I'm trying to make an alert box pop up with one random sentence from an array of sentences inside it. This is my code:
var tasks = [
"This is the first task",
"And this is the second task",
"Third task..."
];
var randomTask = Math.floor((Math.random() * tasks.length) - 1);
alert(tasks[randomTask]);If you run it, the only thing the pop says is "undefined". Why doesn't it work?
Thanks to anyone that answers! :-)
Math.random returns a random number between 0 (inclusive) and 1 (exclusive), you multiply it by 3 and subtract 1, so you can get a number between -1 and 2 (where 2 is exclusive - value will always be lower than 2). And when you floor a negative value, you get -1. That's why you get undefined sometimes
Basically, remove - 1 and it should work
-1?The reason is that, the Math.random() returns a number between 0 and 1.
When the number is 0.1xxx, it is
computed as
0.1xxxxx * 3 - 1
that is
Math.floor(0.3xxxx - 1) = -1
And array[-1] is undefined.
To get around this issue, you can use % operator on the resulting random number. % will ensure that the number is always between 0 and arr.length - 1.
var tasks = [
"This is the first task",
"And this is the second task",
"Third task..."
];
var randomTask = Math.floor((Math.random() * tasks.length) % tasks.length);
alert(tasks[randomTask]);This will do:
var tasks = [
"This is the first task",
"And this is the second task",
"Third task..."
];
var rand = Math.floor(Math.random() * ((tasks.length -1) - 0 + 1)) + 0;
alert(tasks[rand]);
((tasks.length -1) - 0 + 1) any different from tasks.length?+ 0 or - 0?