Defining an array in C
 

  • Syntax to define an array in C: 

          dataType  arrayName[ arraySize ];              

 E.g.:

    int   A[10];
    float B[20];

  • Notable difference between arrays in C and Java:

      • C has static arrays

        I.e.: the name of the array variable is the (constant) address (= label) of the array

Review: static and dynamic arrays

Static array   (C) Dynamic array   (Java)
The base address of a static array is a constant number (i.e.: a label) The base address of a dynamic array is stored in a variable

Using an array

  • Syntax to access array elements in C: 

          arrayName[ index ]

Example: 

 #include <stdio.h>

 int main( int argc, char *argv[] )
 {
    int A[10], i;

    for (i = 0; i < 10; i++ )
       A[i] = i*i; // Assign a value to A[i] 

    for (i = 0; i < 10; i++ )
       printf("%d\n", A[i]); // Using the value in A[i] 
 }

Warning: C does not perform array bound checks !!!
 

  • C does not perform array bound check to achieve better performance

Example: 

 #include <stdio.h>

 int main( int argc, char *argv[] )
 {
    int A[10], i;

    for (i = 0; i < 10; i++ )
       A[i] = i*i; // Assign a value to A[i] 

    for (i = 0; i < 20 ; i++ ) // index out of bound !!
       printf("%d\n", A[i]); // Using the value in A[i] 
 }

Array parameters in functions

How to specific an array parameter in a function: 

  #include <stdio.h>

  int sumArray( int A[], int n /*array length*/ ) {
     int i, s;

     s = 0;
     for (i = 0; i < n; i++)
        s += A[i];
     return(s);
  }

  int main(int argc, char *argv[]) {
     int B[10] = {1, 3, 5, 7, 9}; // B[5 - 9] are init to 0 
     int r;

     r = sumArray(B, 10); // Pass array B as parameter
     printf("r = %d\n", r);
  }

Note: we must pass the array length as an additional parameter to a function in C (because the array data type in C does not contain the array length !!)

Array variables are passed-by-reference in C

Program that shows that array variables are "passed-by-reference": 

  #include <stdio.h>

  void clearArrayElem( int A[], int i) {
     A[i] = 0;  // Update one array elem...
  }

  int main(int argc, char *argv[]) {
     int B[10] = {1, 3, 5, 7, 9, 11, 13, 15, 17, 19};  

     int k;

     for ( k = 0; k < 10; k++ )  printf("%d ", B[k]);
     printf("\n\n");

     clearArrayElem(B, 5);   // B[5] changed to 0 ! 

     for ( k = 0; k < 10; k++ )  printf("%d ", B[k]);
     printf("\n\n");
  }

Actually, C passes the address of the array B by-value....

How to declare a function with array parameter

Situation where we need to declare a function: 

  #include <stdio.h>

  int main(int argc, char *argv[])
  {
     int B[10] = {1, 3, 5, 7, 9};
     int r;

     r = sumArray(B, 10); // Use function before it was defined  
     printf("r = %d\n", r);
  }  

  int sumArray( int A[], int n /*array length*/ )
  {
     int i, s;

     s = 0;
     for (i = 0; i < n; i++)
        s += A[i];
     return(s);
  }

How to declare a function with array parameter

How to declare a function with array as parameter (the easiest way): 

  #include <stdio.h>

  int sumArray( int A[], int n); // Declaration

  int main(int argc, char *argv[])
  {
     int B[10] = {1, 3, 5, 7, 9};
     int r;

     r = sumArray(B, 10); //Compiler now has the data type info 
     printf("r = %d\n", r);
  }  

  int sumArray( int A[], int n /*array length*/ )
  {
     int i, s;

     s = 0;
     for (i = 0; i < n; i++)
        s += A[i];
     return(s);
  }

Defining a 2 dimensional array in C
 

  • Syntax to define an 2 dimensional array in C: 

          dataType  arrayName[ dim1 ][dim2 ];              

 E.g.:

    int   A[4][3];  
    float B[20][10];

  • Note:

      • C can only define rectangular shaped 2 dimensional arrays     

Using a 2 dimensional array

  • Syntax to access array elements of a 2 dim array in C: 

          arrayName[ index1 ][ index2 ]

Example: 

 #include <stdio.h>

 int main( int argc, char *argv[] )
 {
    int A[4][3], i, j;

    for (i = 0; i < 4; i++ )
       for (j = 0; j < 3; j++ )
          A[i][j] = i*j; // Assign a value to A[i][j] 

    for (i = 0; i < 4; i++ )
    {  for (j = 0; j < 3; j++ )
          printf("%d ", A[i][j]); 
       printf("\n");
    }
 }

2 dimensional array parameters in functions

How to specific a 2-dimensional array parameter in a function: 

  int sumArray( int A[][3], int m, int n)
  {  // Explained in next slide
     int i, j, s;

     s = 0;
     for (i = 0; i < m; i++)
        for (j = 0; j < n; j++)
           s += A[i][j];
     return(s);
  }

  int main(int argc, char *argv[])
  {
     int B[4][3] = {{1,3,5}, {1,1,1}, {2,1,2}, {3,2,4}};

     int r;
     r = sumArray(B, 4, 3); // Pass 2 dim array B as param
     printf("r = %d\n", r);
  }

Note: we must pass the array dimensions (4,3) as additional parameters to functions in C !

Why do we need to specify the # columns in the array variable parameter

C stores a 2-dimensional array row-wise in memory: 

 int A[4][3]:                        Memory:

   +---------+---------+---------+      +---------+
   | A[0][0] | A[0][1] | A[0][2] |   A: | A[0][0] | (4 bytes) 
   +---------+---------+---------+      +---------+
   | A[1][0] | A[1][1] | A[1][2] |      | A[0][1] | (4 bytes)
   +---------+---------+---------+      +---------+
   | A[2][0] | A[2][1] | A[2][2] |      | A[0][2] | (4 bytes)
   +---------+---------+---------+      +---------+
   | A[3][0] | A[3][1] | A[3][2] |      | A[1][0] | (4 bytes)
   +---------+---------+---------+      +---------+
                                        | A[1][1] | (4 bytes)
                                        +---------+
                                        | A[1][2] | (4 bytes)
                                        +---------+
                                           ...
   Offset of A[1][0] = (3×1 + 0) × 4 = 3 × 4
   Offset of A[1][2] = (3×1 + 2) × 4 = 5 × 4

The # columns is necessary to compute the offset of A[i][j] = (#cols × i + j) × sizeof(oneArrayElem)

How to declare a function with array parameter

Situation where we need to declare a function: 

  #include <stdio.h>

  int main(int argc, char *argv[])
  {
     int B[4][3] = {{1,3,5}, {1,1,1}, {2,1,2}, {3,2,4}};

     int r;
     r = sumArray(B, 4, 3); // Used function before def
     printf("r = %d\n", r);
  } 

  int sumArray( int A[][3], int m, int n)
  {
     int i, j, s;

     s = 0;
     for (i = 0; i < m; i++)
        for (j = 0; j < n; j++)
           s += A[i][j];
     return(s);
  }

How to declare a function with array parameter

How to declare a function with 2-dimensional array as parameter (the easiest way): 

  #include <stdio.h>

  int sumArray( int A[][3], int m, int n); // Declaration 

  int main(int argc, char *argv[])
  {
     int B[4][3] = {{1,3,5}, {1,1,1}, {2,1,2}, {3,2,4}};

     int r;
     r = sumArray(B, 4, 3); // Used function after declare
     printf("r = %d\n", r);
  } 

  int sumArray( int A[][3], int m, int n)
  {
     int i, j, s;

     s = 0;
     for (i = 0; i < m; i++)
        for (j = 0; j < n; j++)
           s += A[i][j];
     return(s);
  }