printf( ) function in C Program

1. printf( ) function

In C programming, printf() is one of the main output function. The function sends formatted output to the screen. The “f” in printf stands for “format” which indicates that it can be used to customize the formatting of the output text.

printf("Hello World");

The above command will output the text “Hello World” on the screen.

The printf() function does not automatically add a newline character at the end of the output. After the execution, the cursor remains at the end of the output without automatically moving to the next line. To move the cursor to the beginning of the next line, you can add a newline character “\n” at the end of the output text.

“\n” is an escape sequence

printf("Hello World\n");

above code will move the cursor to the beginning of the next line.

If there are line breaks within the text, they are also achieved by inserting line break characters. The line break character in C is represented by the escape sequence \n. When the \n escape sequence is encountered in a string, it indicates that a new line should begin at that point. For example, the following code would output the string “Hello\nWorld” on two separate lines:

printf("Hello\nWorld\n");

results:

The above example first outputs “Hello” and then adds a newline. On the next line, it outputs “World” at the beginning, followed by another newline.

The above example can also be written using two printf() statements, with the same result.

printf("Hello\n");
printf("World\n");

The printf() function is defined in the standard library header file stdio.h. Before using this function in a source code file, it is necessary to include this header file at the beginning of the file using the #include <stdio.h> directive.

2. A Placeholder in C Program

Detail: A placeholder 

Common formatting output functions include printf() and sprintf().

The placeholder starts with a percent sign (%) followed by a character that represents the type and format of the data to be output. For example, %d represents an integer, %f represents a floating-point number, %s represents a string, and so on.

Here are some common placeholders and their meanings:

• %d: output integer
• %i: output a integer
• %f: output floating-point number
• %c: output character
• %s: output string
• %p: output pointer address
• %e: output floating-point number in scientific notation
• %x: output hexadecimal integer

In addition to representing data types, placeholders can also be used to control output format, such as controlling output width, precision, alignment, and so on. These control characters can be combined with placeholders, such as %5d to output an integer with a width of 5.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("I am %i years old\n", 16);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("I am %i(%s) years old\n", 16, "sixteen");
    return 0;
}

#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("I am %i(%s) years old\n", 16, "sixteen");
    return 0;
}

3. Output Format

Printf() syntax

printf("format string", argument1, argument2, ...);

• The format string is a string that contains two types of objects: ordinary characters and format specifiers.
• Ordinary characters (e.g. letters, digits, punctuation marks) are copied to the output stream.
• Format specifiers begin with a percent sign (%) and are followed by one or more format characters. They specify the type of data to be printed and how it should be formatted.
• Arguments are the values to be printed using the format specifiers in the format string.

With printf(), you can customize the output format of placeholders.

3.1 Limit the Output Widths

example 3.1  Printf() allows you to specify a minimum width for the placeholders right-aligned.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("%8d\n", 4567); // output "    4567"
    return 0;
}

The example above shows that “%8d” indicates that the minimum width of this placeholder is 8 digits. If the value is less than 8 digits, white-spaces will be added in front of it.

The output value is right-aligned by default, which means there are spaces in front of the output content. Adding a “-” after the “%” in the placeholder format will left-align the output value. For example, %-8d would left-align the integer value with a minimum width of 8 characters, adding spaces after the value if necessary.

Example 3.2 Printf() allows you to specify a minimum width for the placeholders, left-aligned.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("%-8dhere\n", 4567); 
    return 0;
}

The example above adds four spaces after the output “4567”. The purpose of adding “Here” is to see the number of spaces clearly.

Example 3.3 For decimals, A Placeholder limits the minimum display width for all numbers.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("here|%12f\n", 567.89);
    return 0;
}

in the above example, %12f means that the floating-point number to be printed should take up at least 12 characters. Since the default display precision for a floating-point number is 6 digits after the decimal point, the resulting output will have 2 spaces added to the beginning to ensure that the total width is at least 12 characters.

3.2 Always Display the Positive and Negative Sign

By default, printf() does not display a + sign for positive numbers, only for negative numbers. To display a + sign for positive numbers as well, you can add a + sign after the % in the format specifier.

Example 3.2.1 Always Display the Positive and Negative Sign

<pre class="EnlighterJSRAW" data-enlighter-language="c">#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("%+d\n", 89); 
    printf("%+d\n", -89); 
    return 0;
}</pre>

In the example above, %+d ensures that the output number always has a sign.

3.3. Limiting the Number of Decimal Places

The precision of the floating-point number can be specified using a dot (.) followed by a number after the percentage symbol (%). The number following the dot represents the number of digits to be displayed after the decimal point. For example, %.2f means to display 2 digits after the decimal point.

Example 3.3.1 – Limiting the Number of Decimal Places

#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("Number is %.3f\n", 0.5);
    return 0;
}

In the example given, the placeholder %.3f is used to display the floating-point number with a precision of 3 decimal places after the decimal point.

this syntax can be combined with width limiters.

Example 3.3.2 – Limiting the Number of Decimal Places and Width Limiters

#include <stdio.h>
#include <stdlib.h>

int main()
{
   printf("Here|%7.3f\n", 0.5);
}

In the example above, “%7.3f” represents a minimum width of 7 characters and 3 decimal places when printing the floating-point number. Therefore, there are two spaces at the beginning of the printed string to fill up the minimum width of 7 characters.

Asterisk (*)

The  width limiters and decimal places can both be replaced with an asterisk (*) through arguments passed into the printf() function.

Example 3.3.3 an asterisk (*) through arguments passed into the printf() function

#include <stdio.h>
#include <stdlib.h>

int main()
{
   printf("%*.*f\n", 7, 3, 0.5);

   // equals to
   printf("%7.3f\n", 0.5);
}

In the above example, the two asterisks in %*.*f are passed into printf() as the two arguments 7 and 3.

3.4. Output Strings

The %s placeholder is used to output a string, and by default, it outputs the entire string. If you only want to output the beginning part of the string, you can use %.[m]s to specify the length of the output, where [m] represents a number that indicates the length to be outputted.

Example 3.4.1  Output part of a string

#include <stdio.h>
#include <stdlib.h>

int main()
{
   // Output My Dea
   printf("%.6s\n", "My Dear Friend!");
}