5.23.2013 How to Generate Random Number (with/without a range)

Random numbers
Do you know what face of dice is going to come up before throwing? That's what random means. We learn how to create a program that shows a random number each time you click a button.

Introduction to "Random"

The Oxford dictionary defines "Random" as "done, chosen, [something] without [somebody] deciding in advance what is going to happen, or without any regular pattern". For example, if you stack books one over the other without maintaining a specific order (such as alphabetically), then you are stacking the books randomly. It is kind of like the shuffling of cards. Nobody knows which card will go where after the shuffling (or randomization).

Randomizing is something when you need something to surprise people or something the result of which is just totally unknown. Such as you roll the dice to see a random face each time. Wikipedia has a Random Article feature which shows you a random Wikipedia article each time you click the link.


There is a Random() function (from the system unit) which can generate random number with a given value. You can use an Integer as a value/parameter.See the syntax below:

function Random(
  l: LongInt

function Random(
  l: Int64

function Random: extended; 

If you write:
-it will return a random number. It will be different each time the code is run, as this is what its designed to do.

You can create a Console program (Project-> NewProject-> Program-> OK) and copy-paste the following code:

program rand_number;

{$mode objfpc}{$H+}

  {$IFDEF UNIX}{$IFDEF UseCThreads}

  i: Integer;

  WriteLn('This program will generate 5 random numbers between 0 and 100');


  for i := 0 to 5 do



Run it (F9 or Run-> Run) and press enter. This will show 5 random numbers ranging from 0 to 99.

Random with Range

What if you want random numbers from 80 to 100? You can try:

WriteLn(Random(20) + 80);

First, we generate a random number of (100-80=)20, because we could have maximum of 20 random numbers between 80 and 100. That will, say, generate a random number like 12.

Now we can't use 12 because our range starts from 80. So we add 80 with 12 and get 92 as a result. 92 is between 80 and 100, right? This way Random(20) could generate other numbers:
6, which would be then turned into (6+80)=86;
0, which would be then turned into (0+80)=80;
19, which would be then turned into (19+80)=99;

But what happens when you don't want random number to start from zero? Instead, you want to start it from 1. Just add 1 to the result of Random() function. For example, if you want to roll a dice (virtually of course :-) ), you will have to:


We add 1 because the Random() function returns random numbers starting from 0 (zero). We don't have a face of dice with 0 dots, so we start from 1. The documentation for Random() function describes:
...Random returns a random number larger or equal to 0 and strictly less than L....
So, when we write Random(6), we would never get 6 as a return. It will always return some number less than 6. If it returns maximum possible number 5, then we have addition of 1 to make it a 6.

So we can put together another way of presenting this with an algorithm:

Random(Range End - Range Start) + Range Start;

Let's see a Quick Tutorial for a GUI program.

Quick Tutorial

Create a new Application Project (Project-> New Project-> Application-> OK). Prepare the form layout like the following:

Random number gen form design Lazarus IDE

Name the Editbox near to "From" as edtFrom and the other one as edtTo. Name the tButton as btnRandom and name also the TLabel which will show the random number as lblRandom. Set the text of both Editboxes as a numeric value.

Now double click the button and enter:

  rFrom, rTo, rRand: integer;

  Randomize; // we generate a new sequence every time
             //  the program is run

  rFrom := StrToInt(edtFrom.Text);
  rTo := StrToInt(edtTo.Text);

  rRand := Random(rTo - rFrom) + rFrom;

  lblRandom.Caption := IntToStr(rRand);


[Note: As the discussion in the comments, the above code has an error. The line rRand := Random(rTo - rFrom) + rFrom; should be rRand := Random(rTo - rFrom + 1) + rFrom; I am testing it further and I will update the code and the project zip download.]

Now Run the project (F9 or Run-> Run). You will see the form like below:

Random number generator program code running

Enter desired values and click the button.

This is a basic implementation of the program. If you enter non-numeric or non-integer inputs, the program will crash. You will have to apply Val() function in order to face this error. Such as:

rFrom: Integer;
Code: Integer; 

  Val(edtFrom.text, rFrom, Code);
  if Code <> 0 then begin
    ShowMessage('Error in input. Please enter a numeric integer as input.');



Download Sample Code ZIP

You can download example source code zip file from here: http://db.tt/uIzAEOfU
Or here: http://bit.ly/195ERZK
Size: 520 KB
The package contains compiled executable EXE file.

Photo credit: en.kioskea.net


ZöBook said...

In the article you assert that Random(x) generate numbers greater or equal than zero and strictly lower than x. There is no consistency in the examples. Random(100) does not give you 0 to 100, gives you 0 to 99, just as the dice example in which Random(6) gives you 0 to 5.

Adnan Shameem said...

Thank you for pointing out this error. Well you are right about Random(100). It generates 0 to 99, not 100. Because random() returns a value that is less than the parameter. I have corrected it and it was a simple mistake that I made unconsciously.

But in the second example I used Random(6)+1. Random(6) returns 0 to 5. Then we add 1 to the result, making it return 1 to 6. That is correct already.

You have made me think about the whole article again, so I found out that the following algorithm has an error:
Random(Range End - Range Start) + Range Start;

It should be:
Random(Range End - Range Start + 1) + Range Start;

I will update the code based on further testing.

ZöBook said...

Yes, indeed the dice example was correct, you say in the article that Random(6) gives you 0 to 5, then add 1 to give you 1 to 6. I know is a small mistake (i already know how Random work on pascal) buy think could confuse the people that are learning ;)
And you are correct about the range random, you should add 1 inside the random and as the result is zero based you add the range start.
I really like your page, was helpful to introduce me to Lazarus because i'm moving from Delphi.

Eddie Bole said...

Thanks Adrian for the nice simple tutorial. I made a random number list maker in VB6, similar to your example.
I have a couple of questions:
How would you make the range EditBoxes numeric only?
How would you add MULTIPLE random numbers to an EditBox instead of a label?

Keep up the good work Adrian.

Adnan Shameem said...

@Eddie Bole
Thanks for reading.
(1) To make it numeric only, the easiest option is to use TSpinEdit (under Misc tab) instead of TEdit. You'll then have to use SpinEdit.Value instead of Edit1.Text. You would have to get rid of StrToInt() functions because SpinEdit.Value is an integer already. Don't forget to set a MaxValue.
(2) How do you want the numbers to be shown in the TEdit? Separated by a space? If you want to get several random numbers in a TEdit you can run a for loop and concatenate the values and a space to the Edit.Text.

Hope it helps.

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