Thanks solved. Learning how Do loop works was necessary. Also I should use AppendTo instead of Append and the problem that I was actually facing is by default all arguments to a function are immutable. So I had to use HoldFirst to get this to work

Thank You that worked. However is the way of declaring local variables correct ? I am also creating a matrix

matx = Table[x[i, j], {i, Length[v]}, {j, Length[p]}]

I want one equation for each of its row and append it to the constraints list. e.g.

x[i, j] + x[i, j+1]+.... +x[i, Length[p]] == K[[i]]   ? i

I have a list of equations

eqs = {}

I can create the inner loop like

C[vecx_] := Plux @@ vecx == 42
C[matx[[[1]]]

However how to run the outer loop ? and also how to pass the iterator i while iterating over the outer loop ?

In[14]:= Apply[C, matx]
Out[14]= C[{x[1, 1], x[1, 2]}, {x[2, 1], x[2, 2]}, {x[3, 1],  x[3, 2]}, {x[4, 1], x[4, 2]}] 

Applying C over matx is not working

I need something like

loop eq in C @@ matx
    Append[eqs, eq]
 end loop

I have tried

Do[
 Print[ac],
 Append[eqs, ac], {
  ac, 
  C @@@ matx
  }
 ]

But that did not work with error

Do::itform: "Argument Append[eqs,assignmentc] at position 2 does not have the correct form for an iterator"

.

In[1]:= vecy = {1, 2, 3, 4};
mysum = Plus @@ Table[y[i], {i, 1, Length[vecy]}]

Out[2]= y[1] + y[2] + y[3] + y[4]

or

mysum = Apply[Plus, Table[y[i], {i, 1, Length[vecy]}]]

and

In[3]:= myprod = Times @@ Table[y[i], {i, 1, Length[vecy]}]

Out[3]= y[1] y[2] y[3] y[4]