[JAVA] A program that calculates factorials from 2 to 100


Recently, in a university programming lecture, I had the task of creating a "program that calculates factorials from 2 to 100", but it was rather difficult, so I would like to leave the program as a memorandum for myself.


Updated on 2019/01/09 (Updated content: Add comment)

public class Report5_2_30114020{
    public static void main(String[] args) {

        int [] answerArray = new int [200];     //An array that stores the calculation result of factorial
        int [] copyArray = new int [200];       //An array that temporarily copies the contents of the array

        //Initialize array values
        for(int i=0; i<answerArray.length; i++){
            answerArray[i] = 0;
        answerArray[0] = 1;

        for(int i=2; i<=100; i++){
            //Copy array
            for(int j=0; j<answerArray.length; j++){
                copyArray[j] = answerArray[j];
            int upNumber1 = 0;
            int upNumber2 = 0;
            int first = 0;
            int second = 0;
            int third = 0;
                first = i;
            }else if(i<100){
                first = i%10;
                second = i/10;
                third = 1;
            for(int j=0; j<answerArray.length; j++){
                int firstPlus = 0;
                int secondPlus = 0;
                int thirdPlus = 0;
                firstPlus = first * copyArray[j];
                    secondPlus = second * copyArray[j-1];
                    thirdPlus = third * copyArray[j-2];

                int sum = firstPlus + secondPlus + thirdPlus + upNumber1;

                answerArray[j] = sum % 10;

                //Preparing for carry
                if(sum <10){
                    upNumber1 = upNumber2;
                    upNumber2 = 0;
                }else if(sum < 100){
                    upNumber1 = upNumber2 + (sum)/10;
                    upNumber2 = 0;
                    upNumber1 = upNumber2 + ((sum)/10)%10;
                    upNumber2 = (sum)/100;

            //View results
            int counter = 0;
            for(int j=0; j<answerArray.length; j++){
                if(answerArray[answerArray.length-1-j] != 0){
            System.out.print(i + "Factorial:");
            for(int j=0; j<answerArray.length; j++){

Program description

Updated on 2019/01/09 (Updated content: "Program description" added) Circled numbers (such as ①) correspond to the circled numbers written in the program.

Program overview

This program is a program that calculates from factorial 2 to factorial 100. Since an ordinary int type cannot store a large number of factorial 100 (158 digits), the calculation result of factorial 100 is calculated one digit at a time in an int type array with a sufficiently large number of elements. The basic policy is to store it. At that time, carry-up etc. will occur, so the processing around that will be a little complicated.

① Prepare an array

An array that stores the calculation result of factorial. Since the factorial calculation result is saved in the elements of the array digit by digit, it is necessary to prepare an array with a larger number of elements, considering that 100! Is 158 digits. copyArray is required when performing the calculation in ⑥.

② Initialize the array values

answerArray [n] corresponds to the n + 1th digit of the factorial calculation result. Since we want to set the initial calculation result to 1, only the first element is set to 1 and the other elements are set to 0 for initialization.

③ Actual processing

This for statement is the outermost process. The variable i that is valid in this for statement corresponds to the factorial number that is being calculated at that time. (Example: In the loop of i = 10, i! Is calculated.)

④ Make a copy of the array

Each time i is updated and loops, it copies the result of the factorial calculated just before. (In ④ of the loop of i = 10, it means that 9! Calculated just before is copied.) This copied array will be used later in ⑥.

⑤ Preparation of variables necessary to update the factorial answer

The variable upNumber1 is a variable that stores the carry number that goes up one digit. The variable upNumber2 is a variable that stores a carry number that goes up two digits. first is the first digit of the i value, second is the second digit of the i value, and third is the third digit of the i value. After this, the product of the calculation results up to the last time is calculated for each of first, second, and third.

⑥ Find the value of the digit of interest

Put the product of the values of first and copyArray [j] in a new variable called firstPlus. Put the value of the product of second and copyArray [j-1] in a new variable called secondPlus. Put the value of the product of third and copyArray [j-2] in a new variable called thirdPlus. Here, the index is j for first, but j-1 and j-2 are different for second and third. Second and third are the numbers and hundreds that represent the tens digit of the variable i, respectively. Because it is the number represented. I think it's easier to understand this area if you imagine the multiplication of multiple-digit numbers and three-digit numbers. The reason that the if statement is used in the process of issuing secondPlus and thirdPlus is to avoid referencing the index where the array does not exist. The variable sum is the sum of the values of firstPlus, secondPlus, thirdPlus and the carry that occurred in the previous loop. Since the first digit of this is the value of the j + 1th digit of i !, the remainder of sum divided by 10 is assigned to answerArray.

⑦ Prepare a variable that saves the carry to be used in the next loop.

At this stage, sum is obtained, so use this value to prepare a variable that stores the carry to be used in the next loop.

⑧ Display calculation result

The calculation result (i!) At that time is displayed. In the variable counter, count the number of empty () arrays. Next, the numbers stored in the array are displayed one digit at a time. By displaying from the one with the largest index, the numerical value of i! Can be output. At this time, it is possible to prevent the unnecessary 0 from being displayed at the beginning by passing through the processing for the number of counters obtained before.


It took me a long time to write a program for the first time in a long time. My personal addiction was that I didn't realize that it was imperative to make a copy of the array in my own way. I would like to post a commentary if I feel like it in the near future.

Recommended Posts

A program that calculates factorials from 2 to 100
To write a user-oriented program (1)
[ruby] Creating a program that responds only to specific conditions
I tried to generate a C language program source from cURL
[Introduction to Java] How to write a Java program
A program that determines whether the entered integer is close to an integer
A story that took time to establish a connection
To become a VB.net programmer from a Java shop
How to create a class that inherits class information
[Updated from time to time] Links that are indebted
I tried to make a program that searches for the target class from the process that is overloaded with Java
[Ruby] I want to make a program that displays today's day of the week!
3. Create a database to access from the web module
How to jump from Eclipse Java to a SQL file
How to deploy to Heroku from a local docker image
[Ruby] A program / concept that combines each_with_index and search
How to launch another command in a Ruby program
Using the database (SQL Server 2014) from a Java program 2018/01/04
[Java] How to erase a specific character from a character string
Program to determine if it is a leap year
I made a Wrapper that calls KNP from Java
Ruby Regular Expression Extracts from a specific string to a string
Call a program written in Swift from Processing (Java)
Send a command to Spigot from an external process
I want to connect to Heroku MySQL from a client
A program that outputs a number greater than or equal to the input integer and a number less than the input integer from an array with 50 elements.
Created a menu program
A memo of the program that allows you to realize that the probability of dice rolling is about 1/6
From Java to Ruby !!
A program that searches for a character string, and when the search character string is found, displays the character string from the beginning of the line to just before the search character string.
Generate a unique collection of values from a collection that contains duplicate values
[iOS] [Objective-C] How to update a widget from an Objective-C app
How to create a form to select a date from the calendar
[Beginner] I made a program to sell cakes in Java
From studying programming for 2 months to releasing a web application
[Java] How to convert a character string from String type to byte type
How to store a string from ArrayList to String in Java (Personal)
Create a method that can retrieve characters from any location
How to Burning a Install Disk of Windows from Ubuntu
Find a Switch statement that can be converted to a Switch expression
A program that counts the number of words in a List
Convert an array that may be null to a stream
Create a data source (connection pool) that connects from WLS (WebLogic) to Autonomous Database (ATP / ADW).
How to remotely debug a javaFX program executed by Raspberry Pi from Windows 10-From environment construction to debug execution ①-
How to remotely debug a javaFX program executed by Raspberry Pi from Windows 10-From environment construction to debug execution ②-
How to deal with the type that I thought about writing a Java program for 2 years