Given the input document:
{"a":1}
{"b":2}
{"c":3,"d":4}
What is the difference between the following jq programs (if any)? They all seem to produce the same output.
jq '[., inputs] | map(to_entries[].value)'jq -n '[inputs] | map(to_entries[].value)'jq -s 'map(to_entries[].value)'
In other words, the following (simplified/reduced) invocations seem identical:
jq '[.,inputs]'jq -n '[inputs]'jq -s '.'.
How are they different? Are there scenarios where one works, but the others don't? Did older versions of jq not support all of them? Is it performance related? Or simply a matter of readability and personal preference?
Bonus points (added later to the question): does the same hold true for the following programs?
jq '., inputs | to_entries[].value'jq -n 'inputs | to_entries[].value'jq -s '.[] | to_entries[].value'jq 'to_entries[].value'
CodePudding user response：
With jq '-n [inputs] ....' and jq '[.,inputs] ....', you are loading the whole file into memory.
A more memory-efficient way to achieve the result as an array is:
jq -n '[inputs | to_entries[].value]'
CodePudding user response：
Those first three programs are equivalent, both functionally and in terms of resource utilization, but they obscure the difference between array-oriented and stream-oriented programming.
In a nutshell, think sed and awk. For more details, see e.g. my A Stream-oriented Introduction to jq, and i.p. the section On the importance of inputs.
Bonus points: does the same hold true for the following programs:
Referring to the last four numbered examples in the Q: (4), (5) and (7) are essentially equivalent; (6) is just silly.
If you're looking for a reason why all these variations exist, please bear in mind that input and inputs were late additions in the development of jq. Perhaps they were late additions because jq was originally envisioned as a very simple and for the most part "purely functional" language.