CODEWITHSUNDEEP
javastring
user9917830
user9917830

Reputation:

How can a number triangle come out of this code?

I'm a beginner in java and this code was used in a book I'm reading, however I can't seem to figure out how it works.

The code:

public class NumberTriangleWhile {
    public static void main(String[] args) {
        int number = 0;
        String output = "";

        while (number < 10) {
        output = output + number;
            System.out.println(output);
            number++;
        }
    }
}

The output: 

0
01
012
0123
01234
012345
0123456
01234567
012345678
0123456789

I don't understand why each number is printed and then somehow stored and reused in the next line, can someone explain this please?

Upvotes: 0

Views: 130

Answers (4)

Rohit BeedKar
Rohit BeedKar

Reputation: 1

Each number is getting printed because its getting concatenated. When number=0, And you are storing the value of number into the output as: Output=output+number Then the number which is an integer gets stored as a string in the output Each time the loop runs the new incremented number gets concatenated with the previous value in the string output. That's why all numbers are getting printed each time.

make this change in your code and you'll get your output

public static void main(String[] args)
{
     int number=0;
     String output="";
     while(number <10)
     {
         output=output+number;
         System.out.println(output);
         number++;
         output="";
     }
}

Upvotes: 0

Yashar Aliabbasi
Yashar Aliabbasi

Reputation: 2719

In every step inside while loop, output added with number, in Java adding something with String will result in String for example: 

String str = "a" + 2;

results in str="a2";.

If we start our loop, in first step number = 0 and output="" hence output = "" + 0 that make output = "0" in second run number=1 hence output = "0" (Old output value) + 1 that make output = "01" and so on

Upvotes: 1

Sweeper
Sweeper

Reputation: 271050

output is a string variable. When you add something to it like this:

output = output + number;

It does not add the numerical value of the number, but instead just joins the number with the original string. For example, if output was originally 1 and number is 2, the above line will change output to 12, not 3.

The loop keeps looping until number is 10. In the first iteration, output changed from an empty string to 0. In the second iteration (number has now increased to 1), output changed to 01 (the original 0 joined with the current value of number - 1). In the third iteration, number is incremented to 2. 2 is then added to the end of output to form 012. This carries on until number is 10.

The misconception you have might be that you think output becomes empty after you print it. It does not. It will still hold the same value.

Upvotes: 2

mariusr
mariusr

Reputation: 163

The string output is defined outside of the while loop. It can be read from and written to inside of the while loop and its contents will be consistent in the next iteration.

If output would be defined inside of the while loop, for example:

while ( number < 10) {
   String output = "";
   output += number;
}

then your output would just be (with new line after each number) 0 1 2 3 4 5 6 7 8 9

Since the new number is consistent throughout each iteration of the while loop, it is "stored" as you say

Upvotes: 0

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