## Friday, April 13, 2012

### Count the number of 1's

I've participated in a Quiz where i had to implement a function that counts the number of 1's required to reach a positive integer passed as a parameter.

Example:
```countOfOnes (12) = 5
1 - one 1
3
4
5
6
7
8
9
10 - two 1's
11 - four 1's
12 - five 1's```

Since countOfOnes (1) = 1, they have requested to find the next number in which the parameter is equal to the return value of countOfOnes ( countOfOnes(n) = n ).

At first glance I've implemented a perl one-liner that returns the number of 1's for an input number. Here it is:

```\$ perl -e '\$sum += () = /1/g for 1..shift; print "NumberOfOnes=\$sum\n"' 111
```
```NumberOfOnes=36
```

However, this solution was inefficient due to the high number of invocations needed to find the solution to the quiz.

So, analyzing the evolution of the mathematical calculation of the function numOfOnes, I outlined the following recursive algorithm:

```# NumOfOnes
sub num_of_ones{
my \$n = shift;

return 0 if \$n < 1;
return 1 if \$n <= 9;

my @a = split('',\$n);

my \$len = length(\$n);
my \$sum = 0;
my \$delta = 0;

if(\$a > 1){
\$delta += 10**(\$len-1);
}else{
\$delta += substr(\$n,1) + 1;
}

\$sum += \$delta + \$a*num_of_ones(9x(\$len-1))+ num_of_ones(substr(\$n,1));
return \$sum;
}
```

Since, recursive functions were used, which were invoked repeatedly with the same parameters, I used the Memoize module, which makes caching for functions with the same parameters.

Although the previous function is more efficient for calculating the numberOfOnes just for a single number, it is not when you want to go all the way incrementing to find the next number that satisfies the requested condition. Once we have the numOfOnes for the previous number, we only need to count the numOfOnes that exists in the current number and increment numOfOnes of the previous number.

The following perl on-liner is a better approach to achieve that

```perl -e 'while(1){\$sum += () = ++\$i =~ /1/g; print "Found:\$i\n"  if \$i == \$sum;}'

Found:1
Found:199981
Found:199982
Found:199983
Found:199984
Found:199985
Found:199986
Found:199987
Found:199988
Found:199989
Found:199990
Found:200000
Found:200001
```

The number after the 1 that satisfies the condition countOfOnes(n) = n is 199981

The source code is here.