Global array variables

Array variables defined inside some function are local array variables
Array variables defined outside a function are global array variables

Example:

int A[10]; // Global array variable int main( ) { double B[10]; // Local array variable }

Declaring global array variables

Recall the variable declaration requirement:
The data type information requirement also apply to array variables:
A example will be presented next

Example of using an global 1-dimensional array variable

In this example, we define the global array variable before it is used:

#include <stdio.h> int A[10]; // Define (global) array before use int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 10; i++) A[i] = i; // Use array for (i = 0; i < 10; i++) printf("%d ", A[i]); // Use array printf("\n\n"); }

There is no compiler error
DEMO: demo/C/set1/declare-array3.c

Example of using an global 1-dimensional array variable

If we define the global array variable after it is used:

#include <stdio.h> int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 10; i++) A[i] = i; // Use array for (i = 0; i < 10; i++) printf("%d ", A[i]); // Use array printf("\n\n"); } int A[10]; // Define (global) array after use

There will be a undeclared identifier compiler error
DEMO: demo/C/set1/declare-array3.c

Example of using an global 1-dimensional array variable

We can fix the error by declaring the global array variable before it is used:

#include <stdio.h> extern int A[10]; // Declare (global) array before use int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 10; i++) A[i] = i; // Use array for (i = 0; i < 10; i++) printf("%d ", A[i]); // Use array printf("\n\n"); } int A[10]; // Define (global) array after use

The declaration "extern int A[10];" tells the C compiler that the name A is an array ([ ]) of integers (int)

DEMO: demo/C/set1/declare-array3.c

The official syntax to declare global 1-dimensional array variable

The array size is optional in the array declaration of a 1-dimensional array:

#include <stdio.h> extern int A[ ]; // Declare (global) array before use int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 10; i++) A[i] = i; // Use array for (i = 0; i < 10; i++) printf("%d ", A[i]); // Use array printf("\n\n"); } int A[10]; // Define (global) array after use

( because the array length information are not retained by the C compiler)

DEMO: demo/C/set1/declare-array3.c

Example of using an global 2-dimensional array variable

In this example, we define the global 2-dimensional array variable before it is used:

#include <stdio.h> int B[4][3]; // Define (global) array before use int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 4; i++) for (j = 0; j < 3; j++) B[i][j] = i+j; // Use array for (i = 0; i < 4; i++) { for (j = 0; j < 3; j++) printf("%d ", B[i][j]); // Use array printf("\n"); } printf("\n\n"); }

There is no compiler error
DEMO: demo/C/set1/declare-array4.c

Example of using an global 2-dimensional array variable

If we define the global 2-dimensional array variable after it is used:

#include <stdio.h> int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 4; i++) for (j = 0; j < 3; j++) B[i][j] = i+j; // Use array for (i = 0; i < 4; i++) { for (j = 0; j < 3; j++) printf("%d ", B[i][j]); // Use array printf("\n"); } printf("\n\n"); } int B[4][3]; // Define (global) array after use

There will be a undeclared identifier compiler error
DEMO: demo/C/set1/declare-array3.c

Example of using an global 2-dimensional array variable

We can fix the error by declaring the global 2-dimensional array variable before it is used:

#include <stdio.h> extern int B[4][3]; // Declare (global) array before use int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 4; i++) for (j = 0; j < 3; j++) B[i][j] = i+j; // Use array for (i = 0; i < 4; i++) { for (j = 0; j < 3; j++) printf("%d ", B[i][j]); // Use array printf("\n"); } printf("\n\n"); } int B[4][3]; // Define (global) array after use

DEMO: demo/C/set1/declare-array4.c

The official syntax to declare global 2-dimensional array variable

The first dimension size is optional in the array declaration of a 2-dimensional array:

#include <stdio.h> extern int B[ ][3]; // Declare (global) array before use int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 4; i++) for (j = 0; j < 3; j++) B[i][j] = i+j; // Use array for (i = 0; i < 4; i++) { for (j = 0; j < 3; j++) printf("%d ", B[i][j]); // Use array printf("\n"); } printf("\n\n"); } int B[4][3]; // Define (global) array after use

(Reason explained next....)
DEMO: demo/C/set1/declare-array4.c

Why is the first dimension not necessary in an array declaration

Recall: elements in 2 dimensional arrays are stored row-wise:
Recall: the C compiler do not record the length of arrays
I.e.:
The C compiler do not record the # rows of a 2-dim array

Why is the 2nd dimension necessary in an array declaration

The array element B[i][j] is accessed using a one-dimensional index as B[i*3 + j]:
The 2nd dimension is necessary to linearize the index used to access array elements:
The C compiler needs to know the # columns to access the array element B[i][j]