Help with understanding java 'for' loops

Question

I have to write a java program where the solution will include the printing of the arrow tip figure depending on the number of rows. Below are example of how the result should look. However, I cannot do this until I understand for loops. I know I have to work with the rows and columns and possibly nested loops. I just dont know how to connect the row with the columns using for loops. Please help me in understanding these loops. Thanks!

Example #1 (odd number of rows)

>
>>>
>>>>>
>>>>>>>
>>>>>
>>>
>

Example #2 (even number of rows)

>
>>>
>>>>>
>>>>>>>
>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>
>>>
>

Answer 1

a for loop will loop through a collection of data, such as an array. The classic for loop looks like this:

  for(counter=0;counter <= iterations;counter++){   }

the first param is a counter variable. the second param expresses how long the loop should last, and the 3rd param expresses how much the counter should be incremented by after each pass.

if we want to loop from 1 - 10, we do the following:

for(counter=1;counter<=10;counter++){ System.out.println(counter); }

if we want to loop from 10 - 1, we do the following:

for(counter=10;counter>=1;counter--){  System.out.println(counter); }

if we want to loop through a 2 dimensional collection, like...

1 2 3
4 5 6
7 8 9

int[][] grid = new int[][] {{1,2,3},{4,5,6},{7,8,9}};

we need 2 loops. The outer loop will run through all the rows, and the inner loop will run through all the columns.

you are going to need 2 loops, one to iterate through the rows, one to iterate through the columns.

 for(i=0;i<grid.length;i++){
    //this will loop through all rows...
    for(j=0;j<grid[i].length;j++){
      //will go through all the columns in the first row, then all the cols in the 2nd row,etc
      System.out.println('row ' + i + '-' + 'column' + j + ':' + grid[i][j]);
    }
 }

In the outer loop, we set a counter to 0 for the first parameter. for the second, to calculate how many times we will loop, we use the length of the array, which will be 3, and for the third param, we increment by one. we can use the counter, i, to reference where we are inside the loop.

We then determine the length of the specific row by using grid[i].length. This will calculate the length of each row as they are being looped through.

Please feel free to ask any questions you may have regarding for loops!

EDIT: understanding the question.....

You are going to have to do several things with your code. Here we will store the number of lines in a variable, speak up if you need to pass in this value to a method.

 int lines = 10; //the number of lines
 String carat = ">";

 for(i=1;i<=lines;i++){
     System.out.println(carat + "\n"); // last part for a newline
     carat = carat + ">>";
 }

The above will print out carats going all the way up. We print out the carat variable then we make the carat variable 2 carats longer.

.... the next thing to do is to implement something that will decide when to decrease the carats, or we can go up half of them and down the other half.

Edit 3:

Class Test {
    public static void main(String[] args) {

        int lines = 7; 

        int half = lines/2;
        boolean even = false;
        String carat = ">";
        int i;

        if(lines%2==0){even = true;} //if it is an even number, remainder will be 0

        for(i=1;i<=lines;i++){
                System.out.println(carat + "\n");                           
                if(i==half && even){System.out.println(carat+"\n");} // print the line again if this is the middle number and the number of lines is even
                if(((i>=half && even) || (i>=half+1)) && i!=lines){ // in english : if the number is even and equal to or over halfway, or if it is one more than halfway (for odd lined output), and this is not the last time through the loop, then lop 2 characters off the end of the string
                        carat = carat.substring(0,carat.length()-2); 
                }else{ 
                        carat = carat + ">>"; //otherwise, going up
                }
        }
    }
}

Explanation and commentary along shortly. Apologies if this is over complicated (i'm pretty sure this is not even close to the best way to solve this problem).

Thinking about the problem, we have a hump that appears halfway for even numbers, and halfway rounded up for the odd numbers.

At the hump, if it is even, we have to repeat the string.

We have to then start taking off "<<" each time, since we are going down.

Please ask if you have questions.

Answer 2

I had the same question for a homework assignment and eventually came to a correct answer using a lot of nested if loops through a single for loop.

There is a lot of commenting throughout the code that you can follow along to explain the logic.

class ArrowTip {

public void printFigure(int n) {      //The user will be asked to pass an integer that will determine the length of the ArrowTip
 int half = n/2;   //This integer will determine when the loop will "decrement" or "increment" the carats to String str to create the ArrowTip
 String str = ">";  //The String to be printed that will ultimately create the ArrowTip
 int endInd;        //This integer will be used to create the new String str by creating an Ending Index(endInd) that will be subtracted by 2, deleting the 2 carats we will being adding in the top half of the ArrowTip

 for(int i = 1; i <= n; i++) {       //Print this length (rows)
    System.out.print(str + "\n");   //The first carat to be printed, then any following carats.
    if (n%2==0) {       //If n is even, then these loops will continue to loop as long as i is less than n.
      if(i <= half) {     //This is for the top half of the ArrowTip. It will continue to add carats to the first carat
         str = str + ">>";    //It will continue to add two carats to the string until i is greater than n.
      }
      endInd = str.length()-2;  //To keep track of the End Index to create the substring that we want to create. Ultimately will determine how long the bottom of the ArrowTip to decrement and whether the next if statement will be called.
      if((endInd >= 0) && (i >= half)){   //Now, decrement the str while j is greater than half
         str = str.substring(0, endInd);  //A new string will be created once i is greater than half. this method creates the bottom half of the ArrowTip
      }
    }
    else {          //If integer n is odd, this else statement will be called.
      if(i < half+1) {  //Since half is a double and the integer type takes the assumption of the one value, ignoring the decimal values, we need to make sure that the ArrowTip will stick to the figure we want by adding one. 3.5 -> 3 and we want 4 -> 3+1 = 4
          str = str + ">>"; //So long as we are still in the top half of the ArrowTip, we will continue to add two carats to the String str that will later be printed.
       }
      endInd = str.length()-2;      //Serves the same purpose as the above if-loop when n is even.
       if((endInd >= 0) && (i > half)) {  //This will create the bottom half of the ArrowTip by decrementing the carats.
        str = str.substring(0, endInd);   //This will be the new string that will be printed for the bottom half of the ArrowTip, which is being decremented by two carats each time.
      }
    }
 }
}
}

Again, this was for a homework assignment. Happy coding.

Answer 3

Here is a simple answer for you hope it helps! Cheers Logan.

public class Loop {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            int count = i;
            int j = 0;

            while (j != count) {
                System.out.print(">");
                j++;
            }
            System.out.println();

        }
        for (int i = 10; i > 0; i--) {
            int count = i;
            int j = 0;

            while (j != count) {
                System.out.print(">");
                j++;
            }
            System.out.println();

        }
    }
}

Answer 4

For making a 'for' loop:

public class Int {
    public static void main(String[] args) {
        
        for (Long num = 1000000L; num >= 9; num++) {
            System.out.print("Number: " + num + " ");
        }
    }
}

Output:

Number: 1008304 Number: 1008305 Number: 1008306 Number: 1008307 ...