Computer Science 3710 -- January 19, 2004

We can pass strings (character arrays terminated with nul bytes) as parameters in much the same way that we pass an array of integers. The address of the first character in the array is passed to the function and any changes that we make to the characters of the string passed into the function will be made to the original string.

The program below defines a function, alternate_caps() that accepts a string parameter and modifies the string so that the case of the letters comprising the string alternates between uppercase and lowercase. The function returns the number of case alterations made to the string. The display() function is passed the numeric index of the string within the string array defined in the main() function, the string itself and the number of changes. The purpose of the display() function is to show these parameters using printf().

#include	<stdio.h>
#include	<ctype.h>

int alternate_caps(char string[]);
void display(int num, char str[], int changes);

#define MAX_COL 40

int
main()
{
	char	strings[][MAX_COL] = {
				"This is an array of CHARACTERS.",
				"tHiS Is aNoThEr aRrAy oF ChArAcTeRs.",
				"thIs sTrInG ReQuIrEs oNe cHaNgE"
				};
	int	i;

	for (i = 0; i < sizeof(strings)/sizeof(strings[0]); i++) {
		int	changes = alternate_caps(strings[i]);
		display(i, strings[i], changes);
	}
	return 0;
}

int
alternate_caps(char str[]) {
	int	i = 0;
	int	changes = 0;
	
	while (str[i] != '\0') {
		if (i % 2 == 0) {
			/* Make characters located at even indices
			   upper case (if they aren't already) */
			if (islower(str[i])) {
				str[i] = toupper(str[i]);
				changes ++;
			}
		}
		else {
			/* Make characters located at odd indices
			   lower case (if they aren't already) */
			if (isupper(str[i])) {
				str[i] = tolower(str[i]);
				changes ++;
			}
		}
		++ i;
	}

	return changes;
}

void
display(int num, char str[], int changes)
{
	printf("string[%d] is \"%s\" (%d change%s)\n",
		num, str, changes, (changes == 1) ? "" : "s");
}
function2.c

More <ctype.h> macros (K&R § 2.7)

In addition to the isupper()/islower() macros, which we saw in the previous lecture, the above code also uses the toupper() and tolower() ctype.h macros. These two macros accept a character argument and returns the uppercase and lowercase version of the same letter, respectivel (if possible). If the conversion is not possible, then the original character is returned.

Iterating over the Characters

This basically says while the character at position i is not the nul byte (i.e. '\0'), execute the loop body. This provides one way to iterate over all the characters in the array supplied to the function. The i index variable is updated at the bottom of the loop. This conditional can be abbreviated by dropping the explicit test against the nul byte. It could also have been implemented with a for loop, as we will see later.

Output from the program

Note that there is no 's' on the word "change" when only one change was made to the string by the alternate_caps().

As mentioned above, when we pass an array, what we are actually passing is a pointer to the array's first element. This implies that there is a very close relationship between arrays and pointers.

Pointers

A pointer is defined by using * before the pointer variable name in its definition. Before a pointer can be used, it must be set to the address of a variable of the appropriate type. We can get the address of the variable by preceding the variable's name with the address-of operator: &. When this is done, we can then get access and/or change the value of the variable by dereferencing the pointer variable. A pointer variable can be dereferenced by placing a * before it.

#include	<stdio.h>

int
main()
{
	int	 i = 0, j = 2;
	int	*ptr = &i;

	/* *ptr is an alias for i */
	printf("Before i = %d\t*ptr = %d\n", i, *ptr);
	*ptr = 1;
	printf("After i = %d\t*ptr = %d\n", i, *ptr);

	ptr = &j;
	/* Now *ptr is an alias for j */
	printf("Before j = %d\t*ptr = %d\n", j, *ptr);
	j = *ptr * 2;
	printf("After j = %d\t*ptr = %d\n", j, *ptr);
	return 0;
}
pointer1.c

Initially, ptr is set to the address of i. This means that the contents of i can be accessed and changed by dereferencing ptr. In other words, *ptr can be thought of as an alias for i. In the second group of statements, ptr is set to the address of j. Now *ptr becomes synonymous with j.

In the first line, the * is being used to define the variable ptr as a pointer to an integer. In the second line, the * operator is being used to dereference the pointer ptr in order to access the contents of the memory location addressed by the pointer.

Pointers are commonly used to efficiently pass arguments to functions, to simulate pass by reference semantics and to store the addresses of blocks of memory allocated dynamically (more on this in later lectures). There is also a very close relationship between pointers and arrays.

Pointers and Arrays

In nearly all cases, an array variable degrades into a pointer to the array's first element. Because of this, we can assign an array to a pointer (of the appropriate type). When we do so, what's actually happening is that the pointer is assigned the address of the array's first element. (Incidentally, one important exception to the arrays degrade into pointers to their first element rule is when an array is used as the argument to the sizeof operator.)

For example, the following code demonstrates three ways of iterating over the characters of a string. One uses the traditional array index variable, the second iterates over the string using for loop that increments a pointer, and the third uses a while loop and an incrementing pointer variable. Note that all loops stop when we reach the nul byte at the end of the string.

#include	<stdio.h>

int
main()
{
	char	 a[] = "Hello";
	char	*p;
	int	 i;

	for (i = 0; a[i] != '\0'; i++)
		printf("a[%d] = %c\n", i, a[i]);

	for (p = a; *p != '\0'; p++)
		printf("*p = %c\n", *p);

	p = a;
	while (*p != '\0')
		putchar(*p++);

	return 0;
}
pointer2.c

In all three cases we can get rid of the != '\0' test, because in C, any zero value (including the nul byte) represents false. This means that the two for loops could have been written as:

By incrementing the pointer in the last two loops, we can move through elements of the array in a sequential fashion and output the characters by dereferencing the pointer.

This is interpreted as *(p++) (K&R p.53). What p++ does is it increments p and returns the previous value of p. This previous value of p is then dereferenced by the * operator and the character located at this address is displayed on standard output by the putchar() function, which is part of the stdio library. This idiom occurs quite frequently in C so you should get used to it.

Incrementing Pointers (K&R § 5.3)

Note that the actual number of bytes added to the pointer variable during each increment depends upon the type of the variable to which it points. For example, pointers to characters are incremented by one byte (because sizeof char == 1). Incrementing a pointer to an integer would increase the value of the pointer by four bytes on a machine where integers are four bytes long.

Fortunately, the compiler takes care of incrementing pointers by the appropriate amount, so as a programmer, you generally don't have to worry about how much is being added to the pointer.

Pointer arithmetic

Because an array degrades to a pointer to its first element, the following relationships hold true after we assign an array a to a pointer p of appropriate type:

If we dereference both sides of the above equations we also get the following equivalences:

These equivalences allow us to use some shortcuts when setting elements of an array. For example, if we wish to set the first element of an array, we can say

We can also subtract pointers (K&R § 5.4), as demonstrated by the following code:

#include	<stdio.h>

int
main()
{
	char	   string[] = "This Is A String Of Characters";
	char	   srch;
	const char start = 'a';
	const char end 	 = 'z';

	for (srch = start; srch != end + 1; srch++) {
		char	*p;

		for (p = string; *p != srch && *p; p++)
			;
		if (*p == srch)
			printf("Found '%c' at index position %d.\n",
				srch, p - string);
		else
			printf("No '%c' in string.\n", srch);
	}
	return 0;
}
pointer3.c

This program introduces the concept of a constant (K&R § 2.3). The constants start and end are set to the begin and end points of a range of characters (in this case, the range of lower case characters). By defining these as constants, we are assuring the C compiler that we will not be changing their values.

We then create a loop which iterates over all the characters delimited by these contants (the two characters at either extreme, a and z, are included in this iteration). We then execute a nested for loop that attempts to search for each of these characters inside the candidate array denoted by string. If we find the search character, then we output the position (starting from zero) of the array at which the character was found. Otherwise, we indicate that the character was not found.

In order to calculate the position, we subtract the address of the first element of the string (given by the variable string, itself) from the current value of p (which represents the address at which the character was found). The result of this subtraction is the offset at which the character was found in string.

Also of interest is the fact that the nested for loop has an empty body (the body is denoted by the null statement, which simply consists of a semicolon). In this case, because all we are doing is searching for a particular character in a string, there is no need to define a body for the loop. The loop will stop searching when it either finds the character or when the end of the string has been reached. Looping constructs with empty bodies are not uncommon in C.

The program outputs each lowercase letter of the alphabet as well as the index position that it first occurs in the search string, specified by the character array string:

Incidentally, a standard library function name strchr() performs a similar function to the inner for loop in the above program.

Differences between Arrays and Pointers

Despite this relationship between arrays and pointers, it is important to remember that arrays and pointers are not the same thing. For example, for an array arr and pointer ptr, the statements arr++ and arr = ptr are invalid. However, ptr++ and ptr = arr are valid.

Another notable difference between arrays and pointers occurs when we initialize each with a string literal (K&R § 5.5):

#include	<stdio.h>

int
main()
{
	char	 a[] = "This is an array";
	char	 *p  = "This is a pointer";

	printf ("sizeof a = %d\n",  sizeof a);
	printf ("sizeof p = %d\n",  sizeof p);

	*a = 't';	/* Valid */
	puts(a);
	*p = 't';	/* Invalid (runtime error) */
	puts(p);

	a = "New array";	/* Invalid (compiler error) */
	p = "New pointer";	/* Valid */

	return 0;
}
pointer4.c

Note that when we assign a string literal to a pointer, the address of the first character of the string literal is assigned to the pointer. This string literal, for all intents and purposes, is read only, i.e. you cannot change any of the characters in the string literal via the pointer p. Attempting to do so may cause your program to crash.

Note that this is very different when we initialize an array with a string literal. All the characters in the string literal (and the implicit nul byte at the end) are copied into the array and the program is free to modify this copy as it sees fit (just as long as it does not access character positions outside the array). For example, the following code fragment is okay:

However, we cannot assign a new string literal to an array after the array has been defined. This is not the case for pointers, which can be assigned a new string literal.

An Array of Pointers

We can also define an array of pointers to characters as shown below (K&R § 5.6, 5.8):

#include	<stdio.h>

int
main()
{
	char	*strings[] = {
			"This is",
			"an array",
			"of pointers",
			"to strings",
			NULL
		};
	char	 **str = strings;

	while (*str)
		puts(*str++);
	return 0;
}
pointer5.c

Note that this is quite different to the 2 dimensional array of characters described in an earlier lecture. There are no superfluous nul bytes stored by this structure, making it more efficient, memory-wise. The array merely contains pointers to characters stored somewhere in memory. Indeed, the strings may not even be adjacent to each other in memory. However, none of the strings stored this way can be modified unless we copy them (either to an array or via dynamic memory allocation which we will see later).

Another important difference is that we are no longer dealing with a rectangular block of memory. In the 2-dimensional array, we could legally access and set any character in the range

strings[0
.. MAX_ROW-1][0 .. MAX_COL-1]

(the byte at that location may be a nul byte, but we are permitted to change it if we want to). In the above "array of character pointers" program, however, attempting to access the array element strings[1][9], for example would yield an undefined character. Attempting to write to strings[1][9] will produce undefined behaviour.

The NULL pointer stored at the end of the array of pointers is analogous to null in Java. It represents a pointer to nothing and is commonly used to represent the end of an array of pointers (as we have done above) or the end of linked list. It can also be used as an initial value for a pointer or as a return value from a function to indicate failure (or maybe even success, depending on how it's used) of the function. Do not confuse the nul byte and the NULL pointer. The former is a character, whereas the latter is a pointer.

By the way, don't let the double pointer in the definition of the str variable throw you off (

char **str =
strings

). If you think of the type char* as being an atomic type (like int, for example), then the definition of str becomes a bit easier to understand. For example, if we replace all char*'s in the above program with just char, replace the initializer list with characters instead of strings and change the puts() to a putchar(), then we get the following program which is analagous to the preceding one except that it deals with char's instead of char*'s.

#include	<stdio.h>

int
main()
{
	char	 chars[] = { 'h', 'e', 'l', 'l', 'o', '\0' };
	char	*ch = chars;

	while (*ch)
		putchar(*ch++);
	return 0;
}
pointer6.c

January 19, 2004 (Monday)

Passing Strings as Parameters (K&R § 1.8)