How does this for loop output a decending number of "#" (top being most amount while botto-CodePudding

I'm attempting to solve some Java questions and I came across a question that asked me to print out a diamond of size n.

Since I got stumped on this question, I decided to look up other's solutions to get an idea as to how I could tackle this problem.

A diamond of size 2 should look like this by the way.

##*##
#***#
##*##

However, one part of the code really stumped me out and that is how does the # get printed out in that manner.

Here is what I found for the top left bit of the diamond, basically this bit:

##
#

Here is the code for that:

 public static void printDiamond(int k){
        for(int i=1; i <= k; i  ) {
            for(int j = i; j <= k; j  ) {
                System.out.print("#");
            }
            System.out.println("");
         }
 }

and the output when k = 5:

#####
####
###
##
#

From my understanding, int j is looping upwards since it's declared as i which also loops upwards until it reaches k.

Shouldn't that print out 1,2,3,4,5 diamonds instead since the loop goes from 0 to k, which is 5?

I was expecting the output to be like:

#
##
###
####
#####

Can someone explain how does the code snippet I attached returns the output from top to bottom?

CodePudding user response：

A simple pen and paper exercise will uncover the details.

for(int i = 1; i <= k; i  ) {
    for(int j = i; j <= k; j  ) {
    ...
   }
}

When i = 1, j runs from 1 to k. So, it will print the star k times. That is how you get k stars in row 1.

When i = 2, j runs from 2 to k. So, it will print the star k - 1 times. That is how you get k - 1 stars in row 2.

And so on...

When i = k, j runs from k to k which is one time and hence you get only one star in the last row.

CodePudding user response：

Initial value of j is set to incrementing i and the inner loop is limited with the fixed value k. That is, the first run prints # from 1 to 5 (5 times), the second run prints from 2 to 5, etc. and the number of prints is decreasing.

To print the lower part in increasing way, the limit should be set to i: for (int j = 1; j <= i; j 0) System.out.print("#").

Also, the inner loops may be replaced with the method String::repeat available since Java 11:

public static void diamond(int k) {
    // upper part, decreasing # increasing *
    for (int i = 1; i <= k; i  ) {
        System.out.println("#".repeat(k - i)   "*".repeat(2 * i - 1)   "#".repeat(k - i));
    }
    
    // lower part, decreasing * increasing #
    for (int i = 1; i < k; i  ) {
        System.out.println("#".repeat(i)   "*".repeat(2 * (k - i) - 1)   "#".repeat(i));
    }
}