Question

I think this is more like Math question rather than programming but since I’m writing Javascript, hope it is forgivable.

I receive user input, 1, 2, 3 or 4.

if 1, then output is 9

if 2, then output is 99

if 3, then output is 999

So basically input represent number of digit.

I admit that I really suck at Math. Tried to break this piece down to some kind of fomula
like 9 * 10 + 9 etc hoping I find some kind of Math approach to this problem but my brain corrupts.

I could do this by String concatenation way:

var userInput = 3;
var output = "";

for(var i=0; i<userInput; i++) {
    userInput = userInput + "9";
}

console.log(userInput); //now this should have string "999"

return parseInt(userInput); //return as integer 999;

while above works, I think it is really… not cool.

Could anyone show me if I can do this using Mathematical way?

Answer 1

output = Math.pow(10,userInput)-1;

That could work. Or:

output = new Array(userInput+1).join("9");

Although that’s probably less efficient.