Nested for loops

Question

I have a decent beginner understanding of regular for loops but I'm having trouble wrapping my head around nested for loops in Java.

In the problem I'm working on, I have a constant integer that is a max number, and then I ask the user for 4 different number inputs. From those 4 inputs, I'm trying to determine which of them I can fit 'inside' the constant integer I declared.

IE: If the constant integer is 30 and the user inputs 5, 9, 3, and 21 it will tell them they can only use the 5, 9, and 3 because the 21 would be too large to add.

The problem in story form is, a user has a knapsack that holds a certain amount of weight. The program asks the user to input 4 different item weights and then decides which items it can fit in the bag.

This is for a school project so I'm required to use nested for loops.

Answer 1

package com.examplehub.basics;

public class ForNestedLoop {
    public static void main(String[] args) {

        /*
         * ####
         * ####
         * ####
         */
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 4; j++) {
                System.out.print("#");
            }
            System.out.println("\n");
        }

        /*
         * Outer loop iteration 1
         * i = 1; j = 1
         * i = 1; j = 2
         * i = 1; j = 3
         * i = 1; j = 4
         * Outer loop iteration 2
         * i = 2; j = 1
         * i = 2; j = 2
         * i = 2; j = 3
         * i = 2; j = 4
         * Outer loop iteration 3
         * i = 3; j = 1
         * i = 3; j = 2
         * i = 3; j = 3
         * i = 3; j = 4
         */
        for (int i = 1; i <= 3; ++i) {

            System.out.println("Outer loop iteration " + i);

            for (int j = 1; j <= 4; ++j) {
                System.out.println("i = " + i + "; j = " + j);
            }
        }

        /*
         * 1
         * 12
         * 123
         * 1234
         * 12345
         */
        for (int i = 1; i <= 5; ++i) {
            for (int j = 1; j <= i; j++) {
                System.out.print("" + j);
            }
            System.out.println();
        }

        /*
         * 1*1=1
         * 1*2=2    2*2=4
         * 1*3=3    2*3=6   3*3=9
         * 1*4=4    2*4=8   3*4=12  4*4=16
         * 1*5=5    2*5=10  3*5=15  4*5=20  5*5=25
         * 1*6=6    2*6=12  3*6=18  4*6=24  5*6=30  6*6=36
         * 1*7=7    2*7=14  3*7=21  4*7=28  5*7=35  6*7=42  7*7=49
         * 1*8=8    2*8=16  3*8=24  4*8=32  5*8=40  6*8=48  7*8=56  8*8=64
         * 1*9=9    2*9=18  3*9=27  4*9=36  5*9=45  6*9=54  7*9=63  8*9=72  9*9=81
         */
        for (int i = 1; i <= 9; ++i) {
            for (int j = 1; j <= i; j++) {
                System.out.print(j + "*" + i + "=" + j * i + "\t");
            }
            System.out.println();
        }

        /*
         * A
         * AB
         * ABC
         * ABCD
         * ABCDE
         */
        for (int i = 1; i <= 5; i++) {
            char letter = 'A';
            for (int j = 1; j <= i; j++) {
                System.out.print(letter++);
            }
            System.out.println();
        }

        /*
         * 1 2 3
         * 4 5 6
         * 7 8 9
         */
        int[][] array = new int[][]{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
        for (int i = 0; i < array.length; i++) {
            for (int j = 0; j < array[i].length; j++) {
                System.out.print(array[i][j] + " ");
            }
            System.out.println();
        }

        /*
         * 1 2 3
         * 4 5 6
         * 7 8 9
         */
        for (int[] ints : array) {
            for (int anInt : ints) {
                System.out.print(anInt + " ");
            }
            System.out.println();
        }
    }
}

source

Answer 2

To understand nested loops, you can start with simple examples, and then try harder one. For example, let's suppose you want to make a counter.

int i, j;
for (i=0; i <= 9; i++)
{
    for (j=0; j <= 9; j++)
    {
        System.out.println(i+""+j)
    }
}

The output is numbers from 00 to 99. You can write the output of the loop in a paper or something to see how it works. Let's take the example of this loop, you have this output:

00 //here your program entered the outer loop, i has now the value 0, after that, you enter to the inner loop, i remains 0, but j will change in the next iteration
01 // you are still in the first iteration of the outer loop, but the inner loop is on the second
02 // and so on ....
03
04
05
06
07
08
09 // ... until the inner loop finished looping
10 // once the inner loop finished looping, the outer loop changes again, and we are back to the inner loop

Once all that is clear on your mind, you can decide how your nested loop will be like. What variables need to be used in the outer loop, and what variables for the inner loop.

Answer 3

I haven't done any JAVA but I know that C# is pretty much the same.

I would do like this:

int max = 30;
int value = 0;
int counter = 0;
int[] input[4] = new int[5, 9, 3, 21];
bool[] canAddInput[4] = new bool[false, false, false, false];

for(value; value <= max; )
{
    for(counter; counter < 4; counter++)
    {
         value += input[i];
         if(value<=max)
             canAddInput[i] = true;
    }

    if(counter >= 4)
        Break;
}

Answer 4

Any easy way to think of nested for loops is to ignore the fact that they are nested. By convention, you will typically use i for the outer loop's increment counter and j for the inner loop's, which is the most important thing to keep straight in the beginning. If that is a point of confusion for you, it would likely benefit you to use more descriptive names for your increment variables than the letters 'i' and 'j', for example outer and inner.

At any given time when you are trying to structure your program's logic you only need to focus on the for loop that you are working most directly inside - at least when you are starting out and learning about them for the first time.