Hi guys, I dont understand why when running my code in the terminal in shows me the following message" Index 2 out of bounds for length 2" Thanks!!!

[rubendelope]

import java.util.List;
import java.util.ArrayList;

public class multiplyingMatrixes{

        public static void main(String[] args) {
            List<Integer> matriz3 = new ArrayList<>();
         
            int [][] m1 = {{1,2,3},{1,2,3}};
            int [][] m2 = {{0,3,4},{1,0,2}};
            int row1 = m1.length;
            int column2 = m2[0].length;
            int [][] m3 = new int[row1][column2];

           if(row1 != column2){
               System.out.println("the rows from the first matrix; "+ row1+",do not match with the columns form the second matrix: "+ column2+",");

           }
            
            // [2] are the rows and [3] are the columns

          
            int x,y,k;
            // rows that is why x goes until 2
            for(x = 0; x < row1 ; x++);
            //columns that is why y goes until 3
                for(y = 0; y < column2; y++);
                m3[x][y] = 0;
            // we use k to sum all the elements multiplied in the row and column
                    for(k  = 0; k < column2; k++){
                      m3[x][y] += m1[x][k] * m2[k][y];
                     }
                     for ( x = 0; x < row1; x++)
                        for( y = 0; y < column2; y++){

                            System.out.println(m3[x][y]);
                            System.out.println("");
                        }
                        
                    }


        
        
    

           

            }

        
    

    
     

   

Delete this before posting. Please make sure to include the language that you're using in the title, and use the <> button for any code.

 

[AbydosOne]

57 minutes ago, rubendelope said:

Delete this before posting. Please make sure to include the language that you're using in the title, and use the <> button for any code.

The short answer is that you have your row count and your column count switched.

[rubendelope]

37 minutes ago, AbydosOne said:

The short answer is that you have your row count and your column count switched.

I am sorry,

what do you mean by that?

[AbydosOne]

20 minutes ago, rubendelope said:

I am sorry,

what do you mean by that?

 

int [][] m1 = {{1,2,3},{1,2,3}};

// column -  1   2   3
// row 1 - [ 1 | 2 | 3 ]
// row 2 - [ 1 | 2 | 3 ]

int row1 = m1.length; // this asks "how many items are in the outside array", which is counting how many rows there are
int column2 = m2[0].length; // this asks "how many items are in the inside array", which is counting how many columns there are

// all good so far, it seems

 

I might have been wrong initially... let's dig into your for-loops...

 

for(x = 0; x < row1 ; x++); // for-loop syntax should have {...} around code to execute after it

// example:

for(x = 0; x < row1 ; x++) {
	System.out.println(x);
}

 

I think that might be more of your issue... try fixing your syntax issues and report back.

 

m3[x][y] += m1[x][k] * m2[k][y]; // if column != row count, "k" will go outside (what your issue refers to) or fail 
                                 // to cover the range of either "rows" or "columns" here

 

...but I think that line above is the issue here.

[aramini]

Most languages start index at 0, not 1. So a 2 element array would have element 0 and element 1. So when you iterate through the array, if you are off by 1 you will get an index out of range or out of bounds error.

 

For example, if there are 2 elements in the array and your iterator is 3, since there is no element at index 3 in the array, you get the index out of bounds/range error.

 

You should check that your iterator variables x,y,k are never bigger than the number of elements of the array they are working with.

 

 

[Sauron]

1 hour ago, rubendelope said:

for(x = 0; x < row1 ; x++); //columns that is why y goes until 3
for(y = 0; y < column2; y++);
m3[x][y] = 0;

You made a mistake here, you forgot the brackets. This way the two loops just cycle and increment x and y until they are equal to the length element, which is out of bound for indexing, and THEN you set m3[x][y] (which now means m3[2][3]) to 0. Remember, the loop breaks when the check is no longer true.

 

Instead you want to nest that command inside the loops using brackets:

for(x = 0; x < row1 ; x++){ //columns that is why y goes until 3
  for(y = 0; y < column2; y++){
    m3[x][y] = 0;
  }
}

 

[Eigenvektor]

19 minutes ago, Sauron said:

You made a mistake here, you forgot the brackets.

Alternatively, you could remove the semicolons after the for-statements:

for (x = 0; x < row1; x++)         // no semicolon here
    for (y = 0; y < column2; y++)  // or here
        m3[x][y] = 0;              // this is now part of the same statement (i.e. the loops)

Written this way, the line following the for-loop is part of the same statement. Strictly speaking, curly braces are only needed when there are multiple statements inside the for-loop. However, for the sake of readability, I would absolutely recommend to go with the style that @Sauron used.

 

It is easy to miss that someone accidentally added those semicolons. Braces make it much more visible what is part of the loop and what isn't. You can also extend the loop later on without having to remember that you now must use braces instead of them being optional. So braces can help avoid bugs like this.