Develop Reference

Copy odd numbers from array into another array

Im new to programming and struggling with this task:
In array X [20] random numbers from 1 to 30 are entered, in array Y enter only odd numbers from array X.
Print down Y.
int[] x = new int[20];
Random rnd = new Random();
int counter = 0;
int[] y;
for (int i = 0; i < x.Length; i++)
{
x[i] = rnd.Next(1, 30);
if (x[i] % 2 !=0 )
{
y = new int[counter];
counter++;
y[counter] = x[i];
}
}
foreach (int number in y)
{
Console.WriteLine(number);
}
Im having problems to fill Y array with odd numbers without defining length of Y, I tried with adding counter but getting some errors all the time,
If someone can help me with some suggestions that would be helpful, thank you!

This looks like homework, so I guess using a more appropriate collection such as a List<int> is out of the question, just as using Linq.
At y = new int[counter]; you're reinitializing the array. This happens each iteration, so your final array only holds the latest added value, and all values before that will be set to their default: 0.
You could've seen this by debugging your code by setting breakpoints, stepping through the code and inspecting your variables. You could then also have provided a more proper problem description than "getting some errors".
If you know the input is never larger than 20, you can initialize the output array to the same size and keep a counter of how many values you copied (the latter of which you already do).
Then when printing, only print the elements up till that count with a for loop instead of foreach.
So something like this:
int[] x = new int[20];
int[] y = new int[x.Length];
Random rnd = new Random();
int counter = 0;
for (int i = 0; i < x.Length; i++)
{
x[i] = rnd.Next(1, 30);
if (x[i] % 2 != 0)
{
y[counter] = x[i];
counter++;
}
}
for (int i = 0; i < counter; i++)
{
Console.WriteLine(y[i]);
}

Your problem is that you create a new y array for each odd number you find. You need to create the array only once and then fill it.
Since you don't know how many odd numbers there will be, I suggest to use a List<int> instead:
int[] x = new int[20];
Random rnd = new Random();
List<int> y = new List<int>(); // create the list before the loop
for (int i = 0; i < x.Length; i++)
{
x[i] = rnd.Next(1, 30);
if (x[i] % 2 !=0 )
y.Add(x[i]); // add odd number to list
}
foreach (int number in y)
{
Console.WriteLine(number);
}

See, In your case you are not aware about the number of odd numbers in that random array. so Array will not be a right choice here if you are following the current implementation. If you want the output as array, then Why not a simple LINQ with Where like this example:
First you collect all random numbers to your array as you are doing currently:
int[] randomIntegers = new int[20];
Random rnd = new Random();
for (int i = 0; i < randomIntegers.Length; i++)
{
randomIntegers[i] = rnd.Next(1, 30);
}
Now you have the all random numbers in x now perform the following operation:
int[] oddValues = randomIntegers.Where(a=> a % 2 !=0).ToArray();

Adding specific numbers from one sequence to another

I feel like this task should not be done this way...
My sequenceY length is not equal to number of steam numbers because you can't assign int[] length with int that have 0 as a starting value.
Therefore my sequenceY have a lot of 0 inside and I can't print the whole sequence. I even tried adding this after for loop:
sequenceY = new int[steamnumbercounter];
But it didn't work ... Why ?
My other question is how do programmers deal with sequences that have unknown length?
I managed to print only steam numbers but the task says print sequenceY not only part of it.
// 4. sequenceX[20] is made of random numbers from 1 to 30 ,
// sequenceY is made of steam numbers from sequenceX. Print sequneceY.
int[] nizx = new int[20];
int[] nizy = new int[20];
int n = 0;
int steamnumbercounter = 0;
Random rnd = new Random();
for (int i = 0; i < nizx.Length; i++)
{
nizx[i] = rnd.Next(1, 30);
if (nizx[i]%2==0)
{
nizy[n] = nizx[i];
n++;
steamnumbercounter++;
}
Console.Write("{0} , ", nizx[i]);
}
for (int i = 0; i < steamnumbercounter; i++)
{
Console.WriteLine("{0} , ",nizy[i]);
}

Partial code review along with an answer.
But it didn't work ... Why ?
That code didn't work because you're reassigning sequenceY to a completely new value.
My other question is how do programmers deal with sequences that have unknown length?
So, with that known we can do a few things here: create an array and use Array.Resize, use a List<T>, fill the initial array then swap it for one of the right size which is filled.
I'm going to assume a "steam" number is an even one.
Your naming is not good: nizx and nizy don't convey the meaning or line up with the problem.
I'm going to demonstrate the last option (since you stated that you don't know how to use many of the moderately complex parts of .NET in this class yet, which is fine): fill the initial array and swap it for a new one. This will run in O(n^2) time (sorta).
So, let's start with our source array.
int[] sequenceX = new int[20];
Next we'll define our destination array to be the same size as our source array. (This is the maximum number of values that could be stored in it, we'll shrink it later.)
int[] sequenceY = new int[sequenceX.Length];
Then we need a variable to hold how many numbers we found that meet our criteria:
int steamNumbers = 0;
And lastly, our Random.
Random random = new Random();
Then, we look through all our sequenceX as you did, but we'll update the logic a bit.
for (int i = 0; i < sequenceX.Length; i++)
{
sequenceX[i] = random.Next(1, 30);
if (sequenceX[i] % 2 == 0)
{
sequenceY[steamNumbers] = sequenceX[i];
steamNumbers++;
}
}
So our code looks almost the same as yours, but we have one more thing to do: since you only want sequenceY to contain steamNumbers we have to shrink it or something.
int[] tempSequenceY = sequenceY;
sequenceY = new int[steamNumbers];
for (int i = 0; i < steamNumbers; i++)
{
sequenceY[i] = tempSequenceY[i];
}
Now sequenceY only has your steam numbers in it.
Final code:
int[] sequenceX = new int[20];
int[] sequenceY = new int[sequenceX.Length];
int steamNumbers = 0;
Random random = new Random();
for (int i = 0; i < sequenceX.Length; i++)
{
sequenceX[i] = random.Next(1, 30);
if (sequenceX[i] % 2 == 0)
{
sequenceY[steamNumbers] = sequenceX[i];
steamNumbers++;
}
}
int[] tempSequenceY = sequenceY;
sequenceY = new int[steamNumbers];
for (int i = 0; i < steamNumbers; i++)
{
sequenceY[i] = tempSequenceY[i];
}
// Print your `sequenceY` here.
You could extract this to a method pretty easily as well:
public int[] GetSteamNumbers(int sequenceCount, int randomMinimum, int randomMaximum)
{
int[] sequenceX = new int[sequenceCount];
int[] sequenceY = new int[sequenceX.Length];
int steamNumbers = 0;
Random random = new Random();
for (int i = 0; i < sequenceX.Length; i++)
{
sequenceX[i] = random.Next(randomMinimum, randomMaximum);
if (sequenceX[i] % 2 == 0)
{
sequenceY[steamNumbers] = sequenceX[i];
steamNumbers++;
}
}
int[] tempSequenceY = sequenceY;
sequenceY = new int[steamNumbers];
for (int i = 0; i < steamNumbers; i++)
{
sequenceY[i] = tempSequenceY[i];
}
return sequenceY;
}
And then call it with:
int[] steamNumbers = GetSteamNumbers(20, 1, 30);
Of course, for the more advanced users (this doesn't help you, but it may help others) we can do something as follows using LINQ:
var random = new Random();
var sequenceY = Enumerable.Range(1, 20)
.Select(x => random.Next(1, 30))
.Where(x => x % 2 == 0)
.ToArray();
Which should have the same effect. (Just demonstrating that there are still things in C# to look forward to in the future.)
Disclaimer: I wrote this entire answer outside of the IDE and without actually compiling it, I make no guarantees to the accuracy of the code but the procedure itself should be fairly straight forward.

The thing with arrays in C# is that they are of fixed size.
You'll have to iterate through and re-create it or use a IEnumerable that has dynamic sizes, such as Lists.
Solution here would be to use a List that contains your integers and then you would use nizx.Add(rnd.Next(1, 30));

Elaborating on my comment above: You can create a 'fake' list by concatenating the values you need in a string, separated by commas. The string.Split(',') will give you the resulting array that you need.
Given a string of form "a,b,c,d" string.Split(',') will create the array ["a","b,"c","d"]. The code:
{
int[] nizx = new int[20];
string numberString = string.Empty;
int n = 0;
Random rnd = new Random();
for (int i = 0; i < nizx.Length; i++)
{
nizx[i] = rnd.Next(1, 30);
if (nizx[i] % 2 == 0)
{
numberString += nizx[i] + ",";
n++;
}
}
var numberArray = numberString.Split(',');
for (int i = 0; i < n; i++)
{
Console.WriteLine("{0} , ", numberArray[i]);
}
}

C# WinForm Enum ToString

// in general i take random colors from an Enum and insert that into a new array.
when i ask by BoxMessage inside the loop For each Array[i] i get random colors as expected.
If i comment (\) the message box and ask for Array[1-5] outside the loop, i get the same color for all 5 message box.
I believe it related to the fact that my message box inside the loop successfully convert the Enum into a string, when i tried to convert the whole array in the loop ( array[i].ToString(); ) it didn't quite work.
please advise....
public void GetArray()
{
array = new EnumColor[5];
for (int i = 0; i < 5; i++)
{
rnd = new Random();
int rndnum = rnd.Next(0, 4);
array[i] = (EnumColor)rndnum;
MessageBox.Show(array[i].ToString());
}
MessageBox.Show(array[0].ToString());
MessageBox.Show(array[1].ToString());
MessageBox.Show(array[2].ToString());
MessageBox.Show(array[3].ToString());
MessageBox.Show(array[4].ToString());
}

for (int i = 0; i < 5; i++)
{
rnd = new Random();
int rndnum = rnd.Next(0, 4);
array[i] = (EnumColor)rndnum;
MessageBox.Show(array[i].ToString());
}
You're overwriting elements in the array, don't do this.
Retrieve elements into a temporary local variable instead (and use .Length instead of a hardcoded length)
The bug itself is caused by Random being incorrectly seeded, it's being regerated on every iteration.
Random rnd = new Random();
for (int i = 0; i < array.Length; i++)
{
int rndnum = rnd.Next(0, 4);
EnumColor selected = (EnumColor)array[i];
MessageBox.Show( selected .ToString() );
}

You are getting the same color because you are declaring random instance each time in your loop, just move the declaration outside:
rnd = new Random();
for (int i = 0; i < 5; i++)
{ ... }
If you like to learn more about why Random class works that way you can take a look at this answer.

The problem is indeed the Random being re-created for every loop. But here's the full explanation.
When you create a Random, it uses the Environment.TickCount as seed value. (milliseconds since system startup). So if your loop is fast enough, this will always be the same value, so the rnd.Next function will always return the same value.
So now for the tricky part.. why did it work with the messagebox inside the loop? By showing the message, the loop became slower by waiting for you clicking the ok button. Therefore the next loop, the seed was indeed changed. That's why you got another value.

How to generate an example of valid input using Regex?

I want to generate an example of a valid input by Regex pattern. I'm programming with C# .Net . Like this:
//this emthod doesn't exists, its an example of funcionality that I want.
Regex.GenerateInputExample("^[0-9]{15}$");
So, this example gives-me a possible value, like 000000000000000. How to do this?

So, this problem would take some time to solve, since the functionality is not built in. I'll give a general way to solve it:
Using an ascii (or unicode) chart find out the character codes that correspond to the characters you are using for your regex (65 =A, 69 = D, etc)
Create a random function with those bounds. Multiple bounds would take a little more trickery (A-Z =26, 0-9 = 10, so a random number from 0- 35)
Random random = new Random();
int randomNumber = random.Next(65, 70); // this generates a random number including the bounds of 65-69)
char temp = (char)random;
Next you would take the randomly generated characters and add them together into a string.
int lowerBound = 65, upperBound =69;
int length = 6;
char temp;
int randomNumber;
string result= "";
Random rand = new Random();
for (int a = 0; a <= length; a++)
{
randomNumber = rand.Next(lowerBound, upperBound);
temp = (char)randomNumber;
result = result + temp;
} //result is the indirect regex generated string
Indirectly giving you a regex generated string.
The next step is parsing information out of a regex. I've provided a simple case below that will not work for every regex, due to regex complexity.
Regex bob = new Regex("[A-Z]");
int lowerBound = Convert.ToInt32(bob.ToString()[1]);
int upperBound = Convert.ToInt32(bob.ToString()[3]);
int length = 6; //length of the string to be generated
char temp;
int randomNumber;
string result= "";
Random rand = new Random();
for (int a = 0; a <= length; a++)
{
randomNumber = rand.Next(lowerBound, upperBound);
temp = (char)randomNumber;
result = result + temp;
}
( This process could be streamlined into class and utilized etc)

Avoiding random duplicates

System.Random generator = new Random(DateTime.Now.Millisecond);
int[] lotteryNumber = new int[7];
Console.WriteLine("Your lottery numbers: ");
for (int i = 0; i<7; i++)
{
lotteryNumber[i] = generator.Next(1, 37);
Console.Write("{0} ",lotteryNumber[i]);
}
Console.ReadLine();
I need to make a program that prints 7 lottery numbers, but without duplicates. The code above prints 7 random numbers in the range of (1-37), but duplicates appaer. I need a way to prevent duplicate numbers from appearing.

The simplest approach IMO would be to generate a sequence of all the possible numbers (i.e. 1-37), shuffle the collection, then take the first seven results.
Searching on Stack Overflow for "Fisher-Yates shuffle C#" will find lots of examples.
In fact, you could modify the Fisher-Yates shuffle to yield results as you took them, so you could write a method such as:
var numbers = Enumerable.Range(1, 37).Shuffle().Take(7).ToList();

You could take a dictionary but make sure that you prevent duplicate key insertion. Keys of dictionary would serve as the unique numbers you need

You could toss them into a HashSet<int>. If you Add and it returns false, generate a new number.

If you're trying to pick numbers from a range without repetitions, you need to create an array of all the possible numbers and then "shuffle" a random selection out:
int[] allPossibleNumbers = Enumerable.Range(1, 37).ToArray();
int[] lotteryNumber = new int[7];
for (int i = 0; i < 7; i++)
{
int index = r.Next(i, 37);
lotteryNumber[i] = allPossibleNumbers[index];
allPossibleNumbers[index] = allPossibleNumbers[i];
// This step not necessary, but allows you to reuse allPossibleNumbers
// rather than generating a fresh one every time.
// allPossibleNumbers[i] = lotteryNumber[i];
}

Generate a list with your 37 items.
Then in your for, select one and delete the selected

Maybe this could help, if you get the existing number just try to find new one that isn't in the array:
static void Main(string[] args)
{
System.Random generator = new Random(DateTime.Now.Millisecond); int[] lotteryNumber = new int[7];
Console.WriteLine("Your lottery numbers: ");
for (int i = 0; i < 7; i++)
{
int lNumber = 0;
do
{
lNumber = generator.Next(1, 37);
}
while (lotteryNumber.Contains(lNumber));
lotteryNumber[i] = lNumber;
Console.Write("{0} ", lotteryNumber[i]);
}
Console.ReadLine();
}

HashSet<int> set = new HashSet<int>();
System.Random generator = new Random(DateTime.Now.Millisecond);
while(set.Count < 7){
set.Add(generator.Next(1,37);
}
That should work, since a HashSet will automatically ignore duplicates. Just loop until the set reaches the number of units you need. Only potential problem is it has the POTENTIAL (unlikely) to loop for a long time, but it should eventually respond.

so I took your original code...found some logic errors and added the fix you were looking for to prevent random number duplicates.
Enjoy!
System.Random generator = new Random(DateTime.Now.Millisecond);
int[] lotteryNumber = new int[7];
int lowerBounds = 1;
int upperBounds = 8;
int maxNumberLotteryValues = 7;
if ( ( upperBounds - lowerBounds ) < (maxNumberLotteryValues))
{
Console.Write("Warning: Adjust your upper and lower bounds...there are not enough values to create a unique set of Lottery numbers! ");
}
else
{
Console.WriteLine("Your lottery numbers: ");
for (int i = 0; i < maxNumberLotteryValues; i++)
{
int nextNumber = generator.Next(lowerBounds, upperBounds);
int count = lowerBounds; //Prevent infinite loop
while ((lotteryNumber.Contains(nextNumber))
&& (count <= upperBounds))
{
nextNumber = generator.Next(lowerBounds, upperBounds);
count++; //Prevent infinite loop
}
lotteryNumber[i] = nextNumber;
Console.Write("{0} ", lotteryNumber[i]);
}
}
Console.ReadLine();

const int nBalls = 37;
const int nPicks = 6;
int[] balls = new int[nPicks];
Random rnd = new Random(DateTime.Now.Millisecond);
int remainingBalls=nBalls;
int remainingPicks=nPicks;
for (int i = 1; i <= nBalls; i++)
{
if (rnd.Next(1, remainingBalls+1) <= remainingPicks)
balls[--remainingPicks]=i;
remainingBalls--;
}
Console.WriteLine(string.Join(",",balls));
Will outperform Shuffle and HashSet methods as nPicks/nBalls gets larger.