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each, peach

Syntax: v1 each x, each[v1;x], v1 peach x, peach[v1;x]
Syntax: (vv)each x, each[vv;x], (vv)peach x, peach[vv;x]

Where

applies v1 or vv as a unary to each item of x and returns a result of the same length.

That is, the projections each[v1;], each[vv;], each[v1;], and peach[vv;] are uniform functions.

q)count each ("the";"quick";" brown";"fox")
3 5 5 3
q)(+\)peach(2 3 4;(5 6;7 8);9 10 11 12)
2 5 9
(5 6;12 14)
9 19 30 42

each and peach perform the same computation and return the same result. peach will divide the work between available secondary tasks.

each is a wrapper for the Each iterator. peach is a wrapper for the Each Parallel iterator. It is good q style to use each and peach for unary values.

each is redundant with atomic functions. (Common qbie mistake.)

Maps for uses of Each with binary and higher-rank values
.Q.fc parallel on cut
Parallel processing
Table counts in a partitioned database

Higher-rank values

peach applies only unary values. For a values of rank ≥2, use Apply to project v as a unary value.

For example, suppose m is a 4-column matrix and each row has values for the arguments of v4. Then .[v4;]peach m will apply v4 to each list of arguments.

Alternatively, suppose t is a table in which columns b, c, and a are arguments of v3. Then .[v3;]peach flip t `b`c`a will apply v3 to the arguments in each row of t.

Blocked within peach

hopen socket
websocket open
socket broadcast (25!x)
amending global variables
load master decryption key (-36!)

And any system command which might cause a change of global state.

Generally, do not use sockets within peach, unless it is encapsulated via one-shot or HTTP client request.

If you are careful to manage your file handles/file access so that there is no parallel use of the same handle (or file) across threads, then you can open and close files within peach.

Streaming execute (-11!) should also be fine. However updates to global variables are not possible, so use cases might be quite restricted within peach.