new to coding,
Quick Homework problem Im having trouble on.
I must find the sum of the first 100 natural numbers. 1+2+3+4, and so on. While using "while loops".
This is what I have so far.
Dim sum, count As Integer
Const num As Integer = 1
Console.WriteLine("The Sum of the first 100 natural numbers is:")
count = num + 1
While count <= 100
Count = num + count
End While
Sum = count
Console.WriteLine("{0}", sum)
Console.ReadLine()
I know for a fact the math and while loop is incorrect, but Im not sure how to fix it. I keep getting 101 as my answer.
You do not need the num constant and the line: count = num + 1
While count <= 100
sum += count
count += 1
End While
I know you are learning and the solution must use a While loop but for fun see what you can do in 2 lines of code.
Dim arrNumbers = Enumerable.Range(1, 100).ToArray
Console.WriteLine(arrNumbers.Sum)
Since your question title asked for sums of all [1 - 100] as well as the evens and odds, here is some LINQ for each one
Dim sum = Enumerable.Range(1, 100).Sum()
Dim sumEvens = Enumerable.Range(1, 100).Sum(Function(i) If(i Mod 2 = 0, i, 0))
Dim sumOdds = Enumerable.Range(1, 100).Sum(Function(i) If(i Mod 2 = 0, 0, i))
Console.WriteLine($"Sum: {sum}, Sum evens: {sumEvens}, Sum odds: {sumOdds}")
Sum: 5050, Sum evens: 2550, Sum odds: 2500
Enumerable.Range(1, 100) creates your sequence of all [1 - 100]. Call the LINQ function Sum to simply get the sum, or you can pass a lambda function to Sum to transform the numbers. Use Mod to select evens or odds. The method above sets odds to 0 for the evens, and vice-versa for the odds.
An alternative to this method of transforming the sequence is to use Where to select only even numbers then Sum, which may be more readable.
Dim sumEvens = Enumerable.Range(1, 100).Where(Function(i) i Mod 2 = 0).Sum()
With your code you've done a few things in a weird way.
Let's simplify the code and see what's going wrong.
To start with you are setting num as a constant to the value of 1. Since it's not changing, we'll put the value in directly.
You'd then have this:
count = 1 + 1
While count <= 100
count = 1 + count
End While
sum = count
So that means count will start at 2 and goes up by one until the value of count is greater than 100 - and that's when it is 101.
Then you set sum to count and you get 101.
What you needed to do was start count as 1 and sum up the values inside the loop.
count = 1
While count <= 100
sum = sum + count
count = count + 1
End While
And that gives the correct result of 5050.
While count <= 100
num += count
count += 1
End While
Sum = num
I'm looking at the documentation for random():
https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man3/srandomdev.3.html#//apple_ref/c/func/random
It returns successive pseudo-random numbers in the range from 0 to (2**31)-1.
I don't want it to return 0 ever.
I'm thinking about writing:
long rand = random() + 1;
But if I'm not mistaken, long can be 32-bits on a 32-bit processor. I guess I would risk stack overflow then.
What is the best approach to getting a random number between 1 and (2**31)-1?
NSUInteger r = arc4random_uniform(N) + 1;
This will generate a number between 1 and N. arc4random_uniform(N) generates a number between 0 and N-1.
You should have no problem with overflow.
long rand = 0;
while (rand == 0) {
rand = random();
}
This will almost absolutely certainly run exactly once. In a very, very rare case (that will never happen), it will run twice.
(Note that this is just a simplified version of how arc4random_uniform works. If you can use that function, as suggested by Jeff, you should.)
The maximum value returned by random() is RAND_MAX, so you can do this:
long rand = 1 + (random() % RAND_MAX);
When random() returns a value between zero and RAND_MAX-1, inclusive, you offset it by adding 1. When random() returns exactly RAND_MAX, modulo operator % converts the result to zero, so rand would be 1 again.
The drawback of this approach is that the probability of getting 1 becomes roughly twice as high as that of getting any other number.
I'd like to generate random numbers in a range (say between 0 and 1) - but with a certain step size (say 0.05).
There's a python function that does exactly that:
random.randrange ( [start,] stop [,step] )
So my problem is not how to generate random numbers in a range - instead I need a way to do this with a certain step size.
How can this be done in .NET?
You could generate a random integer between 0 and 20 and divide the result by 20
Dim rnd = New Random()
Dim nextValue = rnd.Next(21) / 20
This will give you a random number between 0 and 1 (inclusive) in 0.05 increments
You can try something like that:
Dim objRandom As New System.Random
Label1.Text = Math.Round(objRandom.NextDouble() * 2, 1) / 2
So you create a random double and you round it to one digit (example: 0.8).
Then you divided it with 2 and you get what you want
I'm sorry the title is so confusingly worded, but it's hard to condense this problem down to a few words.
I'm trying to find the minimum value of a specific equation. At first I'm looping through the equation, which for our purposes here can be something like y = .245x^3-.67x^2+5x+12. I want to design a loop where the "steps" through the loop get smaller and smaller.
For example, the first time it loops through, it uses a step of 1. I will get about 30 values. What I need help on is how do I Use the three smallest values I receive from this first loop?
Here's an example of the values I might get from the first loop: (I should note this isn't supposed to be actual code at all. It's just a brief description of what's happening)
loop from x = 1 to 8 with step 1
results:
x = 1 -> y = 30
x = 2 -> y = 28
x = 3 -> y = 25
x = 4 -> y = 21
x = 5 -> y = 18
x = 6 -> y = 22
x = 7 -> y = 27
x = 8 -> y = 33
I want something that can detect the lowest three values and create a loop. From theses results, the values of x that get the smallest three results for y are x = 4, 5, and 6.
So my "guess" at this point would be x = 5. To get a better "guess" I'd like a loop that now does:
loop from x = 4 to x = 6 with step .5
I could keep this pattern going until I get an absurdly accurate guess for the minimum value of x.
Does anybody know of a way I can do this? I know the values I'm going to get are going to be able to be modeled by a parabola opening up, so this format will definitely work. I was thinking that the values could be put into a column. It wouldn't be hard to make something that returns the smallest value for y in that column, and the corresponding x-value.
If I'm being too vague, just let me know, and I can answer any questions you might have.
nice question. Here's at least a start for what I think you should do for this:
Sub findMin()
Dim lowest As Integer
Dim middle As Integer
Dim highest As Integer
lowest = 999
middle = 999
hightest = 999
Dim i As Integer
i = 1
Do While i < 9
If (retVal(i) < retVal(lowest)) Then
highest = middle
middle = lowest
lowest = i
Else
If (retVal(i) < retVal(middle)) Then
highest = middle
middle = i
Else
If (retVal(i) < retVal(highest)) Then
highest = i
End If
End If
End If
i = i + 1
Loop
End Sub
Function retVal(num As Integer) As Double
retVal = 0.245 * Math.Sqr(num) * num - 0.67 * Math.Sqr(num) + 5 * num + 12
End Function
What I've done here is set three Integers as your three Min values: lowest, middle, and highest. You loop through the values you're plugging into the formula (here, the retVal function) and comparing the return value of retVal (hence the name) to the values of retVal(lowest), retVal(middle), and retVal(highest), replacing them as necessary. I'm just beginning with VBA so what I've done likely isn't very elegant, but it does at least identify the Integers that result in the lowest values of the function. You may have to play around with the values of lowest, middle, and highest a bit to make it work. I know this isn't EXACTLY what you're looking for, but it's something along the lines of what I think you should do.
There is no trivial way to approach this unless the problem domain is narrowed.
The example polynomial given in fact has no minimum, which is readily determined by observing y'>0 (hence, y is always increasing WRT x).
Given the wide interpretation of
[an] equation, which for our purposes here can be something like y =
.245x^3-.67x^2+5x+12
many conditions need to be checked, even assuming the domain is limited to polynomials.
The polynomial order is significant, and the order determines what conditions are necessary to check for how many solutions are possible, or whether any solution is possible at all.
Without taking this complexity into account, an iterative approach could yield an incorrect solution due to underflow error, or an unfortunate choice of iteration steps or bounds.
I'm not trying to be hard here, I think your idea is neat. In practice it is more complicated than you think.
I know the modulus (%) operator calculates the remainder of a division. How can I identify a situation where I would need to use the modulus operator?
I know I can use the modulus operator to see whether a number is even or odd and prime or composite, but that's about it. I don't often think in terms of remainders. I'm sure the modulus operator is useful, and I would like to learn to take advantage of it.
I just have problems identifying where the modulus operator is applicable. In various programming situations, it is difficult for me to see a problem and realize "Hey! The remainder of division would work here!".
Imagine that you have an elapsed time in seconds and you want to convert this to hours, minutes, and seconds:
h = s / 3600;
m = (s / 60) % 60;
s = s % 60;
0 % 3 = 0;
1 % 3 = 1;
2 % 3 = 2;
3 % 3 = 0;
Did you see what it did? At the last step it went back to zero. This could be used in situations like:
To check if N is divisible by M (for example, odd or even)
or
N is a multiple of M.
To put a cap of a particular value. In this case 3.
To get the last M digits of a number -> N % (10^M).
I use it for progress bars and the like that mark progress through a big loop. The progress is only reported every nth time through the loop, or when count%n == 0.
I've used it when restricting a number to a certain multiple:
temp = x - (x % 10); //Restrict x to being a multiple of 10
Wrapping values (like a clock).
Provide finite fields to symmetric key algorithms.
Bitwise operations.
And so on.
One use case I saw recently was when you need to reverse a number. So that 123456 becomes 654321 for example.
int number = 123456;
int reversed = 0;
while ( number > 0 ) {
# The modulus here retrieves the last digit in the specified number
# In the first iteration of this loop it's going to be 6, then 5, ...
# We are multiplying reversed by 10 first, to move the number one decimal place to the left.
# For example, if we are at the second iteration of this loop,
# reversed gonna be 6, so 6 * 10 + 12345 % 10 => 60 + 5
reversed = reversed * 10 + number % 10;
number = number / 10;
}
Example. You have message of X bytes, but in your protocol maximum size is Y and Y < X. Try to write small app that splits message into packets and you will run into mod :)
There are many instances where it is useful.
If you need to restrict a number to be within a certain range you can use mod. For example, to generate a random number between 0 and 99 you might say:
num = MyRandFunction() % 100;
Any time you have division and want to express the remainder other than in decimal, the mod operator is appropriate. Things that come to mind are generally when you want to do something human-readable with the remainder. Listing how many items you could put into buckets and saying "5 left over" is good.
Also, if you're ever in a situation where you may be accruing rounding errors, modulo division is good. If you're dividing by 3 quite often, for example, you don't want to be passing .33333 around as the remainder. Passing the remainder and divisor (i.e. the fraction) is appropriate.
As #jweyrich says, wrapping values. I've found mod very handy when I have a finite list and I want to iterate over it in a loop - like a fixed list of colors for some UI elements, like chart series, where I want all the series to be different, to the extent possible, but when I've run out of colors, just to start over at the beginning. This can also be used with, say, patterns, so that the second time red comes around, it's dashed; the third time, dotted, etc. - but mod is just used to get red, green, blue, red, green, blue, forever.
Calculation of prime numbers
The modulo can be useful to convert and split total minutes to "hours and minutes":
hours = minutes / 60
minutes_left = minutes % 60
In the hours bit we need to strip the decimal portion and that will depend on the language you are using.
We can then rearrange the output accordingly.
Converting linear data structure to matrix structure:
where a is index of linear data, and b is number of items per row:
row = a/b
column = a mod b
Note above is simplified logic: a must be offset -1 before dividing & the result must be normalized +1.
Example: (3 rows of 4)
1 2 3 4
5 6 7 8
9 10 11 12
(7 - 1)/4 + 1 = 2
7 is in row 2
(7 - 1) mod 4 + 1 = 3
7 is in column 3
Another common use of modulus: hashing a number by place. Suppose you wanted to store year & month in a six digit number 195810. month = 195810 mod 100 all digits 3rd from right are divisible by 100 so the remainder is the 2 rightmost digits in this case the month is 10. To extract the year 195810 / 100 yields 1958.
Modulus is also very useful if for some crazy reason you need to do integer division and get a decimal out, and you can't convert the integer into a number that supports decimal division, or if you need to return a fraction instead of a decimal.
I'll be using % as the modulus operator
For example
2/4 = 0
where doing this
2/4 = 0 and 2 % 4 = 2
So you can be really crazy and let's say that you want to allow the user to input a numerator and a divisor, and then show them the result as a whole number, and then a fractional number.
whole Number = numerator/divisor
fractionNumerator = numerator % divisor
fractionDenominator = divisor
Another case where modulus division is useful is if you are increasing or decreasing a number and you want to contain the number to a certain range of number, but when you get to the top or bottom you don't want to just stop. You want to loop up to the bottom or top of the list respectively.
Imagine a function where you are looping through an array.
Function increase Or Decrease(variable As Integer) As Void
n = (n + variable) % (listString.maxIndex + 1)
Print listString[n]
End Function
The reason that it is n = (n + variable) % (listString.maxIndex + 1) is to allow for the max index to be accounted.
Those are just a few of the things that I have had to use modulus for in my programming of not just desktop applications, but in robotics and simulation environments.
Computing the greatest common divisor
Determining if a number is a palindrome
Determining if a number consists of only ...
Determining how many ... a number consists of...
My favorite use is for iteration.
Say you have a counter you are incrementing and want to then grab from a known list a corresponding items, but you only have n items to choose from and you want to repeat a cycle.
var indexFromB = (counter-1)%n+1;
Results (counter=indexFromB) given n=3:
`1=1`
`2=2`
`3=3`
`4=1`
`5=2`
`6=3`
...
Best use of modulus operator I have seen so for is to check if the Array we have is a rotated version of original array.
A = [1,2,3,4,5,6]
B = [5,6,1,2,3,4]
Now how to check if B is rotated version of A ?
Step 1: If A's length is not same as B's length then for sure its not a rotated version.
Step 2: Check the index of first element of A in B. Here first element of A is 1. And its index in B is 2(assuming your programming language has zero based index).
lets store that index in variable "Key"
Step 3: Now how to check that if B is rotated version of A how ??
This is where modulus function rocks :
for (int i = 0; i< A.length; i++)
{
// here modulus function would check the proper order. Key here is 2 which we recieved from Step 2
int j = [Key+i]%A.length;
if (A[i] != B[j])
{
return false;
}
}
return true;
It's an easy way to tell if a number is even or odd. Just do # mod 2, if it is 0 it is even, 1 it is odd.
Often, in a loop, you want to do something every k'th iteration, where k is 0 < k < n, assuming 0 is the start index and n is the length of the loop.
So, you'd do something like:
int k = 5;
int n = 50;
for(int i = 0;i < n;++i)
{
if(i % k == 0) // true at 0, 5, 10, 15..
{
// do something
}
}
Or, you want to keep something whitin a certain bound. Remember, when you take an arbitrary number mod something, it must produce a value between 0 and that number - 1.