Java do-while细节

Question

My code does an error but I don't know how to correct it:

public class Cenas {
public static void main(String[] args) {
    //gera matrizes de tamanho aleatório com uns e zeros
    int a = 2;//(int)(Math.random() *3) + 1;
    int b  = 2;//(int)(Math.random() *3) + 1;
    int[][]matriz = new int [a][b];
    do{
        for (int i=0; i<a; i++) {
            for (int j=0; j<b; j++) {
                matriz[i][j] = (int) Math.round(Math.random());
                System.out.print(matriz[i][j] + " ");
            }
            System.out.println("");
        }
    }while(matrizIdentidade(matriz)==true); //the error is in here!!! the ";"


public static boolean matrizIdentidade (int[][]m){
    boolean diagonal = false;
    if (m.length==m[0].length) //matriz.Testada[0] é o comprimento da matriz
        for (int i = 0; i < m.length; i++)
            for (int j = 0; j < m[0].length; j++)
                if(i==j && m[i][j]==1)
                    if(i!=j && m[i][j]==0)
                        diagonal = true;
    return diagonal;
}
}

It generates random matrices and tells me if they are an identity matrix or not. I put the System.out.print and dimension 2 by 2 just for testing. The error makes my loop infinite...

";" on the line commented appears underlined in red (in Eclipse) giving me an error.

I humbly think that you are missing my question. I don't know if the statements in my methods are correct (I'm working on it), but what brings me here is that the ";" gives me an error: "Syntax error, insert "}" to complete MethodBody". If that is due to my badly coded logic I apologized. But I think, instead, that indicates that I'm doing some syntax error on the do-while loop.

Answer 1

You are defining the return value only by the last member

  diagonal = true;

Should be the opposite, you start assuming the matrix is the identity and return false when you check that it is not true.

if((i==j && m[i][j]!=1) || (i!=j && m[i][j]!=0)) {
   // this is not an identity matrix, so you can stop
   return false;
}

if the loop ends, the matrix is the identity so return true .

Answer 2

The loop do-while is infinite because it never get matrizIdentidade(matriz)==true .

Try to browse the elements of the matrix twice to check the two properties of a diagonal matrix. Try this please :

public static boolean matrizIdentidade (int[][]m){

    boolean diagonal1 = false, diagonal2 = false  ;

    if (m.length==m[0].length) //matriz.Testada[0] é o comprimento da matriz
        for (int i = 0; i < m.length; i++)
            for (int j = 0; j < m[0].length; j++)
                    if(i!=j && m[i][j]==0)
                        diagonal1 = true;

        for (int i = 0; i < m.length; i++)
            for (int j = 0; j < m[0].length; j++)
                    if(i==j && m[i][j]==1)
                        diagonal2 = true;

        if(diagonal1 == true && diagonal2 == true ) return true
        else return false; 
}

If the problem is related to the syntax of do-while check this documentation Snippet :

       do {
            System.out.println("Count is: "
                               + count);
            count++;
        } while (count < 11);