Input and Output in C

As you may already know stdio.h header file is required for input and output operations in C. In this chapter we will discuss some input functions like scanf(), getchar() and some output functions like printf(), putchar(). But first we need to learn about something called conversion specification because functions like scanf() and printf() use this facility.

Conversion Specification

Conversion specifications are used to specify datatype. Each conversion specification begins with (% ) sign. Here are some common conversion specifications:

CONVERSION SPECIFICATION DESCRIPTION
%c a single character
%d an integer
%f floating point number
%x a hexadecimal integer
%o an octal integer
%i an integer, hexadecimal or octal
%s a string
%u an unsigned integer
%h a short integer
%lf a long range floating point number

Outputting data

The printf() function is used to output data to the console.

Syntax: printf("Control string", variable1, variable2 , ...);

Control string: It contains conversion specification and text enclosed in double quotes. This argument controls how the output will appear on the screen.

Variables: variables whose data we want to print in the console. Instead of passing variables we can also pass constants and expressions too. This parameter is optional.

If control string does not contain any conversion specification, then variables are not specified.

Example 1: Printing Strings

The following program prints a string to the console using printf() statement.

Expected Output:

Here control string contains only text no conversion specification. So there is no need to specify any variable or expression.

Example 2: Printing integers

Expected Output:

Here control string contains a single %d character which means that an integer value will be displayed. We can also use text inside control string along with conversion specification.

Example 3: Printing integers with along with some text

Expected Output:

Here control string contains text "Basic salary: " along with conversion specification %d. The text will be displayed as it is, and %d is replaced by the actual value of the variable sal.

Example 4: Printing floating point numbers

Expected Output:

Here control string contains a single %f conversion specification character which means a floating point value will be displayed.

Example 5: Printing characters

Expected Output:

Here control string contains a single %c conversion specification which means a character will be displayed.

Example 6: Printing arrays

Expected Output:

Here control string contains single %s conversion specification which means a string will be displayed.

Example: 7

Expected Output:

Here control string contains text along with three conversion specification. In the general number of conversion specification and variable are equal, this is true for both scanf() as well as printf() function.

Example: 8

We already know that just like %d is used to denote a decimal number. Similarly %o and %x is used to denote octal and hexadecimal numbers respectively.

Expected Output:

Example 9: Printing newline

We have learned about escape sequences in earlier chapters. Let’s see how we can use them to properly format our output.

The following program demonstrates how we can properly format our data using escape sequences.

Expected Output:

When \n newline character is encountered it moves the cursor to the beginning of next line and printing starts from there.

Example 10: Printing tabs

Expected Output:

\t is known as a tab character. When \t is encountered it moves the cursor to the next tab stop. \t is commonly used to display data in tabular form.

Example 11:

Another commonly used escape sequence is \" , which represent " character. Since " character marks the beginning and end of a string, we can’t use it directly inside a string.

Expected Output:

Example 12:

We know that marks the beginning of escape sequence that’s why we can’t use it directly inside a string because the compiler will assume that it’s is the beginning of escape sequence. To print a single we have to use two characters inside a string.

Expected Output:

is commonly used to display Windows pathname.

Reading input from keyboard

The scanf() function is used to read input from the keyboard.

Syntax: scanf("Control string", address1, address2 , ...);

You must pass atleast two arguments to this function.

Control string: A string which contains one or more conversion specification enclosed in double quotes. The number of conversion specification depends on the number of variables we want to input.

The next parameter, address1 is the address of the variable, scanf() function expects at least one address. The address of the variable can be found by preceding a variable name with (&) sign.

In scanf() function syntax ... (known as ellipsis) indicates that scanf() can accept variable number of arguments.

The following program demonstrates how to receive input from the keyboard using scanf() function.

Expected Output:

In the above program, we want the user to enter a number that’s why a single %d conversion specification is used in scanf(). If we want the user to enter a string we should use %s. Similarly use %c for a single character and %f for float.

Reading a character

Expected Output:

Here control string contains a single %c conversion specification, which means that a single character should be entered. Similarly, you can ask user to enter a floating point number.

Expected Output:

Accepting more than one values

A single scanf() function can also be used to input more than one values.

Expected Output:

Here two %d conversion specification characters are used which means two integer values should be entered. To enter two numbers simultaneously, you need to enter first number press space then enter the second number.

When more than one value are input by scanf(), these values can be separated by whitespace characters like space, tab or newline(default), but you can change this behaviour by placing a specific character between conversion specification. Let’s take an example to illustrate this point.

Expected Output:

Here colon(:) character is used between two %d‘s. It means now you have to enter first number then colon(:), followed by the second number.

The following program asks the user to enter three integers separated by comma (,).

Expected Output:

For better readability we can include some spaces between conversion specification. For example:

Other than better readability they don’t have any significance. So the above code is essentially same as:

Character I/O

The getchar() and putchar() macros are used for character i/o in C, we will discuss what macros are in later chapters, but for now treat them as functions. The getchar() reads a single character from the standard input i.e keyboard and putchar() outputs one character to standard outputs i.e console.

Expected Output:

In the next chapter, we will learn about Formatted Input and Output in C.

Deadly scanf()

The scanf() function contains several traps which we haven’t discussed. Consider the following program:

Nothing extraordinary is going on here just a simple program asking the user to enter a number and a character, but that’s a deception, run the program and see it yourself.

Expected Output:

As soon as you enter a number program displays it without waiting for you to enter a character, why it is so ?

Let’s discuss the scanf() function in little more detail.

When input is entered it is stored in a temporary memory called input buffer. Consider the following scanf() call:

Let’s say user entered 445\n.

So now contents in the input buffer is:

Here we have provided %d conversion specification which means we want to scanf() to read a number. But since scanf() doesn’t know how long your number will be, so it keeps reading digits until it encounters a non-digit character ( in this case \n ).  The scanf() reads the \n character since it’s not a number, it pushes \n back to input buffer.

At this point contents of input buffer is:

Here is the rule 1: The character that is pushed back to the input buffer will be read first by the subsequent calls of scanf().

So now we can explain what’s going on in our program.

Let’s say the user entered 100. So now contents of the input buffer are:

First, the scanf() call in line 9 reads 100 and pushes \n character back to the input buffer. Now the contents of the buffer are:

The second scanf() statement in line 12 reads the \n character. So now variable ch contains a newline character. We can verify this fact by printing the ASCII value of newline \n character which is 10. Add the following print() statement after the printf() statement in line 17.

Run the program and enter the input as follows:

Expected Output:

This verifies the fact that ch contains a newline(\n) character. Obviously, the question arises how to solve this problem ?

It turns out that if there are one or more white-space characters in the control string, scanf() repeatedly reads white-space characters from input buffer until a non-space character is encountered. A white-space character in a format string matches any number of white-space character in the input including none.

So if we add one or more white-space character just before %c, this causes scanf() to read all the white-space character before reading a character.

Another solution is that just before reading the character flush the buffer using the following function.

Calling this function removes all the data from the input buffer.

Here is our modified program:

Expected Output:

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