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Arrays revisited - array declaration and array parameter

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• Review of array
  • Previously, we learned:

    How to define an array: int A[10]; // Defines an array of 10 int's float B[20]; // Defines an array of 20 float's How to use an array (it's just like Java): for (i = 0; i < 10; i++) A[i] = i;

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  • Since we discussed arrays, we have learned:

    Variable declaration: extern int x; Function declaration (i.e.: prototyping): int f(int x);

    We have not discussed:

    How to declare an array How to pass an array (as parameter) to a function

    This webpage will discuss these topics on array to "fill in the gap".

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• Declaring array variables
  • Recall from this webpage: click here), that the syntax to declare a variable is:

    extern dataType varName ;

    You can basically use this method to declare an array variable

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  • How I usually declare an array variable:

    If the array variables are: int A[10]; float B[4][7]; // 2 dimensional array I will declare the array variables by prepending the extern keyword as follows: extern int A[10]; extern float B[4][7]; // 2 dimensional array

    Example:

    #include <stdio.h> extern int A[10]; // Declare array extern float B[4][7]; // Declare array int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 10; i++) A[i] = i; for (i = 0; i < 10; i++) printf("%d ", A[i]); printf("\n\n"); for (i = 0; i < 4; i++) for (j = 0; j < 7; j++) B[i][j] = i+j; for (i = 0; i < 4; i++) { for (j = 0; j < 7; j++) printf("%f ", B[i][j]); printf("\n"); } printf("\n\n"); } int A[10]; // Define array float B[4][7]; // Define array

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/Declare-array/declare-array1.c

    How to run the program:

    To compile:   gcc declare-array1.c          To run: ./a.out

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• Alternate way to declare an array in C
  • Fact:

    The first dimension of an array declaration is optional !!!

    In other words, you can declare array variables as follows:

    #include <stdio.h> extern int A[ ]; // Declare array extern float B[ ][7]; // Declare array int main(int argc, char *argv[] ) { int i, j; for (i = 0; i < 10; i++) A[i] = i; for (i = 0; i < 10; i++) printf("%d ", A[i]); printf("\n\n"); for (i = 0; i < 4; i++) for (j = 0; j < 7; j++) B[i][j] = i+j; for (i = 0; i < 4; i++) { for (j = 0; j < 7; j++) printf("%f ", B[i][j]); printf("\n"); } printf("\n\n"); } int A[10]; // Define array float B[4][7]; // Define array

    The reason for this is:

    The first dimension is not needed to find the address (= location) of an array element (The first dimension is only used for array "out-of-bound" check. Since C does not check array-out-bound errors, the first dimension is optional)

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• Passing an array to a function as parameter
  • Here is an example function that uses arrays as parameters:

    #include <stdio.h> void printArray1( int X[10] ) { for (int i = 0; i < 10; i++) printf("%d ", X[i]); printf("\n\n"); } void printArray2( float X[4][7] ) { for (int i = 0; i < 4; i++) { for (int j = 0; j < 7; j++) printf("%f ", X[i][j]); printf("\n"); } printf("\n\n"); } int main(int argc, char *argv[] ) { int A[10]; // Define array float B[4][7]; // Define array int i, j; for (i = 0; i < 10; i++) A[i] = i; printArray1(A); // Pass array parameter for (i = 0; i < 4; i++) for (j = 0; j < 7; j++) B[i][j] = i+j; printArray2(B); // Pass array parameter }

    (This is how I usually specify array parameters -- because they "look normal")

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/Declare-array/pass-array1.c

    How to run the program:

    To compile:   gcc pass-array1.c          To run: ./a.out

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• Alternate way to define array parameter
  • Fact:

    The first dimension of an array parameter specification is optional !!!

    In other words, you can specify array parameters as follows:

    #include <stdio.h> void printArray1( int X[ ] ) { for (int i = 0; i < 10; i++) printf("%d ", X[i]); printf("\n\n"); } void printArray2( float X[ ][7] ) { for (int i = 0; i < 4; i++) { for (int j = 0; j < 7; j++) printf("%f ", X[i][j]); printf("\n"); } printf("\n\n"); } int main(int argc, char *argv[] ) { int A[10]; // Define array float B[4][7]; // Define array int i, j; for (i = 0; i < 10; i++) A[i] = i; printArray1(A); // Pass array parameter for (i = 0; i < 4; i++) for (j = 0; j < 7; j++) B[i][j] = i+j; printArray2(B); // Pass array parameter }