How to replace only a single instance of a substring?

Hi everybody,

for an assignment, I am supposed to turn a user inputted string into an "encrypted" message; My program is heavily dependent on the 'replace' function and my problem is that since I am using a loop with an iteration of 1:length(user_inputted_str) when it comes to words with repeated letters such as "hello", 'replace' will replace all 'l's with the corresponding encryption letter and then when the next iteration of the loop occurs, it replaces it again.

TL;DR Is there a way to replace only one instance of the substring using the strrep function? if not, I am open to other suggeestions.

Thank you so much

Code below:

inp = input('please input string to be converted: ', 's');
for i = 1:length(inp)

    disp(inp(1,i))
    switch inp(1,i)
        case('a')
            inp = replace(inp, inp(1,i),'n')
        case('b')
            inp = replace(inp, inp(1,i),'o')
        case('c')
           inp = replace(inp, inp(1,i),'p')    
        case('d')
            inp = replace(inp, inp(1,i),'q')
        case('e')
            inp = replace(inp, inp(1,i),'r')
        case('f')
            inp = replace(inp, inp(1,i),'s')
        case('g')
            inp = replace(inp, inp(1,i),'t')
        case('h')
            inp = replace(inp, inp(1,i),'u')
        case('i')
            inp = replace(inp, inp(1,i),'v')
        case('j')
            inp = replace(inp, inp(1,i),'w')
        case('k')
            inp = replace(inp, inp(1,i),'x')
        case('l')
            inp = replace(inp, inp(1,i),'y')
        case('m')
            inp = replace(inp, inp(1,i),'z')
        case('o')
            inp = replace(inp, inp(1,i),'b')
        case('p')
            inp = replace(inp, inp(1,i),'c')
        case('q')
            inp = replace(inp, inp(1,i),'d')
        case('r')
            inp = replace(inp, inp(1,i),'e')
        case('s')
            inp = replace(inp, inp(1,i),'f')
        case('t')
            inp = replace(inp, inp(1,i),'g')
        case('u')
            inp = replace(inp, inp(1,i),'h')
        case('v')
            inp = replace(inp, inp(1,i),'i')
        case('w')
            inp = replace(inp, inp(1,i),'j')
        case('x')
            inp = replace(inp, inp(1,i),'k')
        case('y')
            inp = replace(inp, inp(1,i),'l')
        case('z')
            inp = replace(inp, inp(1,i),'m')
    end

end

Answer 1

[Walter Roberson]

Lines such as

 inp = replace(inp, inp(1,i),'n')

should be

 inp(1,i) = 'n';

This only affects a single location, and prevents positions from being changed twice. With your existing code, suppose your text started with 'a' and got replaced by 'n'. Suppose there was also an 'n' in the input. Then when you go to work on that 'n' on input and you go to replace the first 'n' with 'a', you would end up replacing the 'n' that you had generated from the 'a' . Your code should not be replacing inside anything it has already replaced.

[John Doe]

Oh my god, I am an idiot. Thank you so much Walter!