LECTURE 18-19: Arrays as Function Arguments / Advanced Topics about Arrays

Using Array Elements as Function Arguments (Section 8.4)

Let us assume an array tictactoe is declared as

char tictactoe[3][3];

Array elements, such as tictactoe[0][2] can be used as function arguments in the same way as single-valued variables.

Example:

scanf(“%d”,&tictactoe[0][2]);

Array Arguments (Section 8.5)

Arrays can also be used as function arguments! Their use is relatively simple. Let us look at the following example.

Using one-dimensional arrays as arguments: Fig 8.15

# include<stdio.h>

# define NOT_FOUND -1 /* Value returned by search function if target not found */

int main(void)

{

int scores[3] = {100, 60, 80};

int pos;

pos = search(scores, 60, 3);

}

/*

* Searches for target item in first n elements of array arr

* Returns index of target or NOT_FOUND

* Pre: target and first n elements of array arr are defined and n>=0

*/

int

search(const int arr[], /* input - array to search */

int target, /* input - value searched for */

int n) /* input - number of elements to search */

{

int i,

found = 0, /* whether or not target has been found */

where; /* index where target found or NOT_FOUND */

/* Compares each element to target */

i = 0;

while (!found && i < n) {

if (arr[i] == target)

found = 1;

else

i++;

}

/* Returns index of element matching target or NOT_FOUND */

if (found)

where = i;

else

where = NOT_FOUND;

return (where);

}

Explanation of the program:

- in the main function, the goal is to find if any element of the first 3 elements of scores array has value equal to 60. If the answer is affirmative, the function returns the position of the first element equal to the search value

- search function has 3 arguments: the first is an integer array arr, the second is search variable target, and the third is variable n that tells how many elements of array arr are considered

- const in expression “const int arr[]” means that values of arr array cannot be changed within the function search

Using two-dimensional arrays as arguments: Fig 8.19

/* Checks whether a tic-tac-toe board is completely filled. */

int

filled(char ttt_brd[ ][3]) /* input - tic-tac-toe board */

{

int r, c, /* row and column subscripts */

ans; /* whether or not board filled */

/* Assumes board is filled until blank is found */

ans = 1;

/* Resets ans to zero if a blank is found */

for (r = 0; r < 3; r++)

for (c = 0; c < 3; c++)

if (ttt_brd[r][c] == ' ')

ans = 0;

return (ans);

}

Explanation of the function filled:

- function filled has only one argument, array ttt_brd

- expression “char ttt_brd[][3]” tells us that

o ttt_brd is two-dimensional array

o ttt_brd is of of char type

o ttt_brd has three columns. It is very important that content of first bracket is empty and the content of the second bracket is the number of columns!!

Advanced topic: Arrays as Pointers

Declaration

int scores[3];

reserves 3 consecutive spaces in memory to store 3 integers

Surprisingly, if you printf the value of scores as

printf(“%d \n”,scores);

you will see the address of the first element of array scores!!

- Therefore, scores is a pointer to the first element of array scores[3]!!

- Following this reasoning, scores+1 is a pointer to the second element of array scores[3].

- Therefore *scores equals scores[0], *(scores+1) equals scores[1], etc…

Important consequence, and source of programming errors:

If you declare

int scores[3];

int result;

and in your program you write

result = scores[2] + scores[3];

the value of result will become some junk value!!