Chapter 3 Basic Input-Output Statements

In order to learn how to program effectively in the C or C++ language, a student must learn how these languages manage the input and output of data to and from the screen and keyboard.

3.1 Basic Screen and Keyboard I/O in C

The C language provides several functions that give different levels of input and output capability. These functions are, in most cases, implemented as routines that call lower level input/output functions.

The input and output functions in C are built around the concept of a set of standard data streams being connected from each executing program to the basic input/output devices. These standard data streams or files are opened by the operating system and are available to every C and assembler program to use without having to open or close the files. These standard files or streams are called:

 
 - stdin : connected to the keyboard
 - stdout : connected to the screen
 - stderr : connected to the screen

The following two data streams are also available on MS-DOS based computers, but not on UNIX or other multi-user based operating systems.

 
   - stdaux : connected to the first serial communications port
   - stdprn : connected to the first parallel printer port

The input/output functions fall into two categories, formatted display and read functions, and non-formatted display and read functions. The following are descriptions of the formatted display and read functions.

int printf( const char *format [,argument, ...] );

where format is composed of literal text, escape sequences used as carriage control, and format specifiers for conversion of data in the arguments to a display format. This function returns the number of characters printed.

printf() returns the number of bytes output. In the event of error, printf returns EOF. Example:

 
main()
{
 char name[30];

 printf("\nEnter your name:");
 gets(name);
 printf("\nHello %s",name);
}

The printf() function has the capability to manage conversion control.

General form

   %[-][width][flags]format

where:

%      : marks the start of the conversion control string 
-      : specifies that the data is to be printed left-justified 
width  : the width of field or number of spaces to allot on the display
flags  : precision of output to be displayed
format : the format specifier desired

Listing 3-1

#include "stdio.h"
int main()
{
 printf("/%d/\n",336);
 printf("/%2d/\n",336);
 printf("/%10d/\n",336);
 printf("/%-10d/\n",336);
 return 0;
}

Fig 3-1:

 
 /336/
 /336/
 / 336/
 /336 /

Listing 3-2

#include "stdio.h"
int main()
{
 printf("/%f/\n",1234.56);
 printf("/%e/\n",1234.56);
 printf("/%4.f/\n",1234.56);
 printf("/%3.1f/\n",1234.56);
 printf("/%10.3f/\n",1234.56);
 printf("%10.3e/\n",1234.56);
 return 0;
}

Fig 3-2:

 
 /1234.560059/
 /1.23456E+03/
 /1234.56/
 /1234.6/
 / 1234.560/
 / 1.234E+03/

Listing 3-3

#include "stdio.h"
#define BLURB "Outstanding Program!"

int main()
{
 printf("/%2s/\n",BLURB);
 printf("/%22s/\n",BLURB);
 printf("/%22.5s/\n",BLURB);
 printf("/%-22.5s/\n",BLURB);
 return 0;
}

results:

 /Outstanding Program!/
 / Outstanding Program!/
 /Outst/
 /Outst/

3.2 Formatted Input and Output

where format is a list of format specifiers indicating the format and type of data to be read from the keyboard and stored in the corresponding address. There must be the same number of format specifiers and addresses as there are input fields.

scanf returns the number of input fields successfully scanned, converted, and stored. The return value does not include scanned fields that were not stored. If scanf attempts to read end-of- file, the return value is EOF. If no fields were stored, the return value is 0.

Listing 3-4

#include "stdio.h"

int main()
{
char last_name[30];
int age, ret;

 printf("\nEnter Last_name and age");
 scanf("%s %d%c", last_name, &age, &ret);
 return 0;
}

The scanf() function scans the data input through the keyboard and by default delimits values by whitespace. Whitespace is defined as being a TAB, a blank or the newline character ('\n'). Therefore, data that is input with the intention of having embedded blanks as part of the data value will be broken into several values and distributed among the input variables specified in the scanf() statement. The result will more than likely not be what was desired.

In the above example, notice that although the prompt asks for the input of a name value and an age value, the scanf() function is told to read values for three arguments. The argument ret is an integer variable but scanf() is reading a %c or character value from the keyboard. The ret variable will hold the newline character input by the user pressing the RETURN or ENTER key on the keyboard. If the newline character is not extracted from the keyboard buffer, the newline will be picked up as the first argument of the next input statement, which could be a scanf(), a gets() or a getchar() function.

The format string is a character string that contains three types of specifiers: whitespace characers, non-whitespace characters and format-specifiers.

The format-specifiers have the following form:

   %[*] [width] [h|l|L] type-character

Each format begins with the percent character, %, after which come the following, in this order: - An optional assignment-suppression character, [*]. This states that the value being read will not be assigned to an argument, but will be dropped. - An optional width specifier, [width]. This designates the maximum number of characters to be read that compose the value for the associated argument. Encountering whitespace before the width is satisfied terminates the input of this value and moves to the next. - An optional argument-type modifier, [h|l|L]. This modifies the type-character specifier to accept format for a type of : h = short int l = long int, if the type-character specifiers an integer conversion l = double, if the type-character specifiers a floating-point conversion L = long double, which is valid only with floating- point conversions

NOTE: Simple data objects must be passed by reference in order for scanf() to be able to store data in the correct memory location. To pass by reference means to pass the memory address of a variable. The & operator in front of a variable name signifies that the address of the following variable is to obtained.

The following are format specifiers which apply only to printf() and scanf().

Table 3-1

NOTE:Numerics with scanf() an h or l or L can be used with each of the following in order to modify the format; h = short; l and L = long.

3.3 Non-formatted Input and Output

puts() displays a string literal or a stored character string on the screen. The function automatically carriage return and line feeds at the end of the display. The string can contain escape sequences but not format specifiers.

On successful completion, puts() returns a nonnegative value. Otherwise, it returns a value of EOF.

Listing 3-5

#include "stdio.h"
int main()
{
char name[30];

 printf("\nEnter your name:");
 gets(name);
 printf("\nHello ");
 puts(name);
 return 0;
}

gets() reads characters from the keyboard and stores them in a passed character array. The reading of keyboard is terminated when the '\n' (RETURN/ENTER) key is pressed.

On success, gets() returns the string argument s; it returns NULL on end-of-file or error.

Listing 3-6

#include "stdio.h"
int main()
{
char name[30];

 printf("\nEnter your name:");
 gets(name);
 printf("\nHello %s",name);
 return 0;
}

putchar() writes a character to the stdout data stream.

On success, putchar() returns the character c. On error, putchar() returns EOF.

int putch( int c );

putch() writes the character directly to the screen. This function is available only on PC based compilers.

On success, putch returns the characer printed, c. On error, it returns EOF.

Listing 3-7

#include "stdio.h"
int main()
{
int c;

 c = 'A';
 putchar(c);
 putch(c);
 return 0;
}

result:

 AA

getchar() reads a single character the from the input data stream; but does not return the character to the program until the '\n' (RETURN/ENTER) key is pressed.

getch() reads, without echoing, a single character from the keyboard and immediately returns that character to the program; available only on PC compilers.

getche() reads, with echo, a single character from the keyboard and immediately returns that character to the program; available only on PC compilers.

Listing 3-8

#include "stdio.h"
int main()
{
int ch;

 printf("\nContinue(Y/N)?");
 ch = getchar();
 return 0;
}

The result:

 Continue(Y/N)?Y < RETURN >

NOTE: The '\n' (RETURN/ENTER) key must be pressed after the response in order for the character to be stored in 'ch'. Also, the character pressed is automatically displayed on the screen.

Listing 3-9

#include "stdio.h"
int main()
{
int ch;

 printf("\nContinue(Y/N)?");
 ch = getch();
 return 0;
}

The result:

 Continue(Y/N)? (Y pressed)

NOTE: Upon pressing the 'Y' or 'N' key the character is immediately stored in 'ch', but the character pressed is not automatically shown on the screen. This is available only with PC based compilers.

Listing 3-10

#include "stdio.h"
int main()
{
int ch;

 printf("\nContinue(Y/N)?");
 ch = getche();
 return 0;
}

The result:

 Continue(Y/N)?N

NOTE: Upon pressing the 'Y' or 'N' key the character is immediately stored in 'ch' and also is echoed on the screen. This is available only with PC based compilers.

3.4 Predefined Classes in C++

Like C, C++ has no built-in facilities for I/O. Instead, you must rely on a library of functions, for performing I/O. In ANSI C, the I/O functions are a part of the standard library, but C++ does not have any standard library yet. Of course, you can call the ANSI C library routines in C++, but for I/O, C++ release 2.0 and above provides an alternative to printf and scanf. C++ comes with the iostream library, which handles I/O through a class of objects.

The C++ iostream library is an object-oriented implementation of the abstraction of a stream as a flow of bytes from source (producer) to a sink (consumer). The iostream library includes input streams (istream class), output streams (ostream class), and streams (iostream class) that can handle both input and output operations. The istream class provides the functionality of scanf and fscanf, and ostream includes capabilities similar to those of printf and fprintf. Like the predefined C streams stdin, stdout, and stderr, the iostream library includes four predefined streams:

  cin is an input stream connected to the standard input. It is analogous to C's stdin.

  cout is an output stream connected to the standard output and is analogous to stdout in C.
 
  cerr is an output stream set up to provide unbuffered output to the standard error device. This is the same as C's stderr.

  clog is like cerr, but it is a fully buffered stream like cin and cout.

To use the iostream library, your C++ program must include the header file "iostream.h". This file contains the definitions of the classes that implement the stream objects and provides the buffering. The file "iostream.h" is analogous to "stdio.h" in ANSI C.

Instead of defining member functions that perform I/O, the iostream library provides an operator notation for input as well as output. It uses C++'s ability to overload operators and defines << and >> as the output and input operators, respectively.

When you see the << and >> operators in use, you will realize their appropriateness. For example, consider the following program that prints some variables to the cout stream, which is usually connected to standard output:

Listing 3-11

#include "iostream.h"

int main()
{
int count = 2;
double result = 5.4;
char *id = "Trying out iostream: ";

 cout << id;
 cout << "count = " << count << '\n' ;
 cout << "result = " << result << endl;
 return 0;
}

When you run this program, it prints the following:

 Trying out iostream: count = 2
 result = 5.4

You can make three observations from this example:

1. The << operator is a good choice to represent the output operation, because it points in the direction of data movement that, in this case, is toward the cout stream.
2. You can concatenate multiple << operators in a single line, all reading the same stream.
3. You use the same syntax to print all the basic data types on a stream. The << operator automatically converts the internal representation of the variable into a textual representation. Contrast this with the need to use different format strings for printing different data types using printf.

Accepting input from the standard input is also equally easy. Here is a small example that combines both input and output:

Listing 3-12

#include "iostream.h"

int main()
{
int count;
float price;
char *prompt = "Enter count (int) and unit price (float): ";

 // display the prompt string
 cout << prompt ;
 // read from standard input
 cin >> count >> price;
 // display total cost
 cout << count << " at " << price << " will cost: ";
 cout << (price * count) << endl;
 return 0;
}

When you run the program and enter the input shown in boldface, the program interacts as follows:

 
Enter count (int) and unit price (float): 5 2.5
 5 at 2.5 will cost: 12.5

Ignoring, for the moment, items that you do not recognize, notice how easy it is to read values into variables from the cin stream. You simply send the data from cin to the variables using the >> operator. Like the << operator, you can also concatenate multiple >> operators. The >> operator automatically converts the strings into the internal representations of the variables according to their types. The simple syntax of input from cin is in sharp contrast with ANSI C's rather complicated scanf function, which serves the same purpose but needs proper format strings and addresses of variable as arguments. Also, cin has the same limitations on input of string type data as scanf.

3.5 Manipulators

Among the new items in the last example, you may have noticed the identifier endl in the last one line. This is a special function known as a manipulator. Manipulators are functions which are written in such a way that by placing a manipulator in the chain of << operators, you can alter the state of the stream. The endl manipulator sends a newline to the stream, forcing the cursor to the beginning of a newline.

The following table summarizes some of the manipulators available in the iostream package. The manipulators that take arguments are declared in the file iomanip.h the rest are in iostream.h

Table 3-2

3.6 Formatted I/O

You can use the manipulators for some simple formatted I/O. Formatting refers to the process of converting to and from the internal binary representation of a variable and its character string representation. For example, if a 16-bit integer variable holds the bit pattern 0000 0000 0110 0100, its character string representation in the decimal number system is 100 and 64 in hexadecimal. If the base of conversion is octal, the representation will be 144. You can display all three forms on separate lines using the following output statements:

Listing 3-13

#include "iostream.h"

int main()
{
int i = 100;

 cout << dec << i << endl;
 cout << hex << i << endl;
 cout << oct << i << endl;
 return 0;
}

This produces the following output:

 100
 64
 144

What if you want to use a fixed field width of six characters to display each value? You can do this by using the setw manipulators as follows:

Listing 3-14

#include "iostream.h"
#include "iomanip.h"

int main()
{
int i = 100;

 cout << setw(6) << dec << i << endl;
 cout << setw(6) << hex << i << endl;
 cout << setw(6) << oct << i << endl;
 return 0;
}

This produces the following output:

 100
 64
 144

Here each variable is displayed in a six-character field aligned at the right and padded with blanks at the left. You can change both the padding and the alignment. To change the padding character, you can use the setfill manipulator. For example, just before the cout statements just shown, insert the following line:

Listing 3-15

#include "iostream.h"
#include "iomanip.h"

int main()
{
int i = 100;

 cout << setfill('.');
 cout << setw(6) << dec << i << endl;
 cout << setw(6) << hex << i << endl;
 cout << setw(6) << oct << i << endl;
 return 0;
}

This produces the following output:

 ...100
 ....64
 ...144

The default alignment of fixed-width output fields is to pad on the left, resulting in right-justified output. The justification information is stored in a bit pattern called the format bits in a class named ios, which forms the basis of all stream classes. You can set or reset specific bits by using the setiosflags and resetiosflags manipulators, respectively. Following is a sample use of these manipulators:

Listing 3-16

#include "iostream.h"
#include "iomanip.h"

int main()
{
int i = 100;

 cout << setfill('.');
 // left-justified labels followed by
 // right-justified values
 cout << setiosflags(ios::left); // left justification
 cout << setw(20) << "Decimal";
 cout << resetiosflags(ios::left); // turn off left just
 cout << setw(6) << dec << i << endl;

 cout << setiosflags(ios::left);
 cout << setw(20) << "Hexadecimal";
 cout << resetiosflags(ios::left);
 cout << setw(6) << hex << i << endl;

 cout << setiosflags(ios::left);
 cout << setw(20) << "Octal";
 cout << resetiosflags(ios::left);
 cout << setw(6) << oct << i << endl;
 return 0;
}

This produces the following output:

 Decimal................100
 Hexadecimal.............64
 Octal..................144

This output amply illustrates how the setiosflags and resetiosflags manipulators work and how the they should be used. All you need to know are the names of the enumerated list of formatting flags so that you can use them as arguments to the setiosflags and resetiosflags manipulators.

To use any of the format flags in the following table, insert the manipulator setiosflags with the name of the flag as the argument. Use resetiosflags with the same argument to revert to the format state before you use the setiosflags manipulator.

Table 3-3:

In addition to the above method of manipulating output and input, another method can be used which involves accessing methods (functions) of the cout and cin objects of the iostream class. This manipulation or formatting can be done with the following set of functions:

Table 3-4:

Listing 3-17

#include "iostream.h"

int main()
{
 cout.setf(ios::right|ios::showpoint|ios::fixed);
 cout.precision(2);
 cout.width(20);
 cout << 500000.0 << endl;
 return 0;
}

The correspondence between iostream.h methods (functions) and iomanip.h manipulators is as follows:

Table 3-5: