What does this line of C# code do?

int hour = DateTime.Now.Hour; 
ViewData["greeting"] = (hour < 12 ? "Good morning" : "Good aftern开发者_运维知识库oon"); 

Sorry for the noob question, but the ASP.NET MVC book I'm currently reading assumes that I already know C# (but I don't).

I understand the first part - it assigns current date and time to the hour variable. But I am lost on the 2nd line. What are those <, ?, : symbols for? ViewData["greeting"] is this an array of some sorts?

Thanks!

If the hour is less than 12, ViewDate["greeting"] is assigned a value of "Good morning". Otherwise, it is assigned a value of "Good afternoon".

The ?: bit is a conditional operator. MSDN gives a good description:

The conditional operator (?:) returns one of two values depending on the value of a Boolean expression.

The ? is called the conditional operator.

It acts as an if-then-else statement, only difference is that it actually returns something upon evaluation.

Take this simple example:

// ex 1
if (condition)
    name = "Female";
else
    name = "Male";

// ex 2
name = condition ? "Female" : "Male";

These two examples are identical. The conditional operator consists of two clauses (other than the condition), one before and one after the colon : -- the before is evaluated on condition == true, and the after is evaluated on condition == false.

The < is simply an operator that checks whether the left hand side is less than the right hand side. 1 < 2 == true, whereas 3 < 2 == false.

ViewData["greeting"] is just the item with index greeting in the array ViewData.

The whole code simply assigns a greeting message based on the current time.

EDIT: As noted above, int hour = DateTime.Now.Hour; assigns the current hour (0-23) to the integer variable hour

it is a short version of an if statement.

if (hour < 12)
    ViewData["Greeting"] = "Good Morning";
else
    ViewData["Greeting"] = "Good afternoon";

I understand the first part - it assigns current date and time to the hour variable.

That's not quite right. Let's analyze each part of the right hand side of the expression:

DateTime - This is a type (class) used for storing and operating on values that have a date and time component.

Now - This is a static member of the DateTime type. That means you don't use a particular instance to access it, but rather the name of the type itself. This static member returns a new DateTime instance populated with the current date and time. Note that once created this instance does not change; a few lines of code later it is already out of date.

Hour - This returns a number indicating the Hour portion of a DateTime instance in 24 hour format (0 is midnight, 13 is 1pm).

So, taken together, this returns a number representing the current hour. Combine this with the other answers explaining the conditional and less than operators, and the code should make a little more sense. If the hour is greater than 12, it's after noon.

That still leaves the ViewData[]. ViewData is a special kind of Dictionary type (key/value pairs indexed by key) used for passing data from an MVC controller to an MVC view.

Basically saying if hour < 12 use "Good Morning" otherwise use "Good Afternoon" - like an inline if statement.

It's another form of "if else." The statement on the left of the question mark stands as the if statement, and ViewData["greeting"] is assigned the value on the left of the ":" if it's true, and the value to the right of ":" if it's false.

Larsenal's answer is correct. More generally, ? ... : ... is a shorthand notation for if in C based languages. It can detract from readability, but the nice thing about it is that you can use it after an assignment operator, as in your example.

Just to answer the other part of your question you asked what the symbols were: < = less than (so is hour less than 12) ?: = ternary (http://en.wikipedia.org/wiki/Ternary_operation) basically saying if the if statement (hour < 12) = true then use value after question makr, if not use value after colon.

the < symbol is the good 'ol boy from math, known as "Less Then" the ? together with the : is what we call the "Ternary Operator" or "Conditional Operator" basically, its like an inline if-then-else statment.

if ( 5 > 2 ) 
   dosomething();
else
  dosomethingelse();

could be written as

(5 > 2 ) ? dosomething() : dosomethingelse();

so, what this code is doing is getting the current hour according to the clock, checking to see if it is before 12pm (aka noon), and assigning a greeting accordingly.

well im not good in c#, but its pretty clear what this code does:

the first line gets the current hour. meaning: if its 7 o clock, the value of the varialble "hour" will be set to 7:

int hour = DateTime.Now.Hour;

"ViewData" is apparently an array holding some String data. thevalue for index "greeting" will be set to "Good morning", if it is before 12 o clock, to "Good afternoon" elsewhen...

ViewData["greeting"] = (hour < 12 ? "Good morning" : "Good afternoon"); 

to be exact,

ViewData["greeting"] = (hour < 12 ? "Good morning" : "Good afternoon"); 

is equal to

if(hour < 12)
    ViewData["greeting"] = "Good morning"; 
else
    ViewData["greeting"] = "Good afternoon"; 

if this is easier to understand...