Input Output in C

In C programming, Input provides a set of built-in functions to read the given input and feed it to the program as per requirement. When we say Output, it means to display some data on screen, printer, or in any file.

The getchar() and putchar() Functions

The int getchar(void) function reads the next available character from the screen and returns it as an integer. This function reads only single character at a time. You can use this method in the loop in case you want to read more than one character from the screen.

The int putchar(int c) function puts the passed character on the screen and returns the same character. This function puts only single character at a time. You can use this method in the loop in case you want to display more than one character on the screen. Check the following example −

#include <stdio.h> 
int main( ) {
 int c;
 printf( "Enter a value :");
 c = getchar( ); 
printf( "\nYou entered: "); 
putchar( c ); 
return 0; 
}

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then the program proceeds and reads only a single character and displays it as follows −

$./a.out
Enter a value : this is test
You entered: t

The gets() and puts() Functions

The char *gets(char *s) function reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF (End of File).

The int puts(const char *s) function writes the string ‘s’ and ‘a’ trailing newline to stdout.

#include <stdio.h> 
int main( ) {
 char str[100]; 
printf( "Enter a value :");
 gets( str ); 
printf( "\nYou entered: "); 
puts( str ); 
return 0; 
}

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then the program proceeds and reads the complete line till end, and displays it as follows −

$./a.out
Enter a value : this is test
You entered: this is test

The scanf() and printf() Functions

The int scanf(const char *format, …) function reads the input from the standard input stream stdin and scans that input according to the format provided.

The int printf(const char *format, …) function writes the output to the standard output stream stdout and produces the output according to the format provided.

The format can be a simple constant string, but you can specify %s, %d, %c, %f, etc., to print or read strings, integer, character or float respectively. There are many other formatting options available which can be used based on requirements. Let us now proceed with a simple example to understand the concepts better −

#include <stdio.h>
int main( ) {

   char str[100];
   int i;

   printf( "Enter a value :");
   scanf("%s %d", str, &i);

   printf( "\nYou entered: %s %d ", str, i);

   return 0;
}

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then program proceeds and reads the input and displays it as follows −

$./a.out
Enter a value : seven 7
You entered: seven 7

It should be noted that scanf() expects input in the same format as you provided %s and %d, which means you have to provide valid inputs like “string integer”. If you provide “string string” or “integer integer”, then it will be assumed as wrong input. Secondly, while reading a string, scanf() stops reading as soon as it encounters a space, so “this is test” are three strings for scanf().

Type Casting in C

Converting one datatype into another is known as type casting or, type-conversion. For example, if you want to store a ‘long’ value into a simple integer then you can type cast ‘long’ to ‘int’. You can convert the values from one type to another explicitly using the cast operator as follows −

(type_name) expression

Consider the following example where the cast operator causes the division of one integer variable by another to be performed as a floating-point operation:

#include <stdio.h> 
main() { 
int sum = 17, count = 5;
 double mean;
 mean = (double) sum / count;
 printf("Value of mean : %f\n", mean );
 }

When the above code is compiled and executed, it produces the following result :

Value of mean : 3.400000

It should be noted here that the cast operator has precedence over division, so the value of sum is first converted to type double and finally it gets divided by count yielding a double value.

Type conversions can be implicit which is performed by the compiler automatically, or it can be specified explicitly through the use of the cast operator. It is considered good programming practice to use the cast operator whenever type conversions are necessary.

Memory Management in C

The C programming language provides several functions for memory allocation and management. These functions can be found in the <stdlib.h> header file.

Allocating Memory Dynamically

While programming, if you are aware of the size of an array, then it is easy and you can define it as an array. For example, to store a name of any person, it can go up to a maximum of 100 characters, so you can define something as follows:

char name[100];

But now let us consider a situation where you have no idea about the length of the text you need to store, for example, you want to store a detailed description about a topic. Here we need to define a pointer to character without defining how much memory is required and later, based on requirement, we can allocate memory as shown in the below example:

#include <stdio.h>
 #include <stdlib.h> 
#include <string.h>
 int main() {
 char name[100]; 
char *description;
 strcpy(name, "Zara Ali"); 
/* allocate memory dynamically */ 
description = malloc( 200 * sizeof(char) );
 if( description == NULL ) {
 fprintf(stderr, "Error - unable to allocate required memory\n"); 
} else { 
strcpy( description, "Zara ali a DPS student in class 10th");
 }
 printf("Name = %s\n", name ); 
printf("Description: %s\n", description );
 }

When the above code is compiled and executed, it produces the following result.

Name = Zara Ali
Description: Zara ali a DPS student in class 10th

Same program can be written using calloc(); only thing is you need to replace malloc with calloc as follows:

calloc(200, sizeof(char));

So you have complete control and you can pass any size value while allocating memory, unlike arrays where once the size defined, you cannot change it.

Resizing and Releasing Memory

When your program comes out, operating system automatically release all the memory allocated by your program but as a good practice when you are not in need of memory anymore then you should release that memory by calling the function free().

Alternatively, you can increase or decrease the size of an allocated memory block by calling the function realloc(). Let us check the above program once again and make use of realloc() and free() functions −

#include <stdio.h>
 #include <stdlib.h>
#include <string.h> 
int main() { 
char name[100];
 char *description;
 strcpy(name, "Zara Ali"); /* allocate memory dynamically */
 description = malloc( 30 * sizeof(char) );
 if( description == NULL ) {
 fprintf(stderr, "Error - unable to allocate required memory\n"); 
} else {
 strcpy( description, "Zara ali a DPS student.");
 } /* suppose you want to store bigger description */ 
description = realloc( description, 100 * sizeof(char) ); 
if( description == NULL ) {
 fprintf(stderr, "Error - unable to allocate required memory\n"); 
} else { 
strcat( description, "She is in class 10th"); 
} 
printf("Name = %s\n", name ); 
printf("Description: %s\n", description ); 
/* release memory using free() function */ 
free(description);
 }

When the above code is compiled and executed, it produces the following result.

Name = Zara Ali
Description: Zara ali a DPS student.She is in class 10th

You can try the above example without re-allocating extra memory, and strcat() function will give an error due to lack of available memory in description.