matrix using pointers

Question

hello, i have problem with the third function (GetMatrix) the others are works. i want to catch data from user into the matrix by poiters only

the lines:

typedef int pArr[COLS];

void GetMatrix(pArr* ,int , int );

M[ROWS][COLS];

are constraints

how i need to do the scan, i try to use scanf("%d",*(M+cols*i+j)) but i get error..its probably wrong what is need to do to make it right

thx all

the code :

#pragma warning ( disable: 4996 )
#include <stdio.h> 
#include <string.h>
#define ROWS 2
#define COLS 3

typedef int pArr[COLS];

void MergeStrings(char* str1,char* str2,char* strRes);
void StrReverse(char* strRes,char* strRev);
void GetMatrix(pArr* M ,int rows , int cols);

void main()
{
 char str1[256],str2[256],strRes[256],strRev[256];
 int M[ROWS][COLS];

 printf("Please enter first string:\n");
 flushall();
 gets(str1);
 printf("\nPlease enter second string:\n");
 flushall();
 gets(str2);

 printf("\nMerge of: %s\n",str1);
 printf("with: %s\n",str2);

 MergeStrings(str1,str2,strRes);

 StrReverse(strRes,strRev);

 printf("is:\n");
 printf("==> %s\n",strRes);

 printf("\nthe reversed merged string is:\n");
 printf("==> %s\n\n",strRev);

 GetMatrix(M,ROWS,COLS);
}

void MergeStrings(char* str1,char* str2,char* strRes)
{
 int i=0,j=0,a=0,flag=0,flag2=0;

 do
 {

  while( *(str1+i)==' ')
   i++;

  while( *(str2+j)==' ')
   j++;

  while( *(str1+i)!=' ' && *(str1+i)!='\0')
  {
   *(strRes+a)=*(str1+i);
    i++;
    a++;
  }

  if(flag!=1)
  {
  *(strRes+a)=' ';
  a++;
  }

  if(*(str1+i)=='\0')
  flag=1;

  while( *(str2+j)!=' ' && *(str2+j)!='\0')
  {
   *(strRes+a)=*(str2+j);
   j++;
   a++;
  }

  if(flag2!=1)
  {
  *(strRes+a)=' ';
  a++;
  }

  if(*(str2+j)=='\0')
  flag2=1;

 }while( (*(str1+i)!='\0') || (*(str2+j)!='\0') );

 *(strRes+a)='\0';

}

void StrReverse(char* strRes,char* strRev)
{
 int size,i=0,j=0;
 size=strlen(strRes);

 for(i=size-2 ; i>=0 ;i--)
 {
  *(strRev+j)=*(strRes+i);
  j++;
 }

 *(strRev+size-1)='\0';

}
void GetMatrix(pArr* M ,int rows , int cols )
{


}

Answer 1

#define ROWS 2
#define COLS 3
typedef int pArr[COLS];
void GetMatrix(pArr* M ,int rows , int cols )
{
  for(int i = 0; i < rows; i++)
    for(int j = 0; j < cols; j++)
      scanf("%d", &(M[i][j]));
}

int main()
{
    int M[ROWS][COLS] = {{100, 101, 102},{103, 104, 105}};
    printf("before\n");
    for(int i = 0; i < ROWS; i++){
        for(int j = 0; j < COLS; j++)
            printf("%d ", M[i][j]);
        printf("\n");
    }
    printf("enter values\n");
    GetMatrix(M, ROWS, COLS);
    printf("after\n");
    for(int i = 0; i < ROWS; i++){
        for(int j = 0; j < COLS; j++)
            printf("%d ", M[i][j]);
        printf("\n");
    }
}

Output:

before
100 101 102 
103 104 105 
enter values
0 1 2 3 4  5
after
0 1 2 
3 4 5 

Inside the function, M is a pointer to an array of three integers. M[i] is the i-th array, and M[i][j] is the j-th element in the i-th array. & operator gives its address to the scanf, which happily stores the read number in there.

Here is what happens when you replace scanf("%d", &(M[i][j])); with scanf("%d",*(M+cols*i+j)):

  for(int i = 0; i < rows; i++)
    for(int j = 0; j < cols; j++)
      scanf("%d", *(M + cols * i + j));

snip..

before
100 101 102 
103 104 105 
enter values
0 1 2 3 4 5
after
0 101 102 
1 104 105 

• In the first iteration of the inner loop, i and j are zero, and *(M+cols*i+j) becomes *M. Since M is a pointer to an array, dereferencing it gives you an array of integers which decays into pointer to an integer. Thus the first number get stored at the correct place.
• In the second iteration of the inner loop, j is 1. Thus, it is *(M + 1). Pointer arithmetic comes into picture here. Since M is a pointer to an array of three integers, (M + 1) is a pointer to an array of three integers that points to the second array; dereferencing it gives the array, which decays into pointer and scanf writes the second number there.
• In the third iteration, j is 2 and (M + 2) points to the third (non existent) array and scanf writes the entered number into that memory location in the stack, thus corrupting the stack.
• So on and so forth. It attempts to write into every third integer location, leaving our preinitialized values (101, 102, 104, 105) untouched.

---

If you want to it with pointers, you can cast M as a pointer to int as in:

scanf("%d", (int*)M + cols * i + j);

Once you cast M as int*, pointer arithmetic adds 1 * sizeof(int) to the starting position instead of 1 * sizeof(array of three ints), thus giving the desired results.

---

Update - answer to the comment I received in Facebook.

Consider the snippet:

int a = 10;
scanf("%d", &a);
printf("%d", a);

Scanf stores the entered value at the specified address - hence we pass a pointer to int as its argument. printf prints the passed value - hence we pass an int as the argument. The same difference applies here. ((int*)M + cols * i + j) is of type int*, pointer to an integer. To print the integer at that location we should dereference it using * operator. Thus it becomes:

printf("%d\t",*((int*)M + cols * i + j));

Oh, and by the way, you'd have received a faster reply if you had posted your doubts here itself instead of my FB. There are lots of good people ready to help hanging out in SO - and I check FB only on weekends.

Answer 2

do you mean?

int GetMatrix( pArr* M ,int rows , int cols )
{
  return 1==scanf("%d",&M[rows][cols]);
}