A string can be formatted, sending it out to the console or sending it to a file. This tutorial explains the formatting of a string, using the printf() function, sending it out to the terminal (console). In C++, the printf() function can be used, through the inclusion of the <cstdio> library.
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Arithmetic Types
Arithmetic types are integer and floating types. Consider the following program:
The output is:
The first argument to the printf() function is a string literal. The newline character, “\n” in the string literal, forces anything to be printed next to be printed on the next line, even if that thing is within the double-quotes. The “\n” is also a whitespace character.
With the inclusion of <cstdio>, “#include <iostream>” is not really necessary.
Integer
Consider the following printf() statements:
The output is:
Number is: 52
The %i or %d is an example of a format specifier. %i within the first argument of the printf function, which is a string literal, means replace itself with the integer value that is the next argument of the printf function. %d is a synonym to %i. The “\n” will always play its role in sending what is to be printed next to the next line at the terminal. A specifier such as %i, typed next to \n, poses no conflict of interest. The second argument to the printf() function can be a variable.
By replacing itself with the next argument value of the printf function, the specifier is said to be expanded to the corresponding value. Note: %i is for an integer, while %f is for a float number.
Now, if one zero is inserted between % and the actual specifier, i, i.e., %0i for 1 zero, then 52 will be outputted as 52. If it is %02i, for 2 zeros, 52 will still be outputted as 52. If it is %03i, for 3 zeros, 52 will be outputted as 052. One zero has been added, in the third position, counting from the right end of the number. If it is %04i, for 4 zeros, 52 will be outputted as, 0052. Two zeros have been added, in the fourth position, counting from the right end of the number.
In this context, zero is called a flag. The following program illustrates this:
The output is:
Number is: 52
Number is: 052
Number is: 0052
Zero in this context is called a flag. Another possible flag is the space. The following program illustrates this for the space:
The output is:
Number is: 52
Number is: 52
Number is: 52
Note that with the space flag, no character is used in the format specifier. A flag is added in front of the number to make up the number of positions requested for the presentation field, counting from the right. If the number of positions is less than or equal to the number of digits, no flag is added.
Field Width
A field is the number of characters that can be displayed for that number. A field width is the maximum number of characters that the programmer hopes to get. The field width number is inserted just after the flag (rightward) in the format specifier. It is the same as the previous number. If the number of characters to be displayed is naturally greater than the field width, the printf() function will allow it. The following program illustrates this:
The output is:
Number is: 012
Number is: 123
Number is: 1234
Number is: 12345
Float
A floating point number is a number with the integer part and a decimal part. Note, the integer part is not represented as an integer, internally. The actual specifier for the float is ”f”. The following program illustrates this:
The output for the author’s computer is:
This number naturally has 2 decimal places. Unfortunately, 4 decimal places of 4 zeros were appended. The truth is, the author’s computer rounds the number of decimal places to 6. It will append zeros to make up for 6 decimal places if the natural number of decimal places is less. The number of decimal places can be decided by the programmer. It can be less than or equal to 6, or it can be greater than 6.
This intention needs another code component, called the precision component, for the format specifier. It consists of the dot and a number for the number of decimal places wanted. The following program illustrates this:
The output is:
Number is: 2.5
Number is: 2.53
Number is: 2.530
Number is: 2.53000000
Note that “.0” is not in any of the specifiers.
Variadic Nature of printf
The first argument of the printf() function is a string literal. Format specifiers can be interspersed within the string literal. The first format specifier from the left, in the string literal, corresponds to the second argument of the printf() function. The second format specifier from the left, in the string literal, corresponds to the third argument of the printf() function. The third format specifier corresponds to the fourth argument of the printf() function and so on. The following program illustrates this for the int and float types:
using namespace std;
int main()
{
int it = 52;
float ft = 2.53;
printf("Numbers are, %03i and %03i and %.3f\n", 27, it, ft);
return 0;
}
The output is:
Numbers are 027 and 052 and 2.530
String Type
The basic string format specifier is %s. The following program shows its use:
The output is:
The ‘\n’ character is to send whatever is printed next to the next line. All the first argument here has %s as content.
The different possible ways of using numbers, with the string format specifier, are as follows:
%.nums
%-nums
%.num1-num2s
%-num1.num2s
%nums
There are 11 characters in the string, “I love you.” The dot is a character. If num is less than 11, the output string will not be truncated. If it is greater than 11, extra spaces will be padded on the left to make the total number of characters in the field the number given. The following code illustrates this:
The output is:
I love you.
%.nums
Here, there is a dot in front of the number. The dot here means print the number of characters of the given number, beginning from the first character. This means, if num is less than the total number of characters, truncate the balance on the right. If num is more, add spaces on the right to make up for the given number. The following program illustrates this:
The output is:
This output needs further explanation. In the string “I love you.\n”, there are 12 characters. “\n” is a character. The first 7 characters are “I love ”. The program’s first printf() function prints this: “I love ”, truncating the rest of the string literal, including “\n”. Since “\n” of the first “I love you.\n” has been taken off, whatever has to be printed next, is printed on this line. The second printf() function prints its 11 characters. The twelfth character, which is “\n”, causes the cursor to go to the next line. Then, 4 more spaces should be printed next.
Here, there is a hyphen in front of the number. The hyphen means, print the number of characters of the given number, beginning from the first character. Plus, do not truncate if num is less than the total number of characters. Simply add more spaces on the right to make up for the given number. The following program illustrates this:
The output is:
I love you.
cursor
The cursor appears after 4 extra spaces in the third line.
The interpretation of these two items is left as an exercise for the reader.
Whitespace
The “\n” is an example of a whitespace character. Whitespace characters are escape sequences. They are not printed. They just have their individual effects. For example, “\n” causes the cursor to go to the following line. In the previous code samples, this “\n” has been employed inside the first argument of the printf() function, a string literal. It can still be employed as a variable, as the following program shows:
using namespace std;
int main()
{
char vr = '\n';
printf("first line%csecond line", vr);
return 0;
}
The output is:
second lineCURSOR
The following are whitespaces and their meanings:
\n: add a newline
\t: horizontal tab
\v: vertical tab
\f: form feed
\040: single space by pressing the space bar key
\r: carriage return
Conclusion
String formatting means putting certain characters for a string, in particular positions and particular order. This also involves the insertion of whitespace characters in certain positions. C++20 is the latest version of C++. It has a format library. However, most C++ compilers have not yet implemented this library. Note, the printf() variadic function, incorporated into the C++ language from the C language, is similar to the purpose of the format library. The first argument to this function is a string literal. The format specifiers are interspersed within. The rest of the arguments to the printf() function correspond to these specifiers in order.
