How do I sum the values in an array of maps in jq?

Question

Given a JSON stream of the following form:

{ \"a\": 10, \"b\": 11 } { \"a\": 20, \"b\": 21 } { \"a\": 30, \"b\": 31 }

I would like to sum t

Answer 1

Another approach, which illustrates the power of jq quite nicely, is to use a filter named "sum" defined as follows:

def sum(f): reduce .[] as $row (0; . + ($row|f) );

To solve the particular problem at hand, one could then use the -s (--slurp) option as mentioned above, together with the expression:

{"a": sum(.a), "b": sum(.b) }  # (2)

The expression labeled (2) only computes the two specified sums, but it is easy to generalize, e.g. as follows:

# Produce an object with the same keys as the first object in the 
# input array, but with values equal to the sum of the corresponding
# values in all the objects.
def sumByKey:
  . as $in
  | reduce (.[0] | keys)[] as $key
    ( {}; . + {($key): ($in | sum(.[$key]))})
;