Using multiple genvar in Verilog loop - for-loop

Question

Is It possible using different "genvar" in a loop? Is there an alternative mode to realize it?
I try with this example:
genvar i;
genvar j;
genvar k;
generate
k=0;
for (i = 0; i < N; i = i + 1)
begin: firstfor
for (j = 0; j < N; j = j + 1)
begin: secondfor
if(j>i)
begin
assign a[i+j*N] = in[i] && p[k];
k=k+1;
end
end
end
endgenerate
And when I run "Check Syntax" display this error:
Syntax error near "=". (k=k+1)

Answer 1

I like this question because unless very familiar with generates it looks like it should work, however there is a similar question which tries to use an extra genvar.
The syntax is not allowed because of how the generates are unrolled. Integers can only be used inside always/initial processes.
If it is just combinatorial wiring rather than parametrised instantiation you might be able to do what you need just using integers (I would not normally recommend this):
integer i;
integer j;
integer k;
localparam N = 2;
reg [N*N:0] a ;
reg [N*N:0] in ;
reg [N*N:0] p ;
always #* begin
k=0;
for (i = 0; i < N; i = i + 1) begin: firstfor
for (j = 0; j < N; j = j + 1) begin: secondfor
if(j>i) begin
a[i+j*N] = in[i] && p[k];
k=k+1;
end
end
end
end
Not sure how synthesis will like this but the assignments are static, it might work.

Answer 2

It is possible to avoid always #* when you want to do more advanced math with genvar loops. Use localparam and a function.
Make k a localparam derived from the genvars with a function, and use k as originally intended.
The getk function seems to violate the principles of code reuse by basically recreating the loops from the generate block, but getk allows each unrolled loop iteration to derive the immutable localparam k from genvars i and j. There is no separate accumulating variable k that is tracked across all the unrolled loops. Both iverilog and ncvlog are happy with this.
(Note that the original example could optimize with j=i+1 as well, but there is still an issue with deriving k.)
module top();
localparam N=4;
function automatic integer getk;
input integer istop;
input integer jstop;
integer i,j,k;
begin
k=0;
for (i=0; i<=istop; i=i+1) begin: firstfor
for (j=i+1; j<((i==istop)? jstop : N); j=j+1) begin: secondfor
k=k+1;
end
end
getk=k;
end
endfunction
genvar i,j;
generate
for (i = 0; i < N; i = i + 1) begin: firstfor
for (j = i+1; j < N; j = j + 1) begin: secondfor
localparam k = getk(i,j);
initial $display("Created i=%0d j=%0d k=%0d",i,j,k);
end
end
endgenerate
endmodule
Output:
$ iverilog tmptest.v
$ ./a.out
Created i=0 j=1 k=0
Created i=0 j=2 k=1
Created i=0 j=3 k=2
Created i=1 j=2 k=3
Created i=1 j=3 k=4
Created i=2 j=3 k=5
I discovered the 'trick' of using functions to derive values from the genvars here:
https://electronics.stackexchange.com/questions/53327/generate-gate-with-a-parametrized-number-of-inputs