What does this line of code mean?

Question

Hi, I am wondering what this line of code mean?

b = (gen_rand_uniform()>0.5)?1:0;

The gren_rand_uniform() is a function to generate random 0 and 1 numbers. However I don't get the meaning for >0.5 and 1:0.

I know this is supposed to be a basic question, please bear with me.

Thanks!

Answer 1

it's a way to get a random value which is either 1 or zero, each 50% of the time. The "?" and ":" are the conditional operarator.

Answer 2

It's rounding. The b variable will either be 0 or 1.

Answer 3

It encodes a flip of the coin. (A perfectly balanced coin that is.)

Answer 4

It's shorthand. In the example you gave, it is equivalent to:

if (gen_rand_uniform() > 0.5) {
    b = 1;
} else {
    b = 0;
}

Since gen_rand_uniform() probably generates uniformly distributed random numbers between 1 and 0, there's a 50% chance of the value being higher than 0.5. Which means there's a 50% chance of getting a 1 or a 0

Answer 5

I don't think get_rand_uniform() does what you think it does. It probably looks like this:

float get_rand_uniform(void);

Or maybe double. The point is, it returns a random decimal number between 0 and 1. So this:

get_rand_uniform() > 0.5

Is a check to see if that number is closer to 1 or 0. And this:

x ? y : z

Is the ternary conditional operator, which serves the same function as this:

if(x) { y } else { z }

Except that the ternary operator is an expression. So all of this:

get_rand_uniform() > 0.5 ? 1 : 0

Is basically rounding the random floating point number to 1 or 0, and this:

b = get_rand_uniform() > 0.5 ? 1 : 0;

Assigns that randomly picked 1 or 0 to b. I believe the parenthesis are unnecessary here, but if you like them, go for it.

Answer 6

Conditional assignment:

variable = condition ? value_if_true : value_if_false;

which is equal to:

if (condition) {
    variable = value_if_true;
} else {
    variable = value_if_false;
}

The code you give us is just random bool. It will return either 1 or 0.

Answer 7

What you are seeing here is a ternary expression. http://en.wikipedia.org/wiki/Ternary_operation This is (as others here has pointed out) a conditional construct, but one that is specific to expressions, meaning that a value is returned.

This construct exist in most languages (but not in eg. VB.Net) and has the form of

condition ? valueiftrue: valueiffalse

An example of this in action is:

var foo = true;
var bar = foo ? 'foo is true' : 'foo is false';
// bar = 'foo is true'

Also note that the condition can be any expression (like in your case gen_rand_uniform() > 0.5) and can infact contain a nested ternary expression, all it has to do is evaluate as a non-false value.