How to start a time step function? SWITCH expression must be a scalar or a character vector.

I got this code from this community. This is the code for Newmark's method. Note that I only included part of the script.

switch acceleration
    case 'Average' 
        gaama = 1/2 ;beta = 1/4 ;
    case 'Linear'
        gaama = 1/2 ;beta = 1/6 ;
end
.......
    % Time step starts
for i = 1:nt
    delP = P0+a*vel(:,i)+b*accl(:,i) ;
    delu = Kcap\delP ;
    delv = a1*delu-a4*vel(:,i)-a6*accl(:,i) ;
    dela = a2*delu-a3*vel(:,i)-a5*accl(:,i);
    disp(:,i+1) = disp(:,i)+delu ;
    vel(:,i+1) = vel(:,i)+delv ;
    accl(:,i+1) = accl(:,i)+dela ;
    U(:,i+1) = phi*disp(:,i+1) ;
   
end

I loaded in a txt. file (which is an acceleration data, the matrix size is 2674x1) into the workspace, and then I ran the code. I don't know what went wrong it says: SWITCH expression must be a scalar or a character vector.

[Igor Arkhandeev]

Hello! Can you show full code before switch operation?

[Kim Hao Teong]

function [disp,vel,accl,U,t] = NewmarkMethod(M,K,C,P,phi,dof,acceleration)
% NEWMARK'S METHOD : LINEAR SYSTEM
% Reference : Dynamics of Structures - Anil K. Chopra
%-------------------------------------------------------------------------
% Code written by :Siva Srinivas Kolukula                                 |
%                  Senior Research Fellow                                 |
%                  Structural Mechanics Laboratory                        |
%                  Indira Gandhi Center for Atomic Research               |
%                  INDIA                                                  |
% E-mail : [e-mail]                                         |
%-------------------------------------------------------------------------
% Purpose : Dynamic Response of a system using linear Newmark's Method
% Synopsis :
%       [disp,vel,accl,U,t] = NewmarkMethod(M,K,C,P,phi,dof,acceleration)
% 
% Variable Description :
% INPUT :
%       M - Mass Matrix (in modal coordinates)
%       K - Stiffness Matrix (in modal coordinates)
%       C - Damping Matrix (in modal coordinates)
%       P - Force Matrix (in modal coordinates)
%       dof - system degree's of freedom
%       acceleration - Type of Newmark's Method to be used 
%       
% OUTPUT :
%        disp - modal displacement's 
%        vel - modal velocities
%        accl - modal accelerations 
%        U - system's displacement
%        t - time values at which integration is done
%--------------------------------------------------------------------------
switch acceleration
    case 'Average' 
        gaama = 1/2 ;beta = 1/4 ;
    case 'Linear'
        gaama = 1/2 ;beta = 1/6 ;
end
% Time step
ti = 0;
tf = 53.46 ;
dt = 0.02 ;
t = ti:dt:tf ;
nt = fix((tf-ti)/dt) ;
n = length(M) ;
% Constants used in Newmark's integration
a1 = gaama/(beta*dt) ; a2 = 1/(beta*dt^2) ;
a3 = 1/(beta*dt) ;       a4 = gaama/beta ;
a5 = 1/(2*beta) ;        a6 = (gaama/(2*beta)-1)*dt ;
disp = zeros(n,nt) ;
vel = zeros(n,nt) ;
accl = zeros(n,nt) ;
U = zeros(dof,nt) ;
% Initial Conditions
disp(:,1) = zeros ;
vel(:,1) = zeros ;
U(:,1) = phi*disp(:,1) ;
P0 = zeros(n,1) ;
accl(:,1) = M\(P-C*vel(:,1)-K*disp(:,1)) ;
Kcap = K+a1*C+a2*M ;
a = a3*M+a4*C ;
b = a5*M+a6*C ;
% Time step starts
for i = 1:nt
    delP = P0+a*vel(:,i)+b*accl(:,i) ;
    delu = Kcap\delP ;
    delv = a1*delu-a4*vel(:,i)-a6*accl(:,i) ;
    dela = a2*delu-a3*vel(:,i)-a5*accl(:,i);
    disp(:,i+1) = disp(:,i)+delu ;
    vel(:,i+1) = vel(:,i)+delv ;
    accl(:,i+1) = accl(:,i)+dela ;
    U(:,i+1) = phi*disp(:,i+1) ;
   
end

Basically, I got this Newmark-Method code from this community. I gotta be honest, I don't know how to write the function code.

[Igor Arkhandeev]

Okay, and what input data is?

[Igor Arkhandeev]

I think, your acceleration variable is array.

Answer 1

[Adam Danz]

Take a moment to read the switch/case documentation to become familiar with what that function does.

The acceleration variable needs to match either 'Average' or 'Linear' so it must be a character vector or string. That's case sensitive, too!

To avoid problems with case sensitivity,

switch lower(acceleration) % set to lower case
    case 'average' % lower case!
        gaama = ...
    case 'linear'  % lower case!
        gaama = ...
end

Lastly, I urse people to include an "otherwise" to catch these types of errors. The "otherwise" section can either throw an error or set a default value.

switch lower(acceleration) % set to lower case
    case 'average' % lower case!
        gaama = ...
    case 'linear'  % lower case!
        gaama = ...
    otherwise
        error('No case match for acceleration input.')
        % or 
        % gaama = ...
end

Answer 2

[Jan]

SWITCH expression must be a scalar or a character vector.

The error message tells you, that the 7th input of the function is neither a CHAR vector nor a scalar. So how did you define acceleration in the caller?

NewmarkMethod(M,K,C,P,phi,dof,acceleration)

The method to find the cause of such error is to use the debugger. Type this in the command window:

dbstop if error

Then run your code again. Matlab stops at the error and you cancheck the contents of the concerned variable.