Digit-increasing number test - c++

Question

A number is called digit-increasing if it is equal n + nn + nnn + ... for some digit n between 1 and 9. For example 24 is digit-increasing because it equals 2 + 22 (here n = 2).
Actually, a friend of mine asked me this question and i am stuck thinking about it but couldn't find the exact solution so far. Can anyone help ? I needed the function that returns true if it is digit-increasing else false.

Answer 1

There are only relatively few numbers with this property: Within the range of unsigned long long (64 bits), there are only 172 digit-increasing numbers.
Therefore, in terms of a practical solution, it makes sense to pre-compute them all and put them in a hash. Here is Python code for that:
# Auxiliary function that generates
# one of the 'nnnn' elements
def digits(digit,times):
result = 0
for i in range(times):
result += digit*(10**i)
return result
# Pre-computing a hash of digit-increasing
# numbers:
IncDig = {}
for i in range(1,30):
for j in range(1,10):
number = reduce(lambda x,y:x+y,[digits(j,k) for k in range(1,i+1)])
IncDig[number] = None
Then the actual checking function is just a look-up in the hash:
def IncDigCheck(number):
return (number in IncDig)
This is virtually O(1), and the time and space taken for the pre-calculation is minimal, because there are only 9 distinct digits (zero doesn't count), hence only K*9 combinations of type n + nn + ... for a sum of length K.

Answer 2

General representation is:
n + (n*10 + n) + (n*100+n)...
If number look like sum of same digits then any digit can be represented as
(1+111+...) * base_digit
. Assuming this we can use simple algorithm:
bool isDigitIncreasing(const int num)
{
int n = 1;
int sum = 1; //value to increase n
while (n <= num) {
//if num is (111...) * base_digit and base_digit is < 10
if (num % n == 0 && n * 10 > num) return true;
sum = sum * 10 + 1; //N*10+N where n is 1 as was assumed
n += sum; //next step
}
return false;
}

Answer 3

Simple exhaustive search will work.
def is_digit_increasing_number(x):
# n = 1, 1+11, 1+11+111, ...
n = 1
i = 1
while n <= x:
if x % n == 0 and n * 10 > x:
return True
i += 1
n = n * 10 + i
return False

Answer 4

Simplest possible way is do the addition (bottom-up), I'll use simple for loop:
List<int> numbersSum = new List<int>{1,2,3,4,5,6,7,8,9};
List<int> lastNumber = new List<int>{1,2,3,4,5,6,7,8,9};
for(int i=0;i<= lg n + 1;i++)
{
for(int j=0;j<9;j++)
{
if(list[j] < n)
{
var lastNumberJ = lastNumber[j]*10+j+1;
list[j] += lastNumberJ; // add numbers to see will be same as n.
if (list[j] == n)
return j+1;
lastNumber[j] = lastNumberJ;
}
}
}
return -1;
The important part is you just need at most log n iteration and also you can return sooner if all numbers are bigger than given number, this is O(log n) algorithm.

Answer 5

Here is a python code.The basic logic here is that a digit increasing number if divided by a specific number between 1-9 gives a digit increasing number made of only ones.All the digit increasing numbers of 1 follow a specific pattern ie 12345678...
import sys
for n in range(1,10):
a=1
if k%n!=0:
a=0
else:
g=str(k/n)
j=int(g[0])
for i in range(1,len(g)):
if int(g[i])==j+1:
j=int(g[i])
else:
a=0
break
if a==1:
print "Yes,it is a digit increasing number"
sys.exit(0)
print "No,it is not a digit increasing number"

Answer 6

I have done in this way. Check out once.
int sum = 0, count =0;
bool flag = false;
public bool isDigitIncreasing(int input_number)
{
int n= get_number_of_digit(input_number); // Gets number of digits
int sum = 0;
for(int i=0;i<n;i++)
{
sum = sum*10+1;
count = count + sum;
}
for(int i=1; i<=9;i++)
{
if((input_number)==count*i)
{
flag = true;
break;
}
else
flag = false;
}
return flag;
}
public int get_number_of_digit(int num)
{
int size = 0;
do
{
num = num/10;
size++;
}while(num>0);
return size;
}

Answer 7

Here is the shortest solution
public static int isDigitIncreasing (int n)
{
if(n<10)
{
return 1;
}
for(int i=1;i<=9;i++)
{
int tempsum=i;
int previous=i;
while(tempsum<=n)
{
previous=previous*10 + i;
tempsum=tempsum + previous;
if(tempsum==n)
{
return 1;
}
}
}
return 0;
}

Answer 8

Ambiguitiy: Are the values 1-9 repeating for themselves? (too lazy to google this myself)
If 1-9 are repeating then following should work. If not, and you want the code to work only on values > 10 then you can initialize mult with 10.
int i, mult = 1, result, flag;
for( i=1; i<9; i++ )
{
flag = 0;
while( result < TARGET )
{
result = result+(i*mult);
mult = mult*10;
if( result == TARGET )
{
flag = 1;
break;
}
}
if( flag == 1 )
break;
}
After execution, i must contain the values for which RESULT is a repeating number IF the flag is 1. If flag is zero after execution then the TARGET isn't a repeating number.
I wonder if its possible that a number could be repeating for multiple values, just curious.

Answer 9

Here num is the number and n is the digit
#include<stdio.h>
int f(int num,int n)
{
int d=n;
while(num>0)
{
num-=n;
n=d+n*10;
}
if(num==0)
return 1;
else
return 0;
}
int main()
{
int num;
int n;
int flag;
printf("Enter the number :");
scanf("%d",&num);
printf("Enter the digit :");
scanf("%d",&n);
flag = f(num,n);
if(flag == 1)
printf("It's in n+nn+nnn+...\n");
if(flag ==0)
printf("It's not\n");
return 0;
}

Answer 10

Let d(k) be 1+11+111+...+(11...11) where the last number has k digits. Then d(1)=1, and d(k+1)=10d(k)+k+1.
We want to test if d(k)*i = n, for some k, and for some i=1..9.
If we've computed d(k), then i (if it exists) must be n/d(k). We can check if n/d(k) is correct, by comparing n with ((n/d(k))%10)*d(k). The %10 makes the test fail if i is larger than 9.
This gives us a relatively terse solution: compute subsequent d(k) until they are bigger than n, and at each point check to see if n is a digit-multiple of d(k).
Here's a very lightly code-golfed implementation of that idea:
#include <stdio.h>
int is_digit_increasing(int n) {
for(int d=1,k=1;d<=n;d=d*10+ ++k)if(n==(n/d)%10*d)return 1;
return 0;
}
int main(int argc, char**argv) {
for (int i=0; i<10000; i++) {
if (is_digit_increasing(i)) {
printf("%d\n", i);
}
}
return 0;
}

Answer 11

// Example program
#include <iostream>
#include <string>
int isDigitIncreasingNo(int n) {
if(n<=0)
return 0;
int len = std::to_string(n).length();
int vector1 = 0;
int vector2 = 0;
for(int i=1;i<=len;i++)
vector2 = (vector2*10)+i;
vector1 = vector2/10;
if(n % vector2 == 0 && (n / vector2)<=9 )
return 1;
if(n % vector1 == 0 && (n / vector1)<=9 )
return 1;
return 0;
}
int main()
{
for (int i=0; i<10000000; i++) {
if (isDigitIncreasingNo(i)) {
printf("%d\n", i);
}
}
return 0;
}

Answer 12

public boolean isDigitIncreasing(int number)
{
int sum;
int size=calculateNumberOfDigits(number);
for(int i=1;i<=9;i++)
{
sum=0;
int temp=size;
while(temp>=1)
{
for(int j=temp;j<=1;j--)
{
sum=sum+i*(int)Math.pow(10,j-1);
}
temp--;
}
if(sum==number)
{
return true;//Its a digit increasing
}
}
return false;
}
public int calculateNumberOfDigits(int number)
{
int size=0;
do
{
number=number/10;
size++;
}while(size>0);
return size;
}