Calling a 2D array function?

Question

I first apologize if some of you find my problem stupid and easy to solve, but I am a very beginner in "c".

My task is to create an inverse of a 3x3 matrix using different functions.

What I am trying to do now is to tell the user to input values for the 3x3 matrix and then print them. I made 2 functions which read and print the values, but I have problems with calling them since I cannot directly call an array in printf.

For now I am able to run the program, enter the values and print a wrong result which leads to a not responding program.

#include <stdio.h>
#include <stdlib.h>
#define SIZE 3 //defining the size of the matrix (3x3)

//prototyping the functions used to calculate the inverse of the matrix
void readMatrix(double a[SIZE][SIZE]);
void printMatrix(double a[SIZE][SIZE]);


main()
{
    double a[SIZE][SIZE];
    int i,j;


   printf("Enter the values for the matrix:\n", i, j);
   readMatrix(a);
   printf("Your Matrix:%d\n",a[i][j]);
   printMatrix(a);

   return 0;
}


//function 1
//letting the user to enter a matrix
void readMatrix(double a[SIZE][SIZE]){


    int i,j;

    for(i = 0; i < SIZE; i++){
        for(j = 0; j < SIZE; j++){
            scanf("%d", &a[i][j]);
        } 
    } 
}

//function 2
//outputing the given matrix
void printMatrix(double a[SIZE][SIZE]){

    int i,j;

    for(i = 0; i < SIZE; i++){
        for(i = 0; i < SIZE; j++){
            printf("Your matrix is: %d", a[i][j]);
       }
   }
}

I don't know what "I cannot directly call an array in printf" means, but your printMatrix contains a silly typo. Something you are supposed to be able to check for by yourself. And why are you using %d to print your matrix values? — AnT stands with Russia
– AnT stands with Russia, Commented Jun 23, 2017 at 6:11
for(i = 0; i < SIZE; j++){ printf("Your matrix is: %d", a[i][j]); --> for(j = 0; j < SIZE; j++){ printf("Your matrix is: %f\n", a[i][j]); — BLUEPIXY
– BLUEPIXY, Commented Jun 23, 2017 at 6:11

cs95 · Accepted Answer · 2017-06-23 06:49:14Z

2

In printf and scanf, it is crucial that you pass the exact format specifier that matches the type of the pointer to the variable. If your format specifier doesn't match the supplied argument, the result is undefined behaviour.

In effect, this

scanf("%d", &a[i][j]);

Needs to be replaced with

scanf("%lf", &a[i][j]);

And the same for printf("Your matrix is: %d", a[i][j]); -> printf("Your matrix is: %lf", a[i][j]);

Also, in printMatrix, you've used the loop variable i in the inner loop twice. What you want is

for(i = 0; i < SIZE; i++){
    for(j = 0; j < SIZE; j++){ 
          printf("%lf ", a[i][j]);
    printf("\n");
}

Edit: As pointed out by @cse in the comments, remove this line in main:

printf("Enter the values for the matrix:\n", i, j);

Since at this point, i, and j are not initialised, they'd contain junk.

edited Jun 23, 2017 at 6:49

answered Jun 23, 2017 at 6:16

cs95

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3 Comments

Velizar Kostadinov Over a year ago

Very strange..I succeeded executing it once, but most of the times i get - "A problem caused the program to stop working correctly. Windiows will close the program and notify you if a solution is available. Also, do you have an idea how can I print it in order to look like a real 3x3 matrix.

Baldrickk Over a year ago

You're printing it transposed. Also a good way to pave fault when dealing with larger arrays... granted, the input is transposed too so it isn't evident.

cse Over a year ago

The program is crashing because: In line printf("Your Matrix:%d\n",a[i][j]); of main() function, since variable i and j are not initialized so it contains garbage value. So don't print value at a[i][j] because it may cause of segmentation fault. OR initialize i and j with a valid value i.e. which is a valid index in array double a[][]

Wagner · Accepted Answer · 2017-06-23 06:18:16Z

1

in printMatrix, there is a infinite loop which will definitely lead your program to no responding. Should be:

   for(j = 0; j < SIZE; j++){
        printf("Your matrix is: %d", a[i][j]);
}

answered Jun 23, 2017 at 6:18

Wagner

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Comments

Baldrickk · Accepted Answer · 2017-06-23 06:55:58Z

For printing the output: you will want to actually print out the entire thing rather than one value after another, correct?

printf ("matrix is:\n")
char outstr[64];            //arbitrary but plenty big enough
char * pout;                 //points to a point in the string
for(j = 0;  j < SIZE;  j++){
    pout = outstr;
    for(i = 0;  i < SIZE;  i++){ 
        pout += sprintf (pout, "%lf," a [i][j]);
    *(--pout) = 0;      //end the string one char earlier (dangling ',')
    printf ("[%s]\n", outstr);
}

Will print:

matrix is:
[1,2,3]
[4,5,6]
[7,8,9]

Where the numbers are of course the ones in the array.

Also unless you were intending on filling the matrix in a columnar fashion, you should switch the i and j loops on the input function. You are storing the matrix in memory transposed. (This code assumes that you are not)

cse · Accepted Answer · 2017-06-23 09:41:40Z

There are following problems with above code:

In line printf("Your Matrix:%d\n",a[i][j]); of main() function, since variable i and j are not initialized so it contains garbage value. So don't print value at a[i][j] because it may cause of segmentation fault. OR initialize i and j with a valid value i.e. which is a valid index in array double a[][]. Also you can change line printf("Enter the values for the matrix:\n", i, j); in main() to printf("Enter the values for the matrix:\n");. Because i and j are not being used here.
In line scanf("%d", &a[i][j]); of function void readMatrix(double a[SIZE][SIZE]). Since you are reading a double primitive data type then you should use %lf formatter instead of %d. Same for line printf("Your matrix is: %d", a[i][j]) in function void printMatrix(double a[SIZE][SIZE]).
In line for(i = 0; i < SIZE; j++) of function void readMatrix(double a[SIZE][SIZE]). It should be for(j = 0; j < SIZE; j++) i.e. the variable to be used in inner loop should be j not i.

You can find working code here which is after correcting the code.

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