Random Function In C#

Question

What is the best way to select a random number between two numbers in C#?

For instance, if I have a low value of 23 and a high value of 9999, what would the correct code be to select a random number between and including these two numbers? Thanks in Advance

Solution 1

Use the Random class like this:

Random rnd = new Random(); 
rnd.Next(23, 10000);

Make sure that you only initialize your rnd object once, to make sure it really generate random values for you.

If you make this loop for instance:

for( int i = 0 ; i < 10; i++ ){
  Random rnd = new Random();
  var temp = rnd.Next(23, 10000);
}

temp will be the same each time, since the same seed is used to generate the rnd object, but like this:

Random rnd = new Random();
for( int i = 0 ; i < 10; i++ ){
  var temp = rnd.Next(23, 10000);
}

It will generate 10 unique random numbers (but of course, by chance, two or more numbers might be equal anyway)

Solution 2

It depends on whether you are looking for an integer or a double. For an integer use Random.Next(minValue, maxValue):

Returns a random number within a specified range.

Note that the minValue is inclusive but the maxValue is exclusive.

There is no equivalent method for choosing a random double within a specified range. Instead you should use the NextDouble method to pick a random number between 0.0 and 1.0 and then use a linear transformation to extend, reduce and/or translate the range.

Comments

[Amra]

Are you sure ? i write this code in VS but there is no sub like 'Next' in Random Function . Tnx for your helping

[Mark Byers]

@Eva: It's an instance method - you need a Random object (for example you can create one when your program starts).

[Øyvind Bråthen]

@Eva - There is no static Next function if that is what you are trying to do, so you can't simply write Random.Next. See my answer for details.

[Amra]

Thanks . If i want select a random string from a Text file , what am i going to do ?

[Mark Byers]

@Eva: There are also some examples on the page I linked to. Have you looked at those?

[Jon Skeet]

@Eva: The simplest way would be to read all the lines from the text file, remembering them all and counting them, then select a random number based on the count.

[Amra]

Yes I see Them . Thanks . Nice Example ;)

[codeulike]

Your example makes it look like a static method because of the capital R on Random. Its a little confusing.

[Darth Coder]

@Knobloch: Can you explain why the seed remains the same in the first case and changes in the second condition? Doesn't the loop create a new instance for 'rnd' for iteration of the loop?