Issue applying script to second sheet - google-apps-script

Question

I have no idea how to code. I use to make website on HTML, so my knowledge is limited. I piece together and alter existing codes. I got decent on Excel VBA, but then needed to start using google sheets. So, that said...
I have a spreadsheet with two sheets, 'MIS' and 'Admin'. I have a bunch of code a formula that assigns a value (1,2,3...) based on how many of two drop-down criteria that each line matches (1 if it matches criteria #1, 2 if matches criteria #2, and 3 if it matches both). The code then hides everything and unhides only those with numbers matching the criteria. I need this to be clean and quick, it's for people who can barely use computers.
The problem is, the code only works on the first page. I tried using the same code, tried amending the code, and I tried inserting 'Admin' in about half a million places. Please help. The admin function is my latest attempt. This is where I inserted 'Admin' in a dozen places. Also, if you see anything I'm using that is slowing down the code, I could use some help with that too. There are 6 functions, which basically do the same thing using the same code but corresponding to different number combinations. The one in question is below.
function Admin(){
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getSheets('Admin')[0];
var range = sheet.getRange(1, 1, sheet.getLastRow());
sheet.hideRows(8,sheet.getLastRow());
var values = range.getValues();
for (var i=0; i<values.length; i++){
if(values[i][0] === 3){
sheet.showRows(i+1);
}
}
}

Answer 1

This line is your 'only the first sheet' problem, you're sort of combining two methods of defining a sheet:
var sheet = ss.getSheets('Admin')[0];
You can get a specific sheet by name by using the .getSheetByName() method i.e.
var adm_sheet = ss.getSheetByName('Admin');
var mis_sheet = ss.getSheetByName('MIS');
Or you can get a sheet by index
var adm_sheet = ss.getSheets()[0]; //returns first sheet
var mis_sheet = ss.getSheets()[1]; //returns second sheet
Lastly, you can get all of the sheets in your spreadsheet and list them in an array like this:
var sheets = ss.getSheets();
And then use the index of the sheet you need in this array like so:
sheets[0] //the first sheet
sheets[1] //the second sheet