If I type the example given in help:
Reap[Sow[a]; b; Sow[c]; Sow[d]; e]
the output is:
{e,{{a,c,d}}}
I understand the {a,c,d} part as those were Sown. I don’t understand why there are double braces around it though.
BUT my main question is where did the “e” come from at the beginning? I see it in the Reap input, but it is never Sown, so why is it included?
Thanks
The e is the result of the evaluation in the argument of Reap.
The reason for the extra set of braces in {{a,c,d}} is that for what was Sown, Reap returns a list of lists–in this case there is only one, but this allows for the other argument forms of Reap to work consistent with this simple form.
To add to David’s reply:
Reap[Sow[1, {x, x}]; Sow[2, y]; Sow[3, x]; 2 + 2]
{4, {{1, 1, 3}, {2}}}
1 is ‘sown’ twice with tag x, then 2 is sown once with tag y, and then 3 is sown with tag ‘x’. Now for each tag it is ‘reaped’. That is the reason for the list of the lists…