Apps Script: Construct range of rows from array of row numbers - google-apps-script

Question

I have a list of row numbers in a spreadsheet which I need to change the background colour of. As the spreadsheet is quite large (10+ sheets, each with almost 5000 rows), I am trying to construct a range so I can batch set the background, as doing each row individually was taking over the max time of 6 minutes.
Here's the code I have:
// highlight required rows
var first = -1, last = -1;
for(var j = 0; j < rowNumsToHighlight.length; j++) {
if(first == -1) {
first = rowNumsToHighlight[j];
continue;
}
// if the current row number is one more than the previous, update last to be the current row number
if(rowNumsToHighlight[j] - 1 == rowNumsToHighlight[j - 1]) {
last = rowNumsToHighlight[j];
continue;
}
// otherwise the last row should be the previous one
else {
last = rowNumsToHighlight[j - 1];
}
var numRows = (last - first) + 1;
var range = sheet.getRange(first, 1, numRows, 4);
if(range.getBackground().toUpperCase() != highlightColour.toUpperCase()) {
range.setBackground(highlightColour);
}
first = -1;
last = -1;
}
rowNumsToHighlight is just an array that looks like: [205,270,271,272,278,279]. So, with that as an example, setBackground should be ran on row 205, on rows 270-272, and on 278-279.
I'm fairly sure the solution is simple, but just can't see it. Thanks for any help.
==== Updated Code ====
Based on Serge's code below, I made it more efficient again by reducing the number of getRange() calls made. Time is down from 78 to 54 seconds.
function updateColours(sheet, array, colour){
var columns = sheet.getLastColumn();
var rows = sheet.getLastRow();
var range = sheet.getRange(1, 1, rows, columns);
Logger.log("Resetting highlight on all rows...");
range.setBackground(null);
var backgrounds = range.getBackgrounds();
for(var n = 0; n < backgrounds.length; n++){
var rowIdx = n + 1;
if(array.indexOf(rowIdx) > -1){
for(var c = 0; c < columns; c++){
backgrounds[n][c] = colour;
}
}
}
Logger.log("Highlighting non-translated rows...");
range.setBackgrounds(backgrounds);
}

Answer 1

Maybe this one is faster(?) and built in a way that will make your work easier (function with arguments).
It writes only once to the sheet (or 2 if you clear colors before writing)...
use like below :
function testBG(){
updateColors(0,[7,8,9,18,19,23]);
}
function updateColors(sheetNum,array){
var sh = SpreadsheetApp.getActiveSpreadsheet().getSheets()[sheetNum];
var columns = sh.getMaxColumns();
var range = sh.getRange(1,1,sh.getMaxRows(),columns);
sh.getRange(1,1,sh.getMaxRows(),columns).setBackground(null);// use this if you want to clear all colors before setting them
var backGrounds = range.getBackgrounds();// get all cells BG
for(var n=0;n<backGrounds.length;n++){
var rowIdx = n+1;
if(array.indexOf(rowIdx)>-1){
for(c=0;c<columns;c++){
backGrounds[n][c]="#F00";// if row number is in the array fill in red
}
}
}
sh.getRange(1,1,sh.getMaxRows(),columns).setBackgrounds(backGrounds);//update sheet in one call
}
test sheet in view only, make a copy to test.

Answer 2

This is how I would do it:
function createRanges() {
var rowNumsToHighlight = [5,7,8,9,18,19];
var arrayLength = rowNumsToHighlight.length;
var loopCounter = 0, thisNumberInArray=0, nextNumberInArray=0, crrentNmbrPlusOne=0;
var currentRangeBegin=0, numberOfRowsInRange=1;
currentRangeBegin = rowNumsToHighlight[0];
for(loopCounter=0; loopCounter < arrayLength; loopCounter+=1) {
thisNumberInArray = rowNumsToHighlight[loopCounter];
nextNumberInArray = rowNumsToHighlight[loopCounter+1];
crrentNmbrPlusOne = thisNumberInArray+1;
if (nextNumberInArray===undefined) {
workOnTheRange(currentRangeBegin, numberOfRowsInRange);
return;
};
if (nextNumberInArray!==crrentNmbrPlusOne) {
workOnTheRange(currentRangeBegin, numberOfRowsInRange);
numberOfRowsInRange = 1; //Reset to 1
currentRangeBegin = nextNumberInArray;
} else {
numberOfRowsInRange+=1;
};
};
};
function workOnTheRange(first,numRows) {
var range = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Sheet11').getRange(first, 1, numRows, 4);
range.setBackground("red");
};
I've tested the code and it works.