Input/Output Statements in C

INPUT/OUTPUT STATEMENTS IN C

Before performing input and output in C programs let us first understand the concept from scratch. This section deals with the basic understanding of the streams involved in accepting input and printing output in C programs.

Streams

A stream acts in two ways. It is the source of data as well as the destination of data. Streams are associated with a physical device such as a monitor or with a file stored on the secondary memory. C uses two forms of streams-text and binary, as shown in Figure 2.11.

In a text stream, sequence of characters is divided into lines with each line being terminated with a new- line character (\n). On the other hand, a binary stream contains data values using their memory representation.

We can do input/output from the keyboard/monitor or from any file but in this chapter we will assume that the source of data is the keyboard and destination of the data is the monitor (Figure 2.12). File handling, i.e., handling input and output via C programs, will be discussed later as a separate chapter.

Formatting Input/Output

C language supports two formatting functions printf and scanf. printf is used to convert data stored in the program into a text stream for output to the monitor, and scanf is used to convert the text stream coming from the keyboard to data values and stores them in program variables. In this section, we will discuss these functions.

Background

The most fundamental operation in a C program is to accept input values from a standard input device (keyboard) and output the data produced by the program to a standard output device (monitor). So far we had been assigning values to variables using the assignment operator =. For example,

int a = 3;

But what if we want to assign value to variable that is inputted by the user at run-time. This is done by using the scanf function that reads data from the keyboard. Similarly, for outputting results of the program, printf function is used that sends results to a terminal. Like printf and scanf, there are different functions in C that can carry out the input/output operations. These functions are collectively known as Standard Input/Output Library. A program that uses standard input/output functions must contain the statement

#include <stdio.h>

at the beginning of the program.

printf()

The printf function (stands for print formatting) is used to display information required by the user and also prints the values of the variables. For this, the printf function takes data values, converts them to a text stream using formatting specifications in the control string and passes the resulting text stream to the standard output. The control string may contain zero or more conversion specifications, textual data, and control characters to be displayed (Figure 2.13).

Each data value to be formatted into the text stream is described using a separate conversion specification in the control string. The specification in the control string describes the data value's type, size and specific format information as shown in Figure 2.13.

The syntax of printf function can be given as

printf ("control string", variable list);

The function accepts two parameters-control string and variable list. The control string may also contain text to be printed like instructions to the user, captions, identifiers, or any other text to make the output readable. In some printf statements you may find only a text string that has to be displayed on screen (as seen in the first program in this chapter). The control characters can also be included in the printf statement. These control characters include \n, \t, \r, \a, etc.

After the control string, the function can have as many additional arguments as specified in the control string. The parameter control string in the printf() function is nothing but a C string that contains the text that has to be written on to the standard output device.

Note that there must be enough arguments, otherwise the result will be completely unpredictable. However, if by mistake you specify more number of arguments, the excess arguments will simply be ignored. The prototype of the control string can be given as below.

% [flags] [width] [.precision] [length modifier] type specifier

Each control string must begin with a % sign. The % character specifies how the next variable in the list of variables has to be printed. After % sign follows:

Flags is an optional argument which specifies output justification such as numerical sign, trailing zeros or octal, decimal, or hexadecimal prefixes. Table 2.5 shows the different types of flags with their decription.

Note that when data is shorter than the specified width then by default the data is right justified. To left justify the data use minus sign (-) in the flags field.

When the data value to be printed is smaller than the width specified, then padding is used to fill the unused spaces.

By default, the data is padded with blank spaces. If zero is used in the flag field then the data is padded with zeros. One thing to remember here is that zero flag is ignored when used with left justification because adding zeros after a number changes its value.

Width is an optional argument which specifies the minimum number of positions in the output. If data needs more space than specified, then printf overrides the width specified by the user. However, if the number of output characters is smaller than the specified width, then the output would be right justified with blank spaces to the left. Width is a very important field especially when you have to align output in columns. However, if the user does not mention any width then the output will take just enough room for data.

Precision is an optional argument which specifies the maximum number of characters to print.

• For integer specifiers (d, i, o, u, x, X): precision flag specifies the minimum number of digits to be written. However, if the value to be written is shorter than this number, the result is padded with leading zeros. Otherwise, if the value is longer, it is not truncated.

• For character strings, precision specifies the maximum number of characters to be printed.

• For floating point numbers, the precision flag specifies the number of decimal places to be printed.

Its format can be given as .m, where m specifies the number of decimal digits. When no precision modifier is specified, printf prints six decimal positions.

When both width and precision fields are used, width must be large enough to contain the integral value of the number, the decimal point and the number of digits after the decimal point. Therefore, a conversion specification %7.3f means print a floating point value of maximum 7 digits where 3 digits are allotted for the digits after the decimal point.

Length modifiers can be explained as given in Table 2.6

Type specifiers are used to define the type and the interpretation of the value of the corresponding argument (Table 2.7).

Note that if the user specifies a wrong specifier then some strange things will be seen on the screen and the error might propagate to other values in the printf() list. The most simple printf statement is

printf ("Welcome to the world of C language");

When executed, the function prompts the message enclosed in the quotation to be displayed on the screen.

Note

The minimum field width and precision specifiers are usually constants. However, they may also be provided by arguments to printf(). This is done by using the * modifier as shown in the printf statement below.

printf("%*.*f", 10, 4, 1234.34);

Here, the minimum field width is 10, the precision is 4, and the value to be displayed is 1234.34.

Examples

Programming Tip: Not placing a comma after the format string in a read or write statement is a compiler error.

printf("\n Result: %d%c%f", 12, 'a', 2.3);

Result: 12a2.3

printf("\n Result: %d %c %f", 12, 'a', 2.3);

Result: 12 a 2.3

printf("\n Result: %d\t%c\t%f", 12, 'a', 2.3);

Result:12 a 2.3

printf("\n Result: %d\t%c\t%6.2f", 12, 'a', 245.37154);

Result: 12 a 245.37

printf("\n Result: %5d \t %x \t %#x", 234, 234, 234);

Result: 234 EA OXEA

printf("\n The number is %6d", 12);

The number is 12

printf("\n The number is %2d", 1234);

The number is 1234

printf("\n The number is %6d", 1234);

The number is 1234

printf("\n The number is %-6d", 1234);

The number is 1234_ // 2 indicates 2_ white spaces

printf("\n The number is %06d", 1234);

The number is 001234

printf("\n The price of this item is %09.2f rupees", 123.456);

The price of this item is 000123.45 rupees

printf("\n This is \'so\' beautiful");

This is 'so' beautiful

printf("\n This is \"so\" beautiful");

This is "so" beautiful

printf("\n This is \\ so beautiful ");        

This is \so beautiful

printf("\n a = |%-+7.2f| b = %0+7.2f c = %-0+8.2f", 1.2, 1.2, 1.2);

a = +1.20 b = 0001.20 c = 1.20

(Note that in this example, - means left justify, + means display the sign, 7 specifies the width, and 2 specifies the precision.)

printf("\n %7.4f \n %7.2f \n %-7.2f \n %f\n %10.2e \n %11.4e \n %-10.2e \n %e", 98.7654, 98.7654, 98.7654, 98.7654, 98.7654, 98.7654, 98.7654, 98.7654);

98.7654

  98.77

98.77

98.7654

  9.88e+01

9.8765e+01

9.88e+01

9.876540e+01

char ch = 'A';

printf("\n %c \n %3c \n %5c", ch, ch, ch);

A

 A

  A                  

char str[] = "Good Morning";

printf("\n %s", str);

printf("\n %20s", str);

printf("\n %20.10s", str);

printf("\n %.7s", str);

printf("\n %-20.10s", str);

printf("\n %7s", str);

Programming Tip: Placing an address operator with a vari- able in the printf statement will gener- ate a run-time error.

Good Morning

 Good Morning

  Good Morni

Good Mo

Good Morni

Good Morning

(Note that in the last printf statement the complete string "Good Morning" is printed. This is because if data needs more space than specified, then printf function overrides the width specified by the user.)

scanf()

The scanf() function stands for scan formatting and is used to read formatted data from the keyboard. The scanf function takes a text stream from the keyboard, extracts and formats data from the stream according to a format control string and then stores the data in specified program variables. The syntax of the scanf() function can be given as:

scanf ("control string", arg1, arg2, arg3,…….argn);

The control string specifies the type and format of the data that has to be obtained from the keyboard and stored in the memory locations pointed by arguments arg1, arg2,..., argn, i.e., the arguments are actually the variable addresses where each piece of data are to be stored.

The prototype of the control string can be give as:

% [*] [width] [modifier] type

Here* is an optional argument that suppresses assignment of the input field, i.e., it indicates that data should be read from the stream but ignored (not stored in the memory location).

Width is an optional argument that specifies the maximum number of characters to be read. However, fewer characters will be read if the scanf function encounters a white space or an inconvertible character because the moment scanf function encounters a white space character it will stop processing further.

Modifier is an optional argument that can be h, 1, or L for the data pointed by the corresponding additional arguments. Modifier h is used for short int or unsigned short int, 1 is used for long int, unsigned long int, or double values. Finally, L is used for long double data values.

Type specifies the type of data that has to be read. It also indicates how this data is expected to be read from the user. The type specifiers for scanf function are same as given for printf() function in Table 2.7.

The scanf function ignores any blank spaces, tabs, and newlines entered by the user. The function simply returns the number of input fields successfully scanned and stored.

We will not discuss functions in detial in this chapter. So understanding scanf function in depth will be a bit difficult here, but for now just understand that the scanf function is used to store values in memory locations associated with variables. For this, the function should have the address of the variables. The address of the variable is denoted by an '&' sign followed by the name of the variable.

Note

Whenever data is read from the keyboard, there is always a return character from a previous read operation. So we should always code at least one white space character in the conversion specification in order to flush that whitespace character. For example, to read two or more data values together in a single scanf statement, we must insert a white space between two fields as shown below: scanf("%d %c", &i, &ch);

Now let us quickly summarize the rules to use a scanf function in our C programs.

Rule 1: The scanf function works until:

(a) the maximum number of characters has been processed,

(b) a white space character is encountered, or

(c) an error is detected.

Rule 2: Every variable that has to be processed must have a conversion specification associated with it. Therefore, the following scanf statement will generate an error as num3 has no conversion specification associated with it.

scanf("%d %d", &num1, &num2, &num3);

Rule 3: There must be a variable address for each conversion specification. Therefore, the following scanf statement will generate an error as no variable address is given for the third conversion specification.

scanf("%d %d %d", &num1, &num2);

Remember that the ampersand operator (&) before each variable name specifies the address of that variable name.

Rule 4: An error would be generated if the format string is ended with a white space character.

Rule 5: The data entered by the user must match the character specified in the control string (except white space or a conversion specification), otherwise an error will be generated and scanf will stop its processing. For example, consider the scanf statement given below.

scanf("%d/%d", &num1, &num2);

Here, the slash in the control string is neither a white space character nor a part of conversion specification, so the users must enter data of the form 21/46.

Rule 6: Input data values must be separated by spaces.

Rule 7: Any unread data value will be considered as a part of the data input in the next call to scanf.

Rule 8: When the field width specifier is used, it should be large enough to contain the input data size.

Look at the code given below that shows how we input values in variables of different data types.

int num;

scanf("%d", &num);

The scanf function reads an integer value (because the type specifier is %d) into the address or the memory location pointed by num.

float salary;

scanf("%f", salam &salary);

The scanf function reads a floating point number (because the type specifier is %f) into the address or the memory location pointed by salary.

char ch;

scanf("%c", &ch);

The scanf function reads a single character (because the type specifier is %c) into the address or the memory location pointed by ch.

char str[10]; scanf("%s", str);

The scanf function reads a string or a sequence of characters (because the type specifier is %s) into the address or the memory location pointed by str. Note that in case of reading strings, we do not use the & sign in the scanf function. This will be discussed in the chapter on Strings.

Programming Tip: A compiler error will be generated if the read and write parameters are not separated by commas.

Look at the code given below which combines reading of variables of different data types in one single statement

int num;

float fnum;

char ch;

char str[10];

scanf("%d %f %c %s", &num, &fnum, &ch, str);

Look at the scanf statement given below for the same code. The statement ignores the character variable and does not store it (as it is preceded by *).

scanf("%d %f %*c %s", &num, &fnum, &ch, str);

Remember that if an attempt is made to read a value that does not match the expected data type, the scanf function will not read any further and would immediately return the values read.

Examples of printf/scanf

Look at the codes given below that show how we output values of variables of different data types.

int num;

scanf("%d", &num);

printf("%d", num);

The printf function prints an integer value (because the type specifier is %d) pointed by num on the screen.

float salary;

scanf("%f", &salary);

printf(".2%f", salary);

The printf function prints the floating point number (because the type specifier is %f) pointed by salary on the screen. Here, the control string specifies that only two digits must be displayed after the decimal point.

Programming Tip: A float specifier cannot be used to read an integer value.

char ch;

scanf("%c", &ch);

printf("%c", ch);

The printf function prints a single character (because the type specifier is %c) pointed by ch on the screen.

char str[10];

scanf("%s", str);

The printf function prints a string or a sequence of characters (because the type specifier is %s) pointed by str on the screen.

scanf("%2d %5d", &num1, &num2);

The scanf statement will read two integer numbers. The first integer number will have two digits while the second can have maximum of 5 digits.

Look at the code given below which combines printing all these variables of different data types in one single statement.

int num;

float fnum;

char ch;

char str[10];

double dnum;

short snum;

long int lnum;

printf("\n Enter the values : ");

scanf("%d %f %c %s %e %hd %ld", &num, &fnum, &ch, str, &dnum, &snum, &lnum);

printf("\n num = %d\n fnum = %.2f \n ch = %c \n str %s\n dnum %e \n snum %hd \n lnum = %ld", num, fnum, ch, str, dnum, snum, lnum);

Note

In the printf statement, `\n', is called the newline character and is used to print the succeeding text on the new line. The following output will be generated on execution of the print function.

Enter the values

2 3456.443 a abcde 24.321E-2 1 12345678

num = 2

fnum = 3456.44

ch = a

str = abcde

dnum = 0.24321

snum = 1

lnum = 12345678

Remember one thing that scanf terminates as soon as it encounters a white space character so if you enter the string as abc def, then only abc is assigned to str.

1. Find out the output of the following program.

#include <stdio.h>

int main()

{

int a, b;

printf("\n Enter two four digit numbers: ");

scanf("%2d %4d", &a, &b);

printf("\n The two numbers are: %d and %d", a, b);

return 0;

}

Output

Enter two four digit numbers : 1234 5678

The two numbers are : 12 and 34

Programming Tip: Using an incorrect specifier for the data type being read or written will generate a run-time error.

Here, the variable a is assigned the value 12 because it is specified as %2d, so it will accept only the first two digits. The rest of the number will be assigned to b. The value 5678 that is unread will be assigned to the first variable in the next call to the scanf function.

Note

The %n specifier is used to assign the number of characters read till the point at which the %n was encountered to the variable pointed to by the corresponding argument. The code fragment given below illustrates its use.

int count;

printf("Hello %n World!", &count);

printf("%d", count);

The output would be-Hello World! 6 because 6 is the number of characters read before the %n modifier.

2. Write a program to demonstrate the use of printf statement to print values of variables of different data types.

#include <stdio.h>

int main()

{

// Declare and initialize variables

int num = 7;

float amt = 123.45;

char code = 'A';

double pi = 3.1415926536;

long int population_of_india = 10000000000;

char msg [ ]  "Hi=";

// Print the values of variables

printf("\n NUM %d\n AMT = %f \n CODE = %c \n PI = %e \n POPULATION OF INDIA %ld \n MESSAGE = %s", num, amt, code, pi, population_of_india, msg);

return 0;

}

Output

NUM = 7

AMT = 123.450000

CODE = A

PI = 3.141590e+00

POPULATION OF INDIA = 10000000000

MESSAGE = Hi

3. Write a program to demonstrate the use of printf and scanf statements to read and print values of variables of different data types.

#include <stdio.h>

int main()

{

int num;

float amt;

char code;

double pi;

long int population_of_india;

char msg [10];

printf("\n Enter the value of num : ");

scanf("%d", &num);

printf("\n Enter the value of amt : ");

scanf("%f", &amt);

printf("\n Enter the value of pi : ");

scanf("%e", &pi);

printf("\n Enter the population of India : ");

scanf("%ld", &population_of_india);

printf("\n Enter the value of code : ");

scanf("%c", &code);

printf("\n Enter the message: ");

scanf("%s", msg);

printf("\n NUM = %d\n AMT = %f \n PI = %e \n POPULATION OF INDIA = %ld\n CODE = %c \n MESSAGE = %s", num, amt, pi, population_of_india, code, msg);

return 0;

}

Output

Enter the value of num : 5

Enter the value of amt : 123.45

Enter the value of pi : 3.14159

Enter the population of India : 12345

Enter the value of code : с

Enter the message : Hello

NUM = 5

AMT = 123.450000

PI = 3.141590e+00

POPULATION OF INDIA = 12345

CODE = c

MESSAGE = Hello

4. Write a program to calculate the area of a triangle using Hero's formula.

#include <stdio.h>

#include <conio.h>

#include <math.h>

int main()

float a, b, c, area, S;

printf("\n Enter the lengths of the three sides of the triangle: ");

scanf("%f %f %f", &a, &b, &c);

S = (a + b + c)/2;

// sqrt is a mathematical function defined in math.h header file

area = sqrt (S* (S-a) * (S-b) * (S-c)); printf("\n Area %f", area);

return 0;

}

Output

Enter the lengths of the three sides of the triangle: 12 16 20

Area = 96

5. Write a program to calculate the distance between two points.

#include <stdio.h>

#include <conio.h>

#include <math.h>

int main()

{

int x1, x2, y1, y2;

float distance;

printf("\n Enter the x and y coordinates of the first point: ");

scanf("%d %d", &x1, &y1);

printf("\n Enter the x and y coordinates of the second point :");

scanf("%d %d", &x2, &y2);

// sqrt and pow are mathematical functions defined in math.h header file

distance = sqrt (pow((x2-x1), 2)+pow ((y2-yl), 2));

printf("\n Distance = %f", distance);

return 0;

}

Output

Enter the x and y coordinates of the first point: 2 5

Enter the x and y coordinates of the second point: 3 7

Distance = 2.236068

Detecting Errors During Data Input

When the scanf function completes reading all the data values, it returns number of values that are successfully read. This return value can be used to determine whether there was any error while reading the input. For example, the statement,

scanf("%d %f %c", &a, &b, &c);

will return 3 if the user enters, say,

12 12.34 A

It will return 1 if the user enters erroneous data like

12 ABC 12.34

This is because a string was entered while the user was expecting a floating point value. So, the scanf function reads only first data value correctly and then terminates as soon as it encounters a mismatch between the type of data expected and the type of data entered.