SBEND ignores ByError in backwards compatibility call to atsbend

[oscarxblanco]

Dear all,

I think there is a bug in the backwards compatibility call to atsbend in line 42 of sbend.m, or in the help:

Help output contains ByError : https://github.com/atcollab/at/blob/66a7c92af28bed5900891b9c4fcbf6fae169e1a0/atintegrators/sbend.m#L6-L13

While sbend and atsbend calls do not have ByError: https://github.com/atcollab/at/blob/66a7c92af28bed5900891b9c4fcbf6fae169e1a0/atintegrators/sbend.m#L1 https://github.com/atcollab/at/blob/66a7c92af28bed5900891b9c4fcbf6fae169e1a0/atintegrators/sbend.m#L42

The way this line is written seems to wrongly assign ByError to K. Or maybe ByError was never used and the help output needs to be fixed.

[lfarv]

Here is the original sbend function as imported from the SLAC version:

function z=rbend(fname,L,A,A1,A2,K,method)
%BEND('FAMILYNAME',  Length[m], BendingAngle[rad], EntranceAngle[rad],
%   ExitAngle[rad], K, 'METHOD')
%   creates a new family in the FAMLIST - a structure with fields
%       FamName         family name
%       Length          length of the arc for an on-energy particle [m]
%       BendingAngle        total bending angle [rad]
%       EntranceAngle       [rad] (0 - for sector bends)
%       ExitAngle           [rad] (0 - for sector bends)
%       ByError             error in the dipole field relative to the design value 
%       K                       quadrupole K-value for combined funtion bends
%       PassMethod        name of the function to use for tracking
% returns assigned address in the FAMLIST that is uniquely identifies
% the family

ElemData.FamName = fname;  % add check for identical family names
ElemData.Length         = L;
ElemData.MaxOrder           = 3;
ElemData.NumIntSteps    = 10;
ElemData.BendingAngle   = A;
ElemData.EntranceAngle  = A1;
ElemData.ExitAngle      = A2;
ElemData.ByError        = 0;
ElemData.K                  = K;

ElemData.R1 = diag(ones(6,1));
ElemData.R2 = diag(ones(6,1));
ElemData.T1 = zeros(1,6);
ElemData.T2 = zeros(1,6);

ElemData.PolynomA           = [0 0 0 0];     
ElemData.PolynomB           = [0 K 0 0]; 

ElemData.PassMethod         = method;

global FAMLIST
z = length(FAMLIST)+1; % number of declare families including this one
FAMLIST{z}.FamName = fname;
FAMLIST{z}.NumKids = 0;
FAMLIST{z}.KidsList= [];
FAMLIST{z}.ElemData= ElemData;

ByError is indeed mentioned in the help, as it is now, but it is not in the argument list, and it is set to zero. Then, I could not find a place where this attribute would be used. So:

• the present compatibility function behaves indeed like the original one, except that it does not set a ByError (useless?) attribute with a zero value.
• Unless we find a place where this attribute is used, I would just correct the help section (which in addition documents a BEND function while its name is SBEND)

[oscarxblanco]

Dear @lfarv ,

I think that correcting the help message is enough. Thank you for checking the original function.