C Language Tutorial Part-2
8. Character Arrays
A string constant , such as
"I am a string"
is an array of characters. It is represented internally in C by the ASCII characters in the string, i.e., ``I'', blank, ``a'', ``m'',... for the above string, and terminated by the special null character ``\0'' so programs can find the end of the string.
String constants are often used in making the output of code intelligible using printf ;
printf("Hello, world\n");
printf("The value of a is: %f\n", a);
String constants can be associated with variables. C provides the char type variable, which can contain one character--1 byte--at a time. A character string is stored in an array of character type, one ASCII character per location. Never forget that, since strings are conventionally terminated by the null character ``\0'', we require one extra storage location in the array!
C does not provide any operator which manipulate entire strings at once. Strings are manipulated either via pointers or via special routines available from the standard string library string.h. Using character pointers is relatively easy since the name of an array is a just a pointer to its first element. Consider the following code:
void main()
{
char text_1[100], text_2[100], text_3[100];
char *ta, *tb;
int i;
/* set message to be an arrray */
/* of characters; initialize it */
/* to the constant string "..." */
/* let the compiler decide on */
/* its size by using [] */
char message[] = "Hello, I am a string; what are you?";
printf("Original message: %s\n", message);
/* copy the message to text_1 */
/* the hard way */
i=0;
while ( (text_1[i] = message[i]) != '\0' )
i++;
printf("Text_1: %s\n", text_1);
/* use explicit pointer arithmetic */
ta=message;
tb=text_2;
while ( ( *tb++ = *ta++ ) != '\0' )
;
printf("Text_2: %s\n", text_2);
}
The standard ``string'' library contains many useful functions to manipulate strings; a description of this library can be found in an appendix of the K & R textbook. Some of the most useful functions are:
char *strcpy(s,ct) -> copy ct into s, including ``\0''; return s
char *strncpy(s,ct,n) -> copy ncharcater of ct into s, return s
char *strncat(s,ct) -> concatenate ct to end of s; return s
char *strncat(s,ct,n) -> concatenate n character of ct to end
of s, terminate with ``\0''; return s
int strcmp(cs,ct) -> compare cs and ct; return 0 if cs=ct,
<0>ct
char *strchr(cs,c) -> return pointer to first occurence of c
in cs or NULL if not encountered
size_t strlen(cs) -> return length of cs
(s and t are char*, cs and ct are const char*, c is an char converted to type int, and n is an int.)
Consider the following code which uses some of these functions:
#include <>
void main()
{
char line[100], *sub_text;
/* initialize string */
strcpy(line,"hello, I am a string;");
printf("Line: %s\n", line);
/* add to end of string */
strcat(line," what are you?");
printf("Line: %s\n", line);
/* find length of string */
/* strlen brings back */
/* length as type size_t */
printf("Length of line: %d\n", (int)strlen(line));
/* find occurence of substrings */
if ( (sub_text = strchr ( line, 'W' ) )!= NULL )
printf("String starting with \"W\" ->%s\n", sub_text);
if ( ( sub_text = strchr ( line, 'w' ) )!= NULL )
printf("String starting with \"w\" ->%s\n", sub_text);
if ( ( sub_text = strchr ( sub_text, 'u' ) )!= NULL )
printf("String starting with \"w\" ->%s\n", sub_text);
}
9. I/O Capabilities
Character level I/O
C provides (through its libraries) a variety of I/O routines. At the character level, getchar() reads one character at a time from stdin, while putchar() writes one character at a time to stdout. For example, consider
#include <>
void main()
{
int i, nc;
nc = 0;
i = getchar();
while (i != EOF) {
nc = nc + 1;
i = getchar();
}
printf("Number of characters in file = %d\n", nc);
}
This program counts the number of characters in the input stream (e.g. in a file piped into it at execution time). The code reads characters (whatever they may be) from stdin (the keyboard), uses stdout (the X-terminal you run from) for output, and writes error messages to stderr (usually also your X-terminal). These streams are always defined at run time. EOF is a special return value, defined in stdio.h, returned by getchar() when it encounters an end-of-file marker when reading. Its value is computer dependent, but the C compiler hides this fact from the user by defining the variable EOF. Thus the program reads characters from stdin and keeps adding to the counter nc, until it encounters the ``end of file''.
An experienced C programmer would probably code this example as:
#include <>
void main()
{
int c, nc = 0;
while ( (c = getchar()) != EOF ) nc++;
printf("Number of characters in file = %d\n", nc);
}
C allows great brevity of expression, usually at the expense of readability!
The () in the statement (c = getchar()) says to execute the call to getchar() and assign the result to c before comparing it to EOF; the brackets are necessary here. Recall that nc++ (and, in fact, also ++nc) is another way of writing nc = nc + 1. (The difference between the prefix and postfix notation is that in ++nc, nc is incremented before it is used, while in nc++, nc is used before it is incremented. In this particular example, either would do.) This notation is more compact (not always an advantage, mind you), and it is often more efficiently coded by the compiler.
The UNIX command wc counts the characters, words and lines in a file. The program above can be considered as your own wc. Let's add a counter for the lines.
#include <>
void main()
{
int c, nc = 0, nl = 0;
while ( (c = getchar()) != EOF )
{
nc++;
if (c == '\n') nl++;
}
printf("Number of characters = %d, number of lines = %d\n",
nc, nl);
}
Can you think of a way to count the number of words in the file?
Higher-Level I/O capabilities
We have already seen that printf handles formatted output to stdout. The counterpart statement for reading from stdin is scanf. The syntax
scanf("format string", variables);
resembles that of printf. The format string may contain blanks or tabs (ignored), ordinary ASCII characters, which must match those in stdin, and conversion specifications as in printf.
Equivalent statements exist to read from or write to character strings. They are:
sprintf(string, "format string", variables);
scanf(string, "format string", variables);
The ``string'' argument is the name of (i.e. a pointer to) the character array into which you want to write the information.
I/O to and from files
Similar statements also exist for handling I/O to and from files. The statements are
#include <>
FILE *fp;
fp = fopen(name, mode);
fscanf(fp, "format string", variable list);
fprintf(fp, "format string", variable list);
fclose(fp );
The logic here is that the code must
define a local ``pointer'' of type FILE (note that the uppercase is necessary here), which is defined in <>
``open'' the file and associate it with the local pointer via fopen
perform the I/O operations using fscanf and fprintf
disconnect the file from the task with fclose
The ``mode'' argument in the fopen specifies the purpose/positioning in opening the file: ``r'' for reading, ``w'' for writing, and ``a'' for appending to the file. Try the following:
#include <>
void main()
{
FILE *fp;
int i;
fp = fopen("foo.dat", "w"); /* open foo.dat for writing */
fprintf(fp, "\nSample Code\n\n"); /* write some info */
for (i = 1; i <= 10 ; i++)
fprintf(fp, "i = %d\n", i);
fclose(fp); /* close the file */
}
Compile and run this code; then use any editor to read the file foo.dat.
10. Functions
Functions are easy to use; they allow complicated programs to be parcelled up into small blocks, each of which is easier to write, read, and maintain. We have already encountered the function main and made use of I/O and mathematical routines from the standard libraries. Now let's look at some other library functions, and how to write and use our own.
Calling a Function
The call to a function in C simply entails referencing its name with the appropriate arguments. The C compiler checks for compatibility between the arguments in the calling sequence and the definition of the function.
Library functions are generally not available to us in source form. Argument type checking is accomplished through the use of header files (like stdio.h) which contain all the necessary information. For example, as we saw earlier, in order to use the standard mathematical library you must include math.h via the statement
#include <>
at the top of the file containing your code. The most commonly used header files are
<> -> defining I/O routines
<> -> defining character manipulation routines
<> -> defining string manipulation routines
<> -> defining mathematical routines
<> -> defining number conversion, storage allocation
and similar tasks
<> -> defining libraries to handle routines with variable
numbers of arguments
<> -> defining time-manipulation routines
In addition, the following header files exist:
<> -> defining diagnostic routines
<> -> defining non-local function calls
<> -> defining signal handlers
<> -> defining constants of the int type
<> -> defining constants of the float type
Appendix B in the K & R book describes these libraries in great detail.
Writing Your Own Functions
A function has the following layout:
return-type function-name ( argument-list-if-necessary )
{
...local-declarations...
...statements...
return return-value;
}
If return-type is omitted, C defaults to int. The return-value must be of the declared type.
A function may simply perform a task without returning any value, in which case it has the following layout:
void function-name ( argument-list-if-necessary )
{
...local-declarations...
...statements...
}
As an example of function calls, consider the following code:
/* include headers of library */
/* defined for all routines */
/* in the file */
#include <>
#include <>
/* prototyping of functions */
/* to allow type checks by */
/* the compiler */
void main()
{
int n;
char string[50];
/* strcpy(a,b) copies string b into a */
/* defined via the stdio.h header */
strcpy(string, "Hello World");
/* call own function */
n = n_char(string);
printf("Length of string = %d\n", n);
}
/* definition of local function n_char */
int n_char(char string[])
{
/* local variable in this function */
int n;
/* strlen(a) returns the length of */
/* string a */
/* defined via the string.h header */
n = strlen(string);
if (n > 50)
printf("String is longer than 50 characters\n");
/* return the value of integer n */
return n;
}
Arguments are always passed by value in C function calls. This means that local ``copies'' of the values of the arguments are passed to the routines. Any change made to the arguments internally in the function are made only to the local copies of the arguments. In order to change (or define) an argument in the argument list, this argument must be passed as an address, thereby forcing C to change the ``real'' argument in the calling routine.
As an example, consider exchanging two numbers between variables. First let's illustrate what happen if the variables are passed by value:
#include <>
void exchange(int a, int b);
void main()
{ /* WRONG CODE */
int a, b;
a = 5;
b = 7;
printf("From main: a = %d, b = %d\n", a, b);
exchange(a, b);
printf("Back in main: ");
printf("a = %d, b = %d\n", a, b);
}
void exchange(int a, int b)
{
int temp;
temp = a;
a = b;
b = temp;
printf(" From function exchange: ");
printf("a = %d, b = %d\n", a, b);
}
Run this code and observe that a and b are NOT exchanged! Only the copies of the arguments are exchanged. The RIGHT way to do this is of course to use pointers:
#include <>
void exchange ( int *a, int *b );
void main()
{ /* RIGHT CODE */
int a, b;
a = 5;
b = 7;
printf("From main: a = %d, b = %d\n", a, b);
exchange(&a, &b);
printf("Back in main: ");
printf("a = %d, b = %d\n", a, b);
}
void exchange ( int *a, int *b )
{
int temp;
temp = *a;
*a = *b;
*b = temp;
printf(" From function exchange: ");
printf("a = %d, b = %d\n", *a, *b);
}
The rule of thumb here is that
You use regular variables if the function does not change the values of those arguments
You MUST use pointers if the function changes the values of those arguments
11. Command-line arguments
It is standard practice in UNIX for information to be passed from the command line directly into a program through the use of one or more command-line arguments, or switches. Switches are typically used to modify the behavior of a program, or to set the values of some internal parameters. You have already encountered several of these--for example, the "ls" command lists the files in your current directory, but when the switch -l is added, "ls -l" produces a so-called ``long'' listing instead. Similarly, "ls -l -a" produces a long listing, including ``hidden'' files, the command "tail -20" prints out the last 20 lines of a file (instead of the default 10), and so on.
Conceptually, switches behave very much like arguments to functions within C, and they are passed to a C program from the operating system in precisely the same way as arguments are passed between functions. Up to now, the main() statements in our programs have had nothing between the parentheses. However, UNIX actually makes available to the program (whether the programmer chooses to use the information or not) two arguments to main: an array of character strings, conventionally called argv, and an integer, usually called argc, which specifies the number of strings in that array. The full statement of the first line of the program is
main(int argc, char** argv)
(The syntax char** argv declares argv to be a pointer to a pointer to a character, that is, a pointer to a character array (a character string)--in other words, an array of character strings. You could also write this as char* argv[]. Don't worry too much about the details of the syntax, however--the use of the array will be made clearer below.)
When you run a program, the array argv contains, in order, all the information on the command line when you entered the command (strings are delineated by whitespace), including the command itself. The integer argc gives the total number of strings, and is therefore equal to equal to the number of arguments plus one. For example, if you typed
a.out -i 2 -g -x 3 4
the program would receive
argc = 7
argv[0] = "a.out"
argv[1] = "-i"
argv[2] = "2"
argv[3] = "-g"
argv[4] = "-x"
argv[5] = "3"
argv[6] = "4"
Note that the arguments, even the numeric ones, are all strings at this point. It is the programmer's job to decode them and decide what to do with them.
The following program simply prints out its own name and arguments:
#include <>
main(int argc, char** argv)
{
int i;
printf("argc = %d\n", argc);
for (i = 0; i <> printf("argv[%d] = \"%s\"\n", i, argv[i]);
}
UNIX programmers have certain conventions about how to interpret the argument list. They are by no means mandatory, but it will make your program easier for others to use and understand if you stick to them. First, switches and key terms are always preceded by a ``-'' character. This makes them easy to recognize as you loop through the argument list. Then, depending on the switch, the next arguments may contain information to be interpreted as integers, floats, or just kept as character strings. With these conventions, the most common way to ``parse'' the argument list is with a for loop and a switch statement, as follows:
#include <>
#include <>
main(int argc, char** argv)
{
/* Set defaults for all parameters: */
int a_value = 0;
float b_value = 0.0;
char* c_value = NULL;
int d1_value = 0, d2_value = 0;
int i;
/* Start at i = 1 to skip the command name. */
for (i = 1; i <>
/* Check for a switch (leading "-"). */
if (argv[i][0] == '-') {
/* Use the next character to decide what to do. */
switch (argv[i][1]) {
case 'a': a_value = atoi(argv[++i]);
break;
case 'b': b_value = atof(argv[++i]);
break;
case 'c': c_value = argv[++i];
break;
case 'd': d1_value = atoi(argv[++i]);
d2_value = atoi(argv[++i]);
break;
}
}
}
printf("a = %d\n", a_value);
printf("b = %f\n", b_value);
if (c_value != NULL) printf("c = \"%s\"\n", c_value);
printf("d1 = %d, d2 = %d\n", d1_value, d2_value);
}
Note that argv[i][j] means the j-th character of the i-th character string. The if statement checks for a leading ``-'' (character 0), then the switch statement allows various courses of action to be taken depending on the next character in the string (character 1 here). Note the use of argv[++i] to increase i before use, allowing us to access the next string in a single compact statement. The functions atoi and atof are defined in stdlib.h. They convert from character strings to ints and doubles, respectively.
A typical command line might be:
a.out -a 3 -b 5.6 -c "I am a string" -d 222 111
(The use of double quotes with -c here makes sure that the shell treats the entire string, including the spaces, as a single object.)
Arbitrarily complex command lines can be handled in this way. Finally, here's a simple program showing how to place parsing statements in a separate function whose purpose is to interpret the command line and set the values of its arguments:
/********************************/
/* */
/* Getting arguments from */
/* */
/* the Command Line */
/* */
/********************************/
/* Steve McMillan */
/* Written: Winter 1995 */
#include <>
#include <>
void get_args(int argc, char** argv, int* a_value, float* b_value)
{
int i;
/* Start at i = 1 to skip the command name. */
for (i = 1; i <>
/* Check for a switch (leading "-"). */
if (argv[i][0] == '-') {
/* Use the next character to decide what to do. */
switch (argv[i][1]) {
case 'a': *a_value = atoi(argv[++i]);
break;
case 'b': *b_value = atof(argv[++i]);
break;
default: fprintf(stderr,
"Unknown switch %s\n", argv[i]);
}
}
}
}
main(int argc, char** argv)
{
/* Set defaults for all parameters: */
int a = 0;
float b = 0.0;
get_args(argc, argv, &a, &b);
printf("a = %d\n", a);
printf("b = %f\n", b);
}
12. Graphical Interfaces: Dialog Boxes
Suppose you don't want to deal with command line interpretation, but you still want your program to be able to change the values of certain variables in an interactive way. You could simply program in a series printf/scanf lines to quiz the user about their preferences:
.
.
.
printf("Please enter the value of n: ");
scanf("%d", &n);
printf("Please enter the value of x: ");
scanf("%f", &x);
.
.
.
and so on, but this won't work well if your program is to be used as part of a pipeline (see the UNIX primer), for example using ther graphics program plot_data, since the questions and answers will get mixed up with the data stream.
A convenient alternative is to use a simple graphical interface which generates a dialog box, offering you the option of varying key parameters in your program. Our graphics package provides a number of easy-to-use tools for constructing and using such boxes. The simplest way to set the integer variable n and the float variable x (i.e. to perform the same effect as the above lines of code) using a dialog box is as follows:
/* Simple program to illustrate use of a dialog box */
main()
{
/* Define default values: */
int n = 0;
float x = 0.0;
/* Define contents of dialog window */
create_int_dialog_entry("n", &n);
create_float_dialog_entry("x", &x);
/* Create window with name "Setup" and top-left corner at (0,0) */
set_up_dialog("Setup", 0, 0);
/* Display the window and read the results */
read_dialog_window();
/* Print out the new values */
printf("n = %d, x = %f\n", n, x);
}
Compile this program using the alias Cgfx (see the page on compilation) to link in all necessary libraries.
The two create lines define the entries in the box and the variables to be associated with them, set_up_dialog names the box and defines its location. Finally, read_dialog_window pops up a window and allows you to change the values of the variables
Modify the numbers shown, click "OK" (or just hit carriage return), and the changes are made. That's all there is to it! The great advantage of this approach is that it operates independently of the flow of data through stdin/stdout. In principle, you could even control the operation of every stage in a pipeline of many chained commands, using a separate dialog box for each.
Keywords : Tutorial,C,C++,C tutorial, C++ Tutorial.
