Question

I don’t seem to be able to find the answer to this-

I need to draw a simple triangle using for loops.

    *
   ***
  *****
 *******
*********

I can make a half triangle, but I don’t know how to add to my current loop to form a full triangle.

*
**
***
****
*****

for (int i = 0; i < 6; i++) {
    for (int j = 0; j < i; j++) {
        System.out.print("*");
    }
    System.out.println("");
}

Answer 1

First of all, you need to make sure you’re producing the correct number of * symbols. We need to produce 1, 3, 5 et cetera instead of 1, 2, 3. This can be fixed by modifying the counter variables:

for (int i=1; i<10; i += 2)
{
    for (int j=0; j<i; j++)
    {
        System.out.print("*");
    }
    System.out.println("");
}

As you can see, this causes i to start at 1 and increase by 2 at each step as long is it is smaller than 10 (i.e., 1, 3, 5, 7, 9). This gives us the correct number of * symbols. We then need to fix the indentation level per line. This can be done as follows:

for (int i=1; i<10; i += 2)
{
    for (int k=0; k < (4 - i / 2); k++)
    {
        System.out.print(" ");
    }
    for (int j=0; j<i; j++)
    {
        System.out.print("*");
    }
    System.out.println("");
}

Before printing the * symbols we print some spaces and the number of spaces varies depending on the line that we are on. That is what the for loop with the k variable is for. We can see that k iterates over the values 4, 3, 2, 1 and 0 when ì is 1,3, 5, 7 and 9. This is what we want because the higher in the triangle we are, the more spaces we need to place. The further we get down the triangle, we less spaces we need and the last line of the triangle does not even need spaces at all.