Undefined Function ,,,,, for input arguments of type 'char'

Thank you in advanced. Just creating a function for a student number checker. 'Yes this is an assignment'. I would very much appreciate any help. If the error is easy to spot please just give me a hint ahha. This is my function:

%Question 3, Student Number Success Function
function [ LoginSuccessful ] = ValidSN( Studentnumber )
% ValidSN is a function that asks for the user's Student Number
% and confirms that their input is valid within Griffith College
% The function will return "Login Successful" if the first input
% start's with a 's' or 'S' and is 8 inputs long is size
% and only number after the 's', 'S'. 
% Function[LoginSuccessful/UnSuccessful] = ValidSN(studentnumber)

% Step 1, Check the length.
 StudentNumberLength = size(Studentnumber);
if StudentNumberLength~=8;
    disp('Your student number length is incorrent, it must be 8 charaecters long');
    LoginSuccessful = 0;
end;

% Step 2, Check the input starts with an 's' or 'S'.
S_or_s = Studentnumber(1);
if S_or_s~= 's' && S_or_s~= 'S'
    disp('First input must start with an ''s'' or an ''S''');
    LoginSuccessful = 0;
end

% Step 3, Check input 2-to-8 are numerical inputs only.
for SNLength = 2:8
    if Studentnumber(SNLength)~= 2:8
        disp('Not a numerical input.')
        LoginSuccessful = 0
    else
        disp('Login Successful');
        url = 'https://www.mathsworks.com'
        web = 'www.mathsworks.com'
        LoginSuccessful = 1;
    end
end

And I'm calling it from this script.

%%QUestion 3
% SNumber checker
% This script calls on the function "isValidStudentNumber"

clc
clear all
% Student Number Checker called to the function ValidSN
% Step 1, Requires Input
while 1
s = input('Student Number: ', 's');
 [Login] = ValidSN(s);
disp(Login)
url 'https://www.mathsworks.com'
web = 'www.mathsworks.com'
end

[Jan]

I've edited the code using the "{} Code" button to make it readable. Please do this by your own in the future - thanks.

Answer 1

[Niels]

i run your code, and i did not get any error message

s = input('Student Number: ', 's'); 
[Login] = ValidSN(s); 
disp(Login)
Student Number: s12873654
Your student number length is incorrent, it must be 8 charaecters long
Not a numerical input.
Not a numerical input.
Not a numerical input.
Not a numerical input.
Not a numerical input.
Not a numerical input.
Not a numerical input.
     0

your problem is that your "check" if the input is a number is wrong

if Studentnumber(SNLength)~= 2:8 ???

first of all studentNumber is a string so Studentnumber(2) and so on are all string. So if you want to check if 2:8 are numerical inputs use str2num and check if its true

if isempty(str2num(Studentnumber(SNLength))) % is empty if argument is no number

[Walter Roberson]

To test for '0' to '9' you can use isdigit(). To test against a restricted set of digits such as '2' to '8' you could either do a pair of tests,

if var>= '2' && var <= '8'

Or you can use

   ismember(var, '2':'8')

[Lewis Samuels]

Thank you. I used this in another code.