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C Programming Code Examples

C > Pointers Code Examples

Program to Find Length of the String using Pointer

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/* Program to Find Length of the String using Pointer Write a C Program which will accept string from the user . Pass this string to the function. Calculate the length of the string using pointer. gets() is used to accept string with spaces. we are passing accepted string to the function. Inside function we have stored this string in pointer. (i.e base of the string is stored inside pointer variable). Inside while loop we are going to count single letter and incrementing pointer further till we get null character. */ #include<stdio.h> #include<conio.h> int string_ln(char*); void main() { char str[20]; int length; clrscr(); printf("\nEnter any string : "); gets(str); length = string_ln(str); printf("The length of the given string %s is : %d", str, length); getch(); } int string_ln(char*p) /* p=&str[0] */ { int count = 0; while (*p != '\0') { count++; p++; } return count; }
getch() Function in C
The getch() is a predefined non-standard function that is defined in conio.h header file. It is mostly used by the Dev C/C++, MS- DOS's compilers like Turbo C to hold the screen until the user passes a single value to exit from the console screen. It can also be used to read a single byte character or string from the keyboard and then print. It does not hold any parameters. It has no buffer area to store the input character in a program.
Syntax for getch() Function in C
#include <conio.h> int getch(void);
The getch() function does not accept any parameter from the user. It returns the ASCII value of the key pressed by the user as an input. We use a getch() function in a C/ C++ program to hold the output screen for some time until the user passes a key from the keyboard to exit the console screen. Using getch() function, we can hide the input character provided by the users in the ATM PIN, password, etc. • getch() method pauses the Output Console until a key is pressed. • It does not use any buffer to store the input character. • The entered character is immediately returned without waiting for the enter key. • The entered character does not show up on the console. • The getch() method can be used to accept hidden inputs like password, ATM pin numbers, etc.
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/* wait for any character input from keyboard by getch() function example. */ // C code to illustrate working of // getch() to accept hidden inputs #include <conio.h> #include <dos.h> // delay() #include <stdio.h> #include <string.h> void main() { // Taking the password of 8 characters char pwd[9]; int i; // To clear the screen clrscr(); printf("Enter Password: "); for (i = 0; i < 8; i++) { // Get the hidden input // using getch() method pwd[i] = getch(); // Print * to show that // a character is entered printf("*"); } pwd[i] = '\0'; printf("\n"); // Now the hidden input is stored in pwd[] // So any operation can be done on it // Here we are just printing printf("Entered password: "); for (i = 0; pwd[i] != '\0'; i++) printf("%c", pwd[i]); // Now the console will wait // for a key to be pressed getch(); }
printf() Function in C
Writes the C string pointed by format to the standard output (stdout). If format includes format specifiers (subsequences beginning with %), the additional arguments following format are formatted and inserted in the resulting string replacing their respective specifiers. printf format string refers to a control parameter used by a class of functions in the input/output libraries of C programming language. The string is written in a simple template language: characters are usually copied literally into the function's output, but format specifiers, which start with a % character, indicate the location and method to translate a piece of data (such as a number) to characters. "printf" is the name of one of the main C output functions, and stands for "print formatted". printf format strings are complementary to scanf format strings, which provide formatted input (parsing). In both cases these provide simple functionality and fixed format compared to more sophisticated and flexible template engines or parsers, but are sufficient for many purposes.
Syntax for printf() function in C
#include <stdio.h> int printf ( const char * format, ... );
format
C string that contains the text to be written to stdout. It can optionally contain embedded format specifiers that are replaced by the values specified in subsequent additional arguments and formatted as requested. A format specifier follows this prototype: [see compatibility note below] %[flags][width][.precision][length]specifier Where the specifier character at the end is the most significant component, since it defines the type and the interpretation of its corresponding argument:
specifier
a conversion format specifier.
d or i
Signed decimal integer
u
Unsigned decimal integer
o
Unsigned octal
x
Unsigned hexadecimal integer
X
Unsigned hexadecimal integer (uppercase)
f
Decimal floating point, lowercase
F
Decimal floating point, uppercase
e
Scientific notation (mantissa/exponent), lowercase
E
Scientific notation (mantissa/exponent), uppercase
g
Use the shortest representation: %e or %f
G
Use the shortest representation: %E or %F
a
Hexadecimal floating point, lowercase
A
Hexadecimal floating point, uppercase
c
Character
s
String of characters
p
Pointer address
n
Nothing printed. The corresponding argument must be a pointer to a signed int. The number of characters written so far is stored in the pointed location.
%
A % followed by another % character will write a single % to the stream. The format specifier can also contain sub-specifiers: flags, width, .precision and modifiers (in that order), which are optional and follow these specifications:
flags
one or more flags that modifies the conversion behavior (optional)
-
Left-justify within the given field width; Right justification is the default (see width sub-specifier).
+
Forces to preceed the result with a plus or minus sign (+ or -) even for positive numbers. By default, only negative numbers are preceded with a - sign.
(space)
If no sign is going to be written, a blank space is inserted before the value.
#
Used with o, x or X specifiers the value is preceeded with 0, 0x or 0X respectively for values different than zero. Used with a, A, e, E, f, F, g or G it forces the written output to contain a decimal point even if no more digits follow. By default, if no digits follow, no decimal point is written.
0
Left-pads the number with zeroes (0) instead of spaces when padding is specified (see width sub-specifier).
width
an optional * or integer value used to specify minimum width field.
(number)
Minimum number of characters to be printed. If the value to be printed is shorter than this number, the result is padded with blank spaces. The value is not truncated even if the result is larger.
*
The width is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.
.precision
an optional field consisting of a . followed by * or integer or nothing to specify the precision.
.number
For integer specifiers (d, i, o, u, x, X): precision specifies the minimum number of digits to be written. If the value to be written is shorter than this number, the result is padded with leading zeros. The value is not truncated even if the result is longer. A precision of 0 means that no character is written for the value 0. For a, A, e, E, f and F specifiers: this is the number of digits to be printed after the decimal point (by default, this is 6). For g and G specifiers: This is the maximum number of significant digits to be printed. For s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered. If the period is specified without an explicit value for precision, 0 is assumed.
.*
The precision is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.
length
an optional length modifier that specifies the size of the argument.
... (additional arguments)
Depending on the format string, the function may expect a sequence of additional arguments, each containing a value to be used to replace a format specifier in the format string (or a pointer to a storage location, for n). There should be at least as many of these arguments as the number of values specified in the format specifiers. Additional arguments are ignored by the function. If a writing error occurs, the error indicator (ferror) is set and a negative number is returned. If a multibyte character encoding error occurs while writing wide characters, errno is set to EILSEQ and a negative number is returned.
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/* print formatted data to stdout by printf() function example */ #include <stdio.h> int main() { char ch; char str[100]; int a; float b; printf("Enter any character \n"); scanf("%c", &ch); printf("Entered character is %c \n", ch); printf("Enter any string ( upto 100 character ) \n"); scanf("%s", &str); printf("Entered string is %s \n", str); printf("Enter integer and then a float: "); // Taking multiple inputs scanf("%d%f", &a, &b); printf("You entered %d and %f", a, b); return 0; }
main() Function in C
In C, the "main" function is treated the same as every function, it has a return type (and in some cases accepts inputs via parameters). The only difference is that the main function is "called" by the operating system when the user runs the program. Thus the main function is always the first code executed when a program starts. main() function is a user defined, body of the function is defined by the programmer or we can say main() is programmer/user implemented function, whose prototype is predefined in the compiler. Hence we can say that main() in c programming is user defined as well as predefined because it's prototype is predefined. main() is a system (compiler) declared function whose defined by the user, which is invoked automatically by the operating system when program is being executed. Its first function or entry point of the program from where program start executed, program's execution starts from the main. So main is an important function in c , c++ programming language.
Syntax for main() Function in C
void main() { ......... // codes start from here ......... }
void
is a keyword in C language, void means nothing, whenever we use void as a function return type then that function nothing return. here main() function no return any value. In place of void we can also use int return type of main() function, at that time main() return integer type value.
main
is a name of function which is predefined function in C library. • An operating system always calls the main() function when a programmers or users execute their programming code. • It is responsible for starting and ends of the program. • It is a universally accepted keyword in programming language and cannot change its meaning and name. • A main() function is a user-defined function in C that means we can pass parameters to the main() function according to the requirement of a program. • A main() function is used to invoke the programming code at the run time, not at the compile time of a program. • A main() function is followed by opening and closing parenthesis brackets.
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/* basic c program by main() function example */ #include <stdio.h> #include <conio.h> main() { printf (" It is a main() function "); int fun2(); // jump to void fun1() function printf ("\n Finally exit from the main() function. "); } void fun1() { printf (" It is a second function. "); printf (" Exit from the void fun1() function. "); } int fun2() { void fun1(); // jump to the int fun1() function printf (" It is a third function. "); printf (" Exit from the int fun2() function. "); return 0; }
gets() Function in C
Get string from stdin. Reads characters from the standard input (stdin) and stores them as a C string into str until a newline character or the end-of-file is reached. The newline character, if found, is not copied into str. A terminating null character is automatically appended after the characters copied to str. Notice that gets is quite different from fgets: not only gets uses stdin as source, but it does not include the ending newline character in the resulting string and does not allow to specify a maximum size for str (which can lead to buffer overflows). The gets() function enables the user to enter some characters followed by the enter key. All the characters entered by the user get stored in a character array. The null character is added to the array to make it a string. The gets() allows the user to enter the space-separated strings. It returns the string entered by the user.
Syntax for gets() Function in C
#include<stdio.h> char * gets ( char * str );
str
Pointer to a block of memory (array of char) where the string read is copied as a C string. On success, the function returns str. If the end-of-file is encountered while attempting to read a character, the eof indicator is set (feof). If this happens before any characters could be read, the pointer returned is a null pointer (and the contents of str remain unchanged). If a read error occurs, the error indicator (ferror) is set and a null pointer is also returned (but the contents pointed by str may have changed).
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/* read characters from the standard input (stdin) and stores them as a C string */ #include<stdio.h> #include<conio.h> #include<stdlib.h> void main() { clrscr(); FILE *fp; char fname[20]; printf("Enter filename : "); gets(fname); fp=fopen(fname, "r"); if(fp==NULL) { printf("Error in opening the file..!!\n"); printf("Press any key to exit..\n"); getch(); exit(1); } fclose(fp); getch(); }
While Loop Statement in C
While loop is also known as a pre-tested loop. In general, a while loop allows a part of the code to be executed multiple times depending upon a given boolean condition. It can be viewed as a repeating if statement. The while loop is mostly used in the case where the number of iterations is not known in advance. The while loop evaluates the test expression inside the parentheses (). If test expression is true, statements inside the body of while loop are executed. Then, test expression is evaluated again. The process goes on until test expression is evaluated to false. If test expression is false, the loop terminates.
Syntax of While Loop Statement in C
while (testExpression) { // the body of the loop }
• The while loop evaluates the testExpression inside the parentheses (). • If testExpression is true, statements inside the body of while loop are executed. Then, testExpression is evaluated again. • The process goes on until testExpression is evaluated to false. • If testExpression is false, the loop terminates (ends).
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/* while loop statement in C language */ #include<stdio.h> int main() { int n, num, sum = 0, remainder; printf("Enter a number: "); scanf("%d", &n); num = n; // keep looping while n > 0 while( n > 0 ) { remainder = n % 10; // get the last digit of n sum += remainder; // add the remainder to the sum n /= 10; // remove the last digit from n } printf("Sum of digits of %d is %d", num, sum); // signal to operating system everything works fine return 0; }
clrscr() Function in C
Function clrscr() clears the screen and moves the cursor to the upper left-hand corner of the screen. If you are using the GCC compiler, use system function to execute the clear/cls command. clrscr() function is also a non-standard function defined in "conio.h" header. This function is used to clear the console screen. It is often used at the beginning of the program (mostly after variable declaration but not necessarily) so that the console is clear for our output.
Syntax to Clear the Console in C
#include<conio.h> clrscr(); OR system("cls"); OR system("clear");
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/* clear the screen and moves the cursor to the upper left-hand corner of the screen by clrscr() function example. */ #include <stdio.h> // clrscr() function definition void clrscr(void) { system("clear"); } int main() { clrscr(); //clear output screen printf("Hello World!!!"); //print message return 0; }
#include Directive in C
#include is a way of including a standard or user-defined file in the program and is mostly written at the beginning of any C/C++ program. This directive is read by the preprocessor and orders it to insert the content of a user-defined or system header file into the following program. These files are mainly imported from an outside source into the current program. The process of importing such files that might be system-defined or user-defined is known as File Inclusion. This type of preprocessor directive tells the compiler to include a file in the source code program. Here are the two types of file that can be included using #include: • Header File or Standard files: This is a file which contains C/C++ function declarations and macro definitions to be shared between several source files. Functions like the printf(), scanf(), cout, cin and various other input-output or other standard functions are contained within different header files. So to utilise those functions, the users need to import a few header files which define the required functions. • User-defined files: These files resembles the header files, except for the fact that they are written and defined by the user itself. This saves the user from writing a particular function multiple times. Once a user-defined file is written, it can be imported anywhere in the program using the #include preprocessor.
Syntax for #include Directive in C
#include "user-defined_file"
Including using " ": When using the double quotes(" "), the preprocessor access the current directory in which the source "header_file" is located. This type is mainly used to access any header files of the user's program or user-defined files.
#include <header_file>
Including using <>: While importing file using angular brackets(<>), the the preprocessor uses a predetermined directory path to access the file. It is mainly used to access system header files located in the standard system directories.
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/* #include directive tells the preprocessor to insert the contents of another file into the source code at the point where the #include directive is found. */ // C program to illustrate file inclusion // <> used to import system header file #include <stdio.h> // " " used to import user-defined file #include "process.h" // main function int main() { // add function defined in process.h add(10, 20); // mult function defined in process.h multiply(10, 20); // printf defined in stdio.h printf("Process completed"); return 0; }
Pointers in C Language
Pointers in C are easy and fun to learn. Some C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect C programmer. Let's start learning them in simple and easy steps. As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which prints the address of the variables defined:
#include <stdio.h> int main () { int var1; char var2[10]; printf("Address of var1 variable: %x\n", &var1 ); printf("Address of var2 variable: %x\n", &var2 ); return 0; }
A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable address. The general form of a pointer variable declaration is:
Syntax for Pointer variable declaration in C
type *var-name;
Here, type is the pointer's base type; it must be a valid C data type and var-name is the name of the pointer variable. The asterisk * used to declare a pointer is the same asterisk used for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Take a look at some of the valid pointer declaration:
int *ip; /* pointer to an integer */ double *dp; /* pointer to a double */ float *fp; /* pointer to a float */ char *ch /* pointer to a character */
The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to. There are a few important operations, which we will do with the help of pointers very frequently. (a) We define a pointer variable, (b) assign the address of a variable to a pointer and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. NULL Pointers: It is always a good practice to assign a NULL value to a pointer variable in case you do not have an exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer. The NULL pointer is a constant with a value of zero defined in several standard libraries. In most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing. To check for a null pointer, you can use an 'if' statement as follows:
if(ptr) /* succeeds if p is not null */ if(!ptr) /* succeeds if p is null */
Pointer arithmetic: There are four arithmetic operators that can be used in pointers: ++, --, +, - Array of pointers: You can define arrays to hold a number of pointers. Pointer to pointer: C allows you to have pointer on a pointer and so on. Passing pointers to functions in C: Passing an argument by reference or by address enable the passed argument to be changed in the calling function by the called function. Return pointer from functions in C: C allows a function to return a pointer to the local variable, static variable, and dynamically allocated memory as well.
Advantage of Pointer
1) Pointer reduces the code and improves the performance, it is used to retrieving strings, trees, etc. and used with arrays, structures, and functions. 2) We can return multiple values from a function using the pointer. 3) It makes you able to access any memory location in the computer's memory.
Usage of Pointer
There are many applications of pointers in c language. 1) Dynamic memory allocation: In c language, we can dynamically allocate memory using malloc() and calloc() functions where the pointer is used. 2) Arrays, Functions, and Structures: Pointers in c language are widely used in arrays, functions, and structures. It reduces the code and improves the performance.
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/* working of pointers in C Language */ #include <stdio.h> int main() { int* pc, c; c = 22; printf("Address of c: %p\n", &c); printf("Value of c: %d\n\n", c); // 22 pc = &c; printf("Address of pointer pc: %p\n", pc); printf("Content of pointer pc: %d\n\n", *pc); // 22 c = 11; printf("Address of pointer pc: %p\n", pc); printf("Content of pointer pc: %d\n\n", *pc); // 11 *pc = 2; printf("Address of c: %p\n", &c); printf("Value of c: %d\n\n", c); // 2 return 0; }


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