent definitions of "fruits" will override the previous definitions. Regardless, my question is: Is there a valid way to append the "oranges" to the previously defined "fruits" map? – Neal Kruis Jul 29 '13 at 20:59
This is the definition of YAML. Different parsers may interpret it differently (e.g., some may take the first, some may take the last, some may pick randomly, some may raise an error), but you shouldn't depend on it, since it's invalid YAML. – Jesse Beder Jul 30 '13 at 02:11

Append items to a map within a YAML file

1 Answers1