Benford’s Law C++

[kinglord]

How to write a code for Benfor's Law.
program that loop through the list of numbers and count how many times 1 is the
first digit, 2 is the first digit, etc. For each digit, output the percentage it appears as the first digit.

[keskiverto]

The trivial thing is to divide an integer number by 10 until the number is less than 10. Floating point values less than 1 require multiplication in similar manner.

[kulkarnisr]

You can also use sprintf, and check here is the sample program

// couter for each digit
int digit_counter[9] = { 0th will have no of 1's }

// some random num from the list
num = 10;

// char buffer for 
char buffer[16];

sprintf(buffer, "%d", num);

++digit_counter[buffer[0] - '0']; 

I have not tested but it should work

[vlad from moscow]

const int base = 10;

int all_digits[base] = {};

while ( true )
{
   std::cout << "Enter a number (0 - exit): ";

   int number = 0;
   std::cin >> number;

   if ( !number ) break;

   int digit;
   do { digit = number % base } while ( number /= base );

   all_digits[digit]++;
}

[kinglord]

Thanks for the code, but I am just new to C++ and you two explain more, I will appreciate it.

[kinglord]

Here is more details. I know it's not hard for some people but to me, it takes a lot of my time writting code(1-10hrs) I am new to C++ and I am new comer, Please Help. Here is more details:

It might seem like the leading digit of each number in the list should

be 1–9 with an equally likely probability. However, Benford’s Law states that

the leading digit is 1 about 30% of the time and drops with larger digits. The

leading digit is 9 only about 5% of the time. Write a program that tests

Benford’s Law. Collect a list of at least one hundred numbers from some real-life data source and enter them into a text file.

Your program should loop through the list of numbers and count how many times 1 is the

first digit, 2 is the first digit, etc. For each digit, output the percentage it appears

as the first digit. If you read a number into the string variable named strNum then you can access

the first digit as a char by using strNum[0] .


Thanks for help, I am very appreciated for helping me a step for.

[keskiverto]

A programming course homework? You do learn by doing.

Now you have an excellent chance to learn. Please explain each line in vlad's code sample (which is not a complete program) as well as you can. Yes, you are not familiar with all the details yet, but if you don't try, you will never be.

[kevinkjt2000]

I would use std::stringstream and output the integer to it. Then take the std::string returned by the std::stringstream::str method and look at the first character. If the first character is a negative sign, it would be skipped. If the first character was a number (after optional skipping), then I would have an std::map<int, int> be increased by one corresponding to the number digit.
The percentages would all be calculable at that point. I will go ahead and type some of this code to help you, but it is a homework assignment so do not expect too much free code.

Here is a link to ideone where I typed out a program that can count all the first digits from all the numbers in a vector.
http://ideone.com/0l9oks

It would be easy to modify it to have string variables instead of integer variables.