I have written a console application
Int64 sum = 0;
int T = Convert.ToInt32(Console.ReadLine());
Int64[] input = new Int64[T];
for (int i = 0; i < T; i++)
{
input[i] = Convert.ToInt32(Console.ReadLine());
}
for (int i = 0; i < T; i++)
{
int[,] Matrix = new int[input[i], input[i]];
sum = 0;
for (int j = 0; j < input[i]; j++)
{
for (int k = 0; k < input[i]; k++)
{
Matrix[j, k] = Math.Abs(j - k);
sum += Matrix[j, k];
}
}
Console.WriteLine(sum);
}
When I gave input as
2
1
999999
It gave Out of memory exception. Can you please help.
Look at what you are allocating:
input[] is allocated as 2 elements (16 bytes) - no worries
But then you enter values: 1 and 999999 and in the first iteration of the loop attempt to allocate
Matrix[1,1] = 4 bytes - again no worries,
but the second time round you try to allocate
Matrix[999999, 999999]
which is 4 * 10e12 bytes and certainly beyond the capacity of your computer even with swap space on the disk.
I suspect that this is not what you really want to allocate (you'd never be able to fill or manipulate that many elements anyway...)
If you are merely trying to do the calculations as per your original code, there is not need to allocate or use the array, as you only ever store one value and immediately use that value and then never again.
Int64 sum = 0;
int T = Convert.ToInt32(Console.ReadLine());
Int64[] input = new Int64[T];
for (int i = 0; i < T; i++)
input[i] = Convert.ToInt32(Console.ReadLine());
for (int i = 0; i < T; i++)
{
// int[,] Matrix = new int[input[i], input[i]];
sum = 0;
for (int j = 0; j < input[i]; j++)
for (int k = 0; k < input[i]; k++)
{
//Matrix[j, k] = Math.Abs(j - k);
//sum += Matrix[j, k];
sum += Math.Abs(j - k);
}
Console.WriteLine(sum);
}
But now beware - a trillion sums is going to take forever to calculate - it won't bomb out, but you might like to take a vacation, get married and have kids before you can expect a result.
Of course instead of doing the full squared set of calculations, you can calculate the sum thus:
for (int i = 0; i < T; i++)
{
sum = 0;
for (int j = 1, term = 0; j < input[i]; j++)
{
term += j;
sum += term * 2;
}
Console.WriteLine(sum);
}
So now the calculation is O(n) instead of O(n^2)
And if you need to know what the value in Matrix[x,y] would have been, you can calculate it by the simple expression Math.Abs(x - y) thus there is no need to store that value.
Related
While printing this jagged array I am getting the no of rows to be 1 less than expected. It should start with 0th index to (h-1)th index creating a total of h rows. What am I doing wrong?
h is the no. of rows.
int h = int.Parse(Console.ReadLine());
int[][] arr = new int[h][];
for(int i = 0; i < h; ++i)
{
arr[i] = new int[i+1];
}
for(int i = 0; i < h; i++)
{
Console.WriteLine();
for(int j = 0; j < i; j++)
{
Console.Write(arr[i][j] + " ");
}
}
That's because your inner for-loop has the condition j < i. If i is 0 in the first pass, the inner for-loop will not be passed.
Try it with
for(int j = 0; j < arr[i].Length; j++)
{
Console.Write(arr[i][j] + " ");
}
The arrays have a growing list of elements, starting with 1 so if you want to scan all the items:
for(int i = 0; i < h; i++)
{
Console.WriteLine();
for(int j = 0; j < (i + 1); j++)
{
Console.Write(arr[i][j] + " ");
}
}
Shouldn't it be:
new int[i];
Instead of:
new int[i+1]
Or is it h - 1? Just change that index of the array.
Or you need j < arr[i].Length
I have a quick question: I know that the complexity of both snippets is same. Yet, I want to know which one is comparatively better and why? This is the selection sort code:
This is what I wrote:
for (int i = 0; i < n - 1; i++)
{
for (int j = i + 1; j <= n - 1; j++)
{
if (a[j] < a[i])
{
int temp = a[i];
a[i] = a[j];
a[j] = temp;
}
}
}
This is what my friend wrote:
for (int i = 0; i < n - 1; i++)
{
int iMin = i;
for (int j = i + 1; j <= n - 1; j++)
{
if (a[j] < a[i])
{
iMin = j;
}
int temp = a[i];
a[i] = a[iMin];
a[iMin] = temp;
}
}
Your code is slightly faster because you make swaps only when a[j] < a[i], whereas your friend's code is always making a swap. So, in most cases, your code will make less swaps.
The complexity of both codes are indeed the same, but your "constants" are smaller, so your code is faster.
I'm writing a program to find the largest palindromic number made from product of 3-digit numbers. Firstly,i Create a method which has ability to check if it is a palindromic number. Here is my code :
static int check(string input_number)
{
for (int i = 0; i < input_number.Length/2; i++)
if (input_number[i] != input_number[input_number.Length - i])
return 0;
return 1;
}
After that, it's my main code:
static void Main(string[] args)
{
int k = 0;
for (int i = 0; i < 999; i++)
for (int j = 0; j < 999; j++)
{
k = i * j;
if (check(k.ToString()) == 1)
Console.Write(k + " ");
}
}
But when it has a problem that the length of input_number is zero. So my code doesn't run right way. What can I do to solve the length of input_number?
You have a few bugs in your code:
1. 3-digit-numbers range from `100` to `999`, not from `0` to `998` as your loops currently do.
So your Main method should look like this:
static void Main(string[] args)
{
int k = 0;
for (int i = 100; i <= 999; i++)
for (int j = 100; j <= 999; j++)
{
k = i * j;
if (check(k.ToString()) == 1)
Console.Write(k + " ");
}
}
Now all pairs of three digit numbers are checked. But to improve performance you can let j start at i, because you already checked e.g. 213 * 416 and don't need to check 416 * 213 anymore:
for (int i = 100; i <= 999; i++)
for (int j = i; j <= 999; j++) // start at i
And since you want to find the largest, you may want to start at the other end:
for (int i = 999; i >= 100; i--)
for (int j = 999; j >= 100; j--)
But that still does not guarantee that the first result will be the largest. You need to collect the result and sort them. Here is my LINQ suggestion for your Main:
var results = from i in Enumerable.Range(100, 900)
from j in Enumerable.Range(i, 1000-i)
let k = i * j
where (check(k.ToString() == 1)
orderby k descending
select new {i, j, k};
var highestResult = results.FirstOrDefault();
if (highestResult == null)
Console.WriteLine("There are no palindromes!");
else
Console.WriteLine($"The highest palindrome is {highestResult.i} * {highestResult.j} = {highestResult.k}");
2. Your palindrome-check is broken
You compare the character at index i to input_number[input_number.Length - i], which will throw an IndexOutOfRangeException for i = 0. Strings are zero-based index, so index of the last character is Length-1. So change the line to
if (input_number[i] != input_number[input_number.Length - i - 1])
Finally, I suggest to make the check method of return type bool instead of int:
static bool check(string input_number)
{
for (int i = 0; i < input_number.Length/2; i++)
if (input_number[i] != input_number[input_number.Length - i - 1])
return false;
return true;
}
This seems more natural to me.
You can use method below. Because you are trying to find the largest number you start from 999 and head backwards, do multiplication and check if its palindrome.
private void FindProduct()
{
var numbers = new List<int>();
for (int i = 999; i > 99; i--)
{
for (int j = 999; j > 99; j--)
{
var product = i * j;
var productString = product.ToString();
var reversed = product.Reverse();
if (product == reversed)
{
numbers.Add(product);
}
}
}
Console.WriteLine(numbers.Max());
}
Consider the following code:
int max = 1000;
int min = 0;
// Code sample A
for(int i = 0; i < array.Length; i++)
{
if (array[i] < min) min = array[i];
else if (array[i] > max) max = array[i];
}
and,
// Code Sample B
for (int i = 0; i < array.Length; i++)
{
if (array[i] < min) min = array[i];
}
for (int i = 0; i < array.Length; i++)
{
if (array[i] > max) max = array[i];
}
Of the above two code samples which one is more efficient?
I know some people would say that the first code sample would be faster as it does two instructions in one iteration. while, the second does it in two separate for loops. I am not so sure if that is true. Please explain.
To keep it simple,
in case A,
your program has to initialize i only once.
Assign values to i only array.Length times.
Increase i values only array.Length times.
do the line execution and comparison 2*array.Length times.
In case B,
initialize i twice.
Assign values to i 2*array.Length times.
Increase i values 2*array.Length times.
do line execution and comparison 2*array.Length times.
So, which one you would prefer?
Prajwal's answer is pretty accurate.
It is not difficult to test this (in Java):
public static void main(String[] args) {
int max = 1000;
int min = 0;
long start, stop, diff;
int[] arr = new int[100_000];
for (int i = 0; i < arr.length; i++)
arr[i] = (int) (-500_000 + Math.random() * 1_000_000);
start = System.nanoTime();
// Code sample A
for(int i = 0; i < arr.length; i++) {
if (arr[i] < min)
min = arr[i];
else if (arr[i] > max)
max = arr[i];
}
stop = System.nanoTime();
diff = stop - start;
System.out.println("nanoseconds needed for A: " + diff);
min = 0; max = 1000;
start = System.nanoTime();
// Code Sample B
for (int i = 0; i < arr.length; i++) {
if (arr[i] < min)
min = arr[i];
}
for (int i = 0; i < arr.length; i++) {
if (arr[i] > max)
max = arr[i];
}
stop = System.nanoTime();
diff = stop - start;
System.out.println("nanoseconds needed for B: " + diff);
}
The result of it:
nanoseconds needed for A: 563581
nanoseconds needed for B: 1018710
I'm quite new to C# and I'm having difficult with our arrays, arrays of arrays, jagged arrays, matrixes and stuff. It's quite different from the C++ , since I can't get a reference (unless I use unsafe code) to a row of a matrix, using pointers and stuff.
Anyway, here's the problem: I have a struct/class called "Image" that cointains 1024 columns and 768 lines. For each line/column theres a 'pixel' struct/class that contains 3 bytes. I'd like to get/set pixels in random places of the matrix as fast as possible.
Let's pretend I have a matrix with 25 pixels. That is 5 rows and 5 columns, something like this:
A B C D E
F G H I J
K L M N O
P Q R S T
U V X W Y
And I need to compare M to H and R. Then M to L and N. Then I need to 'sum' G+H+I+L+M+N+Q+R+S.
How can I do that?
Possibilities:
1) Create something like pixel[5][5] (that's a jagged array, right?), which will be slow whenever I try to compare elements on different columns, right?
2) Create something like pixel[25] , which won't be as easy to code/ready because I'll need to do some (simple) math each and everything I want to access a element
3) Create something like pixe[5,5] (that's a multi-dimensional array, right?)... But I don't know how that will be translated to actual memory... If it's going to be a single block of memory, like the pixe[25], or what...
Since I intend to do this operations ('random' sums/comparison of elements that are in different rows/columns) tens of thousands of times per image. And I have 1000+ imagens. Code optimization is a must... Sadly I'm not sure which structure / classe I should use.
TL;DR: Whats the FASTEST and whats the EASIEST (coding wise) way of getting/setting elements in random positions of a (fixed size) matrix?
edit: I do not want to compare C++ to C#. I'm just saying I AM NEW TO C# and I'd like to find the best way to accomplish this, using C#. Please don't tell me to go back to C++.
I have worked on pixel based image processing in C#. I found that pattern #2 in your list is fastest. For speed, you must forget about accessing pixels via some kind of nice abstract interface. The pixel processing routines must explicitly deal with the width and height of the image. This makes for crappy code in general, but unless Microsoft improves the C# compiler, we are stuck with this approach.
If your for loops start at index 0 of an array, and end at array.Length - 1, the compiler will optimize out the array index bounds testing. This is nice, but typically you have to use more than one pixel at a time while processing.
I just finished testing, heres the result:
SD Array Test1: 00:00:00.9388379
SD Array Test2: 00:00:00.4117926
MD Array Test1: 00:00:01.4977765
MD Array Test2: 00:00:00.8950093
Jagged Array Test1: 00:00:03.6850013
Jagged Array Test2: 00:00:00.5036041
Conclusion: Single dimensional array is the way to go... Sadly we lose in readability.
And heres the code:
int[] myArray = new int[10000 * 10000];
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 10000; j++)
{
myArray[(i*10000)+j] = i+j;
}
}
Stopwatch sw = new Stopwatch();
int sum = 0;
sw.Start();
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 10000; j++)
{
sum += myArray[(j * 10000) + i];
}
}
sw.Stop();
Console.WriteLine("SD Array Test1: " + sw.Elapsed.ToString());
sum=0;
sw.Restart();
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 10000; j++)
{
sum += myArray[(i * 10000) + j];
}
}
sw.Stop();
Console.WriteLine("SD Array Test2: " + sw.Elapsed.ToString());
myArray = null;
int[,] MDA = new int[10000, 10000];
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 10000; j++)
{
MDA[i, j] = i + j;
}
}
sum = 0;
sw.Restart();
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 10000; j++)
{
sum += MDA[j, i];
}
}
sw.Stop();
Console.WriteLine("MD Array Test1: " + sw.Elapsed.ToString());
sw.Restart();
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 10000; j++)
{
sum += MDA[i, j];
}
}
sw.Stop();
Console.WriteLine("MD Array Test2: " + sw.Elapsed.ToString());
MDA = null;
int[][] JA = new int[10000][];
for (int i = 0; i < 10000; i++)
{
JA[i] = new int[10000];
}
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 10000; j++)
{
JA[i][j] = i + j;
}
}
sum = 0;
sw.Restart();
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 10000; j++)
{
sum += JA[j][i];
}
}
sw.Stop();
Console.WriteLine("Jagged Array Test1: " + sw.Elapsed.ToString());
sw.Restart();
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 10000; j++)
{
sum += JA[i][j];
}
}
sw.Stop();
Console.WriteLine("Jagged Array Test2: " + sw.Elapsed.ToString());
MDA = null;
Console.ReadKey();