How to Compare a vector with for loop iterations in a nested if statement in a for loop

I'm trying to write a nested if statement within a for loop, where I compare a user input vector of numbers (1,2,3,4,etc.) to the iterations of a for loop (e.g. for i=1:length(a vector)). I need to compare each iteration of the for loop with the user input vector of values, and if they match, the code will return a certain equation; if not, the code will return a different equation. Ultimately, I need a column vector of equations based on the if/for loop. Any thoughts?

function [ DeltaO2Concentration ] = O2Solver_beta(t,O2Conc,c,O2Leak_term,Num_nodes)
%UNTITLED Test function used to solve 'first attempt' code.
%   System of ODE diffusion equations that will be solved using 'ode45'
%   function. Inputs are constant composed of universal and calculated
%   constants, time (independent variable), O2Conc (dependent variable), 
%   O2Leak which is a separate term or something.

%VECTOR LENGTH WILL NEED TO VARY ACCORDING TO USER INPUT%

start= 0;
terminate= Num_nodes;
DeltaO2Concentration= zeros(1,terminate-start);         %Initialize Vector
cnt= 1;
for i= 3:length(DeltaO2Concentration)
    if %THE ITERATION MATCHES A VALUE IN THE USER INPUT VECTOR
       %RETURN EQUATION 1
    ifelse 
       %RETURN EQUATION 2
    end
    cnt= cnt+1;
end
DeltaO2Concentration= DeltaO2Concentration';
end

Here is the main script, for reference

prompt_node= {'Enter Selected Node Numbers (e.g. 1,2,3,4,...)'};
dlog_title= 'User Input';
answer_nodes= inputdlg(prompt_node, dlog_title);
Slt_nodes= strsplit(answer_nodes{1},',');

Answer 1

[dpb]

...
for i= 3:length(DeltaO2Concentration)
  if ismember(i,USER_INPUT_VECTOR)  % USER_INPUT_VECTOR is your list variable, didn't see it???
    %RETURN EQUATION 1
  else 
    %RETURN EQUATION 2
  end
  ...

Alternate way to write the test would be

if any(i==USER_INPUT_VECTOR)  % match
  ...

The first always returns a scalar logical if the index value is first in the argument list whereas the latter returns a logical variable of length(USER_INPUT_VECTOR) with T for the position that matches, if there is a match.

I don't recognize which argument is supposed to be this list from the code; the help text doesn't explain the input variables which isn't of much help to anybody besides the author... :) It, could, I suppose be the last "Num_nodes" I guess, but you didn't show the call to the routine in the higher-level code to do an argument association matching....anyways, use the right variable where I left the placeholder.