Print a sequence of numbers adding one number each time

I need to write a programm that, when inserted a number, it print a sequence of numbers from 1 to that number, in separate lines adding a number each time. Example: if you insert 8, it print

1
1 2
1 2 3
1 2 3 ... 8

import java.util.Scanner;
public class Series {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Scanner sc = new Scanner(System.in);
        System.out.print("Insert number ");
            int a = sc.nextInt();
            int i = 0;
        StringBuilder sb = new StringBuilder();
            for(i=1; i<=a; i++)
                sb.append(i);
        System.out.println(sb.toString());
            
    }

}

I'm using this and obviously it print just one line from 1 to the number. I don't know what i can change, or if this is totally wrong

• You've appended all numbers into StringBuilder but printed the result only once after the loop, which outputs the entire sequence in one single line, not step-by-step as you want.
• Also, the numbers were appended directly without any space, resulting in a single concatenated string like 12345

Here is the corrected version:

import java.util.Scanner;

public class Series {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Insert number: ");
        int a = sc.nextInt();
        sc.close();

        StringBuilder sb = new StringBuilder();
        for (int i = 1; i <= a; i++) {
            sb.append(i).append(" ");
            System.out.println(sb.toString().strip());
        }
    }
}

Result

Insert number: 10
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
1 2 3 4 5 6
1 2 3 4 5 6 7
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9 10

---

• We should have added an if condition and avoided using strip() (or trim()) here for efficiency:

import java.util.Scanner;

public class Series {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Insert number: ");
        int a = sc.nextInt();
        sc.close();

        StringBuilder sb = new StringBuilder();
        for (int i = 1; i <= a; i++) {
            if (i > 1) {
                sb.append(" ");
            }

            sb.append(i);
            System.out.println(sb.toString());
        }
    }
}

---

• Without using StringBuilder():

import java.util.Scanner;

public class Series {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Insert number: ");
        int a = sc.nextInt();
        sc.close();

        if (a < 1) {
            System.out.println("");
            return;
        }

        String res = "1";
        System.out.println(res);
        for (int i = 2; i <= a; i++) {
            res += " " + i;
            System.out.println(res);
        }
    }
}

---

• Using two-loops approach is more memory-efficient as it avoids using StringBuilder(). The numbers are printed directly as they are processed, with no intermediate storage required.

import java.util.Scanner;

public class Series {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Insert number: ");
        int a = sc.nextInt();
        sc.close();

        for (int i = 1; i <= a; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print(j + " ");
            }
            System.out.println();
        }
    }
}

Notes

I would recommend making a habit of calling one of the String#strip… methods rather than trim. See the post: Java’s String.trim has a strange idea of whitespace. And see more discussion on this Answer of mine. – Comment from Basil Bourque

The trim() or if can be avoided by starting with "1" in the StringBuilder and looping from 2 on. (actually I would suggest using a plain String instead of a SringBuilder) – by user85421