How can you get the first digit in an int (C#)? - c#

In C#, what's the best way to get the 1st digit in an int? The method I came up with is to turn the int into a string, find the 1st char of the string, then turn it back to an int.
int start = Convert.ToInt32(curr.ToString().Substring(0, 1));
While this does the job, it feels like there is probably a good, simple, math-based solution to such a problem. String manipulation feels clunky.
Edit: irrespective of speed differences, mystring[0] instead of Substring() is still just string manipulation

Benchmarks
Firstly, you must decide on what you mean by "best" solution, of course that takes into account the efficiency of the algorithm, its readability/maintainability, and the likelihood of bugs creeping up in the future. Careful unit tests can generally avoid those problems, however.
I ran each of these examples 10 million times, and the results value is the number of ElapsedTicks that have passed.
Without further ado, from slowest to quickest, the algorithms are:
Converting to a string, take first character
int firstDigit = (int)(Value.ToString()[0]) - 48;
Results:
12,552,893 ticks
Using a logarithm
int firstDigit = (int)(Value / Math.Pow(10, (int)Math.Floor(Math.Log10(Value))));
Results:
9,165,089 ticks
Looping
while (number >= 10)
number /= 10;
Results:
6,001,570 ticks
Conditionals
int firstdigit;
if (Value < 10)
firstdigit = Value;
else if (Value < 100)
firstdigit = Value / 10;
else if (Value < 1000)
firstdigit = Value / 100;
else if (Value < 10000)
firstdigit = Value / 1000;
else if (Value < 100000)
firstdigit = Value / 10000;
else if (Value < 1000000)
firstdigit = Value / 100000;
else if (Value < 10000000)
firstdigit = Value / 1000000;
else if (Value < 100000000)
firstdigit = Value / 10000000;
else if (Value < 1000000000)
firstdigit = Value / 100000000;
else
firstdigit = Value / 1000000000;
Results:
1,421,659 ticks
Unrolled & optimized loop
if (i >= 100000000) i /= 100000000;
if (i >= 10000) i /= 10000;
if (i >= 100) i /= 100;
if (i >= 10) i /= 10;
Results:
1,399,788 ticks
Note:
each test calls Random.Next() to get the next int

Here's how
int i = Math.Abs(386792);
while(i >= 10)
i /= 10;
and i will contain what you need

Try this
public int GetFirstDigit(int number) {
if ( number < 10 ) {
return number;
}
return GetFirstDigit ( (number - (number % 10)) / 10);
}
EDIT
Several people have requested the loop version
public static int GetFirstDigitLoop(int number)
{
while (number >= 10)
{
number = (number - (number % 10)) / 10;
}
return number;
}

The best I can come up with is:
int numberOfDigits = Convert.ToInt32(Math.Floor( Math.Log10( value ) ) );
int firstDigit = value / Math.Pow( 10, numberOfDigits );

variation on Anton's answer:
// cut down the number of divisions (assuming i is positive & 32 bits)
if (i >= 100000000) i /= 100000000;
if (i >= 10000) i /= 10000;
if (i >= 100) i /= 100;
if (i >= 10) i /= 10;

int myNumber = 8383;
char firstDigit = myNumber.ToString()[0];
// char = '8'

Had the same idea as Lennaert
int start = number == 0 ? 0 : number / (int) Math.Pow(10,Math.Floor(Math.Log10(Math.Abs(number))));
This also works with negative numbers.

If you think Keltex's answer is ugly, try this one, it's REALLY ugly, and even faster.
It does unrolled binary search to determine the length.
... leading code along the same lines
/* i<10000 */
if (i >= 100){
if (i >= 1000){
return i/1000;
}
else /* i<1000 */{
return i/100;
}
}
else /* i<100*/ {
if (i >= 10){
return i/10;
}
else /* i<10 */{
return i;
}
}
P.S. MartinStettner had the same idea.

Very simple (and probably quite fast because it only involves comparisons and one division):
if(i<10)
firstdigit = i;
else if (i<100)
firstdigit = i/10;
else if (i<1000)
firstdigit = i/100;
else if (i<10000)
firstdigit = i/1000;
else if (i<100000)
firstdigit = i/10000;
else (etc... all the way up to 1000000000)

An obvious, but slow, mathematical approach is:
int firstDigit = (int)(i / Math.Pow(10, (int)Math.Log10(i))));

int temp = i;
while (temp >= 10)
{
temp /= 10;
}
Result in temp

I know it's not C#, but it's surprising curious that in python the "get the first char of the string representation of the number" is the faster!
EDIT: no, I made a mistake, I forgot to construct again the int, sorry. The unrolled version it's the fastest.
$ cat first_digit.py
def loop(n):
while n >= 10:
n /= 10
return n
def unrolled(n):
while n >= 100000000: # yea... unlimited size int supported :)
n /= 100000000
if n >= 10000:
n /= 10000
if n >= 100:
n /= 100
if n >= 10:
n /= 10
return n
def string(n):
return int(str(n)[0])
$ python -mtimeit -s 'from first_digit import loop as test' \
'for n in xrange(0, 100000000, 1000): test(n)'
10 loops, best of 3: 275 msec per loop
$ python -mtimeit -s 'from first_digit import unrolled as test' \
'for n in xrange(0, 100000000, 1000): test(n)'
10 loops, best of 3: 149 msec per loop
$ python -mtimeit -s 'from first_digit import string as test' \
'for n in xrange(0, 100000000, 1000): test(n)'
10 loops, best of 3: 284 msec per loop
$

I just stumbled upon this old question and felt inclined to propose another suggestion since none of the other answers so far returns the correct result for all possible input values and it can still be made faster:
public static int GetFirstDigit( int i )
{
if( i < 0 && ( i = -i ) < 0 ) return 2;
return ( i < 100 ) ? ( i < 1 ) ? 0 : ( i < 10 )
? i : i / 10 : ( i < 1000000 ) ? ( i < 10000 )
? ( i < 1000 ) ? i / 100 : i / 1000 : ( i < 100000 )
? i / 10000 : i / 100000 : ( i < 100000000 )
? ( i < 10000000 ) ? i / 1000000 : i / 10000000
: ( i < 1000000000 ) ? i / 100000000 : i / 1000000000;
}
This works for all signed integer values inclusive -2147483648 which is the smallest signed integer and doesn't have a positive counterpart. Math.Abs( -2147483648 ) triggers a System.OverflowException and - -2147483648 computes to -2147483648.
The implementation can be seen as a combination of the advantages of the two fastest implementations so far. It uses a binary search and avoids superfluous divisions. A quick benchmark with the index of a loop with 100,000,000 iterations shows that it is twice as fast as the currently fastest implementation.
It finishes after 2,829,581 ticks.
For comparison I also measured a corrected variant of the currently fastest implementation which took 5,664,627 ticks.
public static int GetFirstDigitX( int i )
{
if( i < 0 && ( i = -i ) < 0 ) return 2;
if( i >= 100000000 ) i /= 100000000;
if( i >= 10000 ) i /= 10000;
if( i >= 100 ) i /= 100;
if( i >= 10 ) i /= 10;
return i;
}
The accepted answer with the same correction needed 16,561,929 ticks for this test on my computer.
public static int GetFirstDigitY( int i )
{
if( i < 0 && ( i = -i ) < 0 ) return 2;
while( i >= 10 )
i /= 10;
return i;
}
Simple functions like these can easily be proven for correctness since iterating all possible integer values takes not much more than a few seconds on current hardware. This means that it is less important to implement them in a exceptionally readable fashion as there simply won't ever be the need to fix a bug inside them later on.

Did some tests with one of my co-workers here, and found out most of the solutions don't work for numbers under 0.
public int GetFirstDigit(int number)
{
number = Math.Abs(number); <- makes sure you really get the digit!
if (number < 10)
{
return number;
}
return GetFirstDigit((number - (number % 10)) / 10);
}

Using all the examples below to get this code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
namespace Benfords
{
class Program
{
static int FirstDigit1(int value)
{
return Convert.ToInt32(value.ToString().Substring(0, 1));
}
static int FirstDigit2(int value)
{
while (value >= 10) value /= 10;
return value;
}
static int FirstDigit3(int value)
{
return (int)(value.ToString()[0]) - 48;
}
static int FirstDigit4(int value)
{
return (int)(value / Math.Pow(10, (int)Math.Floor(Math.Log10(value))));
}
static int FirstDigit5(int value)
{
if (value < 10) return value;
if (value < 100) return value / 10;
if (value < 1000) return value / 100;
if (value < 10000) return value / 1000;
if (value < 100000) return value / 10000;
if (value < 1000000) return value / 100000;
if (value < 10000000) return value / 1000000;
if (value < 100000000) return value / 10000000;
if (value < 1000000000) return value / 100000000;
return value / 1000000000;
}
static int FirstDigit6(int value)
{
if (value >= 100000000) value /= 100000000;
if (value >= 10000) value /= 10000;
if (value >= 100) value /= 100;
if (value >= 10) value /= 10;
return value;
}
const int mcTests = 1000000;
static void Main(string[] args)
{
Stopwatch lswWatch = new Stopwatch();
Random lrRandom = new Random();
int liCounter;
lswWatch.Start();
for (liCounter = 0; liCounter < mcTests; liCounter++)
FirstDigit1(lrRandom.Next());
lswWatch.Stop();
Console.WriteLine("Test {0} = {1} ticks", 1, lswWatch.ElapsedTicks);
lswWatch.Reset();
lswWatch.Start();
for (liCounter = 0; liCounter < mcTests; liCounter++)
FirstDigit2(lrRandom.Next());
lswWatch.Stop();
Console.WriteLine("Test {0} = {1} ticks", 2, lswWatch.ElapsedTicks);
lswWatch.Reset();
lswWatch.Start();
for (liCounter = 0; liCounter < mcTests; liCounter++)
FirstDigit3(lrRandom.Next());
lswWatch.Stop();
Console.WriteLine("Test {0} = {1} ticks", 3, lswWatch.ElapsedTicks);
lswWatch.Reset();
lswWatch.Start();
for (liCounter = 0; liCounter < mcTests; liCounter++)
FirstDigit4(lrRandom.Next());
lswWatch.Stop();
Console.WriteLine("Test {0} = {1} ticks", 4, lswWatch.ElapsedTicks);
lswWatch.Reset();
lswWatch.Start();
for (liCounter = 0; liCounter < mcTests; liCounter++)
FirstDigit5(lrRandom.Next());
lswWatch.Stop();
Console.WriteLine("Test {0} = {1} ticks", 5, lswWatch.ElapsedTicks);
lswWatch.Reset();
lswWatch.Start();
for (liCounter = 0; liCounter < mcTests; liCounter++)
FirstDigit6(lrRandom.Next());
lswWatch.Stop();
Console.WriteLine("Test {0} = {1} ticks", 6, lswWatch.ElapsedTicks);
Console.ReadLine();
}
}
}
I get these results on an AMD Ahtlon 64 X2 Dual Core 4200+ (2.2 GHz):
Test 1 = 2352048 ticks
Test 2 = 614550 ticks
Test 3 = 1354784 ticks
Test 4 = 844519 ticks
Test 5 = 150021 ticks
Test 6 = 192303 ticks
But get these on a AMD FX 8350 Eight Core (4.00 GHz)
Test 1 = 3917354 ticks
Test 2 = 811727 ticks
Test 3 = 2187388 ticks
Test 4 = 1790292 ticks
Test 5 = 241150 ticks
Test 6 = 227738 ticks
So whether or not method 5 or 6 is faster depends on the CPU, I can only surmise this is because the branch prediction in the command processor of the CPU is smarter on the new processor, but I'm not really sure.
I dont have any Intel CPUs, maybe someone could test it for us?

Check this one too:
int get1digit(Int64 myVal)
{
string q12 = myVal.ToString()[0].ToString();
int i = int.Parse(q12);
return i;
}
Also good if you want multiple numbers:
int get3digit(Int64 myVal) //Int64 or whatever numerical data you have
{
char mg1 = myVal.ToString()[0];
char mg2 = myVal.ToString()[1];
char mg3 = myVal.ToString()[2];
char[] chars = { mg1, mg2, mg3 };
string q12= new string(chars);
int i = int.Parse(q12);
return i;
}

while (i > 10)
{
i = (Int32)Math.Floor((Decimal)i / 10);
}
// i is now the first int

Non iterative formula:
public static int GetHighestDigit(int num)
{
if (num <= 0)
return 0;
return (int)((double)num / Math.Pow(10f, Math.Floor(Math.Log10(num))));
}

Just to give you an alternative, you could repeatedly divide the integer by 10, and then rollback one value once you reach zero. Since string operations are generally slow, this may be faster than string manipulation, but is by no means elegant.
Something like this:
while(curr>=10)
curr /= 10;

start = getFirstDigit(start);
public int getFirstDigit(final int start){
int number = Math.abs(start);
while(number > 10){
number /= 10;
}
return number;
}
or
public int getFirstDigit(final int start){
return getFirstDigit(Math.abs(start), true);
}
private int getFirstDigit(final int start, final boolean recurse){
if(start < 10){
return start;
}
return getFirstDigit(start / 10, recurse);
}

int start = curr;
while (start >= 10)
start /= 10;
This is more efficient than a ToString() approach which internally must implement a similar loop and has to construct (and parse) a string object on the way ...

Very easy method to get the Last digit:
int myInt = 1821;
int lastDigit = myInt - ((myInt/10)*10); // 1821 - 1820 = 1

int i = 4567789;
int digit1 = int.Parse(i.ToString()[0].ToString());

This is what I usually do ,please refer my function below :
This function can extract first number occurance from any string you can modify and use this function according to your usage
public static int GetFirstNumber(this string strInsput)
{
int number = 0;
string strNumber = "";
bool bIsContNo = true;
bool bNoOccued = false;
try
{
var arry = strInsput.ToCharArray(0, strInsput.Length - 1);
foreach (char item in arry)
{
if (char.IsNumber(item))
{
strNumber = strNumber + item.ToString();
bIsContNo = true;
bNoOccued = true;
}
else
{
bIsContNo = false;
}
if (bNoOccued && !bIsContNo)
{
break;
}
}
number = Convert.ToInt32(strNumber);
}
catch (Exception ex)
{
return 0;
}
return number;
}

public static int GetFirstDigit(int n, bool removeSign = true)
{
if (removeSign)
return n <= -10 || n >= 10 ? Math.Abs(n) % 10 : Math.Abs(n);
else
return n <= -10 || n >= 10 ? n % 10 : n;
}
//Your code goes here
int[] test = new int[] { -1574, -221, 1246, -4, 8, 38546};
foreach(int n in test)
Console.WriteLine(string.Format("{0} : {1}", n, GetFirstDigit(n)));
Output:
-1574 : 4
-221 : 1
1246 : 6
-4 : 4
8 : 8
38546 : 6

Here is a simpler way that does not involve looping
int number = 1234
int firstDigit = Math.Floor(number/(Math.Pow(10, number.ToString().length - 1))
That would give us 1234/Math.Pow(10, 4 - 1) = 1234/1000 = 1

Related

How to get amount into parts with nearest 100

I am using below code to divide amount into parts
public static IEnumerable<int> DistributeInteger(double total, int divider)
{
if (divider == 0)
{
yield return 0;
}
else
{
double rest = total % divider;
double result = total / (double)divider;
for (int i = 0; i < divider; i++)
{
if (rest-- > 0)
yield return (int)Math.Ceiling(result);
else
yield return (int)Math.Floor(result);
}
}
}
and using it as follows
var test = DistributeInteger(5000, 4).ToList();
above code returning me.
1250
1250
1250
1250
(All four part sum = 5000)
but i need it as nearest 100 of each part like
1300
1300
1300
1100
If I pass
var test = DistributeInteger(5219, 5).ToList();
then it is returning
1044
1044
1044
1044
1043
(All five part sum = 5219)
but it should be
1000
1000
1000
1000
1000
219
if amount 1 to 100 for example 89 then it will return same amount which is 89,
I am trying it from morning but no luck.
as well as i checked may codes from net but it is giving only solution to get nearest 100 value of a given no.
How about this?
public static IEnumerable<int> DistributeInteger(double total, int divider)
{
int part = 100 * ((int)(50 + total / divider) / 100);
if (part == 0)
{
yield return (int)total;
yield break;
}
double runningTotal = 0;
while (runningTotal <= (total - part))
{
yield return part;
runningTotal += part;
}
if (runningTotal < total)
yield return (int) (total - runningTotal);
}
(Note: Error handling omitted for brevity.)
How about:
public static IEnumerable<int> DistributeInteger(double total, int divider)
{
// get increment to the nearest hundred
var increment = (int) Math.Round(total/ 100 / divider, MidpointRounding.AwayFromZero) * 100;
// make sure increment is non-zero
if (increment == 0) increment = 100;
var remainder = (int) total;
// subtract increment while remainder is >= zero
while (remainder >= increment){
remainder -= increment;
yield return increment;
}
// check if remainder is non-zero
if (remainder > 0)
yield return remainder;
}
Now uses Ceiling
Seems to work with midpoint rounding away from zero
What about this approach:
public static IEnumerable<int> DistributeInteger(double total, int divider)
{
if (divider == 0)
{
return null;
}
else
{
var parts = new List<int>();
var singlePart = total / divider;
singlePart = Math.Round(singlePart / 100, MidpointRounding.AwayFromZero) * 100;
while (total - singlePart > 0)
{
parts.Add((int)singlePart);
total -= singlePart;
}
// Add remainnig value.
parts.Add((int)total);
return parts;
}
}
It will work for second scenario only, for the first scenarion you'd need to use Ceiling function instead of Round.

Calculate 2^(n) where 0<n<10000

So, this is my problem to solve:
I want to calculate 2^(n) where 0 < n< 10000
I am representing each element of array as a space where 4digit number should be "living" and if extra digit appears, I am replacing it to the next element of this array.
The principle I am using looks like this:
The code I am using is the following:
static string NotEfficient(int power)
{
if (power < 0)
throw new Exception("Power shouldn't be negative");
if (power == 0)
return "1";
if (power == 1)
return "2";
int[] A = new int[3750];
int current4Digit = 0;
//at first 2 is written in first element of array
A[current4Digit] = 2;
int currentPower = 1;
while (currentPower < power)
{
//multiply every 4digit by 2
for (int i = 0; i <= current4Digit; i++)
{
A[i] *= 2;
}
currentPower++;
//checking every 4digit if it
//contains 5 digit and if yes remove and
//put it in next 4digit
for (int i = 0; i <= current4Digit; i++)
{
if (A[i] / 10000 > 0)
{
int more = A[i] / 10000;
A[i] = A[i] % 10000;
A[i + 1] += more;
//if new digit should be opened
if (i + 1 > current4Digit)
{
current4Digit++;
}
}
}
}
//getting data from array to generate answer
string answer = "";
for (int i = current4Digit; i >= 0; i--)
{
answer += A[i].ToString() + ",";
}
return answer;
}
The problem I have is that it doesn't display correctly the number, which contains 0 in reality. for example 2 ^ (50) = 1 125 899 906 842 624 and with my algorithm I get 1 125 899 96 842 624 (0 is missing). This isn't only for 50...
This happens when I have the following situation for example:
How I can make this algorithm better?
Use BigInteger, which is already included in .Net Core or available in the System.Runtime.Numerics Nuget Package.
static string Efficient(int power)
{
var result = BigInteger.Pow(2, power);
return result.ToString(CultureInfo.InvariantCulture);
}
On my machine, NotEfficient takes roughly 80ms, where Efficient takes 0.3ms. You should be able to manipulate that string (if I'm understanding your problem statement correctly):
static string InsertCommas(string value)
{
var sb = new StringBuilder(value);
for (var i = value.Length - 4; i > 0; i -= 4)
{
sb.Insert(i, ',');
}
return sb.ToString();
}
One way to resolve this is to pad your 4-digit numbers with leading zeroes if they are less than four digits by using the PadLeft method:
answer += A[i].ToString().PadLeft(4, '0') + ",";
And then you can use the TrimStart method to remove any leading zeros from the final result:
return answer.TrimStart('0');

How to do the loop and solve the following

I'm supposed to code a program that writes out a division just like in school.
Example:
13:3=4.333333333333
13
1
10
10
10....
So my approach was:
Solve the division then get the solution in a List.
Then question if the first number (in this case 1) is divisible by 3.
If not put it down and add the second number and so on...
I managed to do this the first time. It's sloppy but works. The problem is that it only works with numbers that when divided get to have a decimal in it.
Exapmle:
123:13
This is the first code:
do
{
for (int number = 1; number <= divNum; number++)
if (number % divisor == 0) countH++;
for (int i = 0; i < count; i++)
Console.Write(" ");
if ((c = divNum % divisor ) < divisor )
{
Console.WriteLine(" " + ((divNum- (countH * divisor ))) * 10);
}
else Console.WriteLine(" " + (divNum- (countH * divisor )));
c = divNum % divisor ;
if (c < divisor )
{
divNum = c * 10;
}
count++; countH = 0;
} while ((divNum >= divisor ) && (count < x));
Any ideas or help? Sorry if this is a bad question.
************ added
Try of a better explanation:
1 cant be divided by 13, so it goes down, we get the 2 down and try 12 divided by 13, still nothing so we get the 3 down and try 123:13, 13 goes 9 times in 123 so we have 123-9*13 = 6 the six goes down we write 9 in the result. We try 6:13 not going so we drop a 0 next to 6. Next we try 60:13, 13 goes 4 times so 60-4*13 = 8, we get the 8 down. And so on..
123:13=9.46153....
123
60
80
20
70
50
....
Something like this should work. Not the fastest solution most likely, but should do the job.
var number = 123;
var b = 12;
int quotient;
double remainder = number;
var x = 10;
do
{
quotient = (int)Math.Floor(remainder / b);
remainder = remainder - (quotient * b);
for (int i = 0; i < count; i++)
Console.Write(" ");
remainder *= 10;
Console.WriteLine(" " + remainder);
count++;
} while ((remainder > 0) && (count < x));

Solving Mathematical logics, placing digits in third number from digits of first and second number

I have two numbers.
First Number is 2875 &
Second Number is 852145
Now I need a program which create third number.
Third Number will be 2885725145
The logic is
First digit of third number is first digit of first number.
Second digit of third number is first digit of second number.
Third digit of third number is second digit of first number.
Fourth digit of third number is second digit of second number;
so on.
If any number has remaining digits then that should be appended at last.
I do not want to convert int to string.
int CreateThirdNumber(int firstNumber, int secondNumber)
{
}
So can anyone suggest me any solution to this problem?
I do not want to convert int to string.
Why?
Without converting to string
Use Modulus and Division operator.
With converting to string
Convert them to string. Use .Substring() to extract and append value in a string. Convert appended string to integer.
Here's a bit that will give you a lead:
Say you have the number 2875. First, you need to determine it's length, and then, extract the first digit
This can be easily calculated:
int iNumber = 2875;
int i = 10;
int iLength = 0;
while (iNumber % i <= iNumber){
iLength++;
i *= 10;
}
// iNumber is of length iLength, now get the first digit,
// using the fact that the division operator floors the result
int iDigit = iNumber / pow(10, iLength-1);
// Thats it!
First a little advice: if you use int in C#, then the value in your example (2885725145) is bigger than int.MaxValue; (so in this case you should use long instead of int).
Anyway here is the code for your example, without strings.
int i1 = 2875;
int i2 = 852145;
int i3 = 0;
int i1len = (int)Math.Log10(i1) + 1;
int i2len = (int)Math.Log10(i2) + 1;
i3 = Math.Max(i1, i2) % (int)Math.Pow(10, Math.Max(i1len, i2len) - Math.Min(i1len, i2len));
int difference = (i1len - i2len);
if (difference > 0)
i1 /= (int)Math.Pow(10, difference);
else
i2 /= (int)Math.Pow(10, -difference);
for (int i = 0; i < Math.Min(i1len, i2len); i++)
{
i3 += (i2 % 10) * (int)Math.Pow(10, Math.Max(i1len, i2len) - Math.Min(i1len, i2len) + i * 2);
i3 += (i1 % 10) * (int)Math.Pow(10, Math.Max(i1len, i2len) - Math.Min(i1len, i2len) + i * 2 + 1);
i1 /= 10;
i2 /= 10;
}
I don't understand why you don't want to use strings (is it homework?). Anyway this is another possible solution:
long CreateThirdNumber(long firstNumber, long secondNumber)
{
long firstN = firstNumber;
long secondN = secondNumber;
long len1 = (long)Math.Truncate(Math.Log10(firstNumber));
long len2 = (long)Math.Truncate(Math.Log10(secondNumber));
long maxLen = Math.Max(len1, len2);
long result = 0;
long curPow = len1 + len2 + 1;
for (int i = 0; i <= maxLen; i++)
{
if (len1 >= i)
{
long tenPwf = (long)Math.Pow(10, len1 - i);
long firstD = firstN / tenPwf;
firstN = firstN % tenPwf;
result = result + firstD * (long)Math.Pow(10, curPow--);
}
if (len2 >= i)
{
long tenPws = (long)Math.Pow(10, len2 - i);
long secondD = secondN / tenPws;
result = result + secondD * (long)Math.Pow(10, curPow--);
secondN = secondN % tenPws;
}
}
return result;
}
This solves it:
#include <stdio.h>
int main(void)
{
int first = 2875,second = 852145;
unsigned int third =0;
int deci,evenodd ,tmp ,f_dec,s_dec;
f_dec = s_dec =1;
while(first/f_dec != 0 || second/s_dec != 0) {
if(first/f_dec != 0) {
f_dec *=10;
}
if( second/s_dec != 0) {
s_dec *= 10;
}
}
s_dec /=10; f_dec/=10;
deci = s_dec*f_dec*10;
evenodd =0;tmp =0;
while(f_dec != 0 || s_dec !=0 ) {
if(evenodd%2 == 0 && f_dec !=0 ) {
tmp = (first/f_dec);
first -=(tmp*f_dec);
tmp*=deci;
third+=tmp;
f_dec/=10;
deci/=10;
}
if(evenodd%2 != 0 && s_dec != 0) {
tmp= (second/s_dec);
second -=(tmp*s_dec);
//printf("tmp:%d\n",tmp);
tmp*=deci;
third += tmp;
s_dec/=10;
deci/=10;
}
evenodd++;
}
printf("third:%u\ncorrct2885725145\n",third);
return 0;
}
output:
third:2885725145
corrct2885725145
#include <stdio.h>
long long int CreateThirdNumber(int firstNumber, int secondNumber){
char first[11],second[11],third[21];
char *p1=first, *p2=second, *p3=third;
long long int ret;
sprintf(first, "%d", firstNumber);
sprintf(second, "%d", secondNumber);
while(1){
if(*p1)
*p3++=*p1++;
if(*p2)
*p3++=*p2++;
if(*p1 == '\0' && *p2 == '\0')
break;
}
*p3='\0';
sscanf(third, "%lld", &ret);
return ret;
}
int main(){
int first = 2875;
int second = 852145;
long long int third;
third = CreateThirdNumber(first, second);
printf("%lld\n", third);
return 0;
}

Round any n-digit number to (n-1) zero-digits

Sorry hard to formulate.
I need to round like this:
12 -> 10
152 -> 200
1538 -> 2000
25000 -> 30000
etc.
Twisting my head, but can't see how to make this. Must work for any n number of digits. Anyone got an elegant method for it?
c# or vb.net
How about this:
double num = 152;
int pow = (int)Math.Log10(num);
int factor = (int)Math.Pow(10, pow);
double temp = num / factor;
double result = Math.Round(temp) * factor;
I think you should try with something like this:
public int Round( int number)
{
int power = number.ToString().Length - 1;
int sz = Math.Pow(10, power);
int rounded = (int)Math.Round( number / sz );
return rounded * sz;
}
The idea is to get the size of the nearest 10 power, available by the length of the number expressed as a string. Then divide the number by that power, leaving it like 1,2 and then round it using the Math.Round method and restore the size by remultiplying it to the power.
Much like the previous answer...
I would do it this way:
double d = 25000;
int power = d.ToString().Length - 1;
double multipler = Math.Pow(10,power);
d = Math.Round(d / multipler) * multipler;
Console.WriteLine(d);
One of the way could be
Convert the number to Decimal
Divide it by 10^(n-1) (where n is number of digits)
Now use round function (Decimal.Round)
Multiply again by 10^(n-1)
Divide the number by 10n and round the result, then multiply the result back with 10n;
int MyRound(int num)
{
double divisor = Math.Pow(10, num.ToString().Length - 1);
return (int)(Math.Round(num / divisor, MidpointRounding.AwayFromZero) * divisor);
}
Note that we should use MidpointRounding.AwayFromZero when rounding because of the default banker's rounding.
int MakeOneSigFig(int value)
{
int neg = 1;
if(value <= 10 && value >= -10) { return value; }
if(value == int.MinValue) { value = int.MaxValue; neg = -1; }
if(value < 0) { value = -value; neg = -1; }
int mult = 10; // start at 10 because we've got 'firstDigit = value / 10' below
while(value > 99) { value /= 10; mult *= 10; }
int firstDigit = value / 10;
if(value % 10 >= 5) firstDigit++;
return neg * firstDigit * mult;
}
This is equivalent to MidpointRounding.AwayFromZero. This method doesn't do any double math or string conversions. If you didn't want to loop, you could replace that with the if block below. That would be more efficient, but more code and not quite as easy to read.
if(value < 100) { mult = 10; }
else if(value < 1000) { mult = 100; value /= 10; }
else if(value < 10000) { mult = 1000; value /= 100; }
else if(value < 100000) { mult = 10000; value /= 1000; }
// etc.

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