8.3.4.
The printf function
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 | 8.3. I/O functions for data types |  |
8.3.4.
The printf function
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| | 8.3. I/O functions for data types | |
printf function
The printf function (the name comes
from “print formatted”) prints a string on the screen using a
“format string” that includes the instructions to mix several
strings and produce the final string to be printed on the
screen. Languages such as Java also include functions similar to this one
(see
printf method of class
PrintStream).
printf is a special function because it
receives a variable number of arguments. The first parameter is fixed and
is the format string. It includes text to be printed literaly and
marks to be replaced by the text obtained from the
additional parameters. Thus, printf is invoked with as many
parameters as marks are included in the format string, plus one (the
format string itself). The following example shows how to print the value
of variable counter.
printf("The value is %d\n", counter);The symbol “%” denotes the
beginning of a format mark. The mark “%d” is
replaced by the value of variable counter and the resulting
string is printed. The symbol “\n” represents a
new line. By default, all output is right-justified, that is, is placed on
the right edge of the field and, by default, the width of the field is the
same width as the string length.
If the format string includes several marks, they are
processed in the same order as they appear. The following figure shows and
example in which the format string has three marks, %s,
%d, %5.2f, that are processed using respectively
the string “red”, the integer 1234567 and the
real number 3.14.
No check is performed to verify that the number of marks
in the format string and the number of remaining parameters are
consistent. In case of an error, the behavior of printf is
undetermined.
The marks in the format string must have the following structure (fields in brackets are optional):
%[parameter][flags][width][.precision][length]type
Thus, every mark starts by the
“%” symbol and finishes with a type. Each of the
names (parameter, flags, width, precision, length and type) represents a
set of possible values that are explained next.
| Parameter | Description |
n$ |
“n” is replaced by a number to
change the order in which the arguments are processed. For
example %3$d refers to the third argument
independently of its place within the format string.
|
| Flags | Description |
| number | Left-pad the output with spaces (or zeros, see next flag) up to the number value. |
| 0 |
Use 0 instead of spaces for left-padding up to
the value given by the previous flag. For example
“%03d” prints a number left padded
with zeros up to three digits.
|
| + | Print the sign of a number |
| - | Align the field to the left (by default it is right aligned) |
| # | Alternative format. For real numbers, it inserts zeroes at the end and always print the comma. For numbers not in base 10, it adds a prefix denoting the base. |
| Width | Description |
| number | Size to left pad the field to print |
| * |
Identical to the previous case, but the number
to be used is given as parameter right before the value. For
example printf("%*d", 5, 10) prints number 10
left-padded up to five digits.
|
| Precision | Description |
| number | Size of the decimal part for real numbers. Number of characters to print for strings. |
| * |
Identical to the previous case, but the number
to be used is given as parameter right before the value. For
example printf("%.*s", 3, "abcdef") prints
“abc”.
|
| Length | Description |
| hh |
Convert a variable of type char
to integer and print
|
| h |
Convert a variable of type
short to integer and print
|
| l |
For integers, a variable of type
long is expected.
|
| ll |
For integers, a variable of type long
long is expected.
|
| L |
For floating point, a variable of type
long double is expected.
|
| z |
For integers, an argument is expected of type
size_t.
|
| Type | Description |
%c | Prints the ASCII character passed as parameter |
%d, %i | Integer signed decimal conversion |
%x, %X | Unsigned hexadecimal conversion |
%p | Memory address (pointer) |
%e, %E | Floating point conversion with sign in scientific notation |
%f, %F | Signed floating point conversion, using decimal point |
%g, %G | Floating point conversion, using the notation that requires less space |
%o | Integer unsigned octal conversion |
%u | Integer unsigned decimal conversion |
%s | Text string (finished with '\0') |
%% | Prints the symbol % |
The format marks included as part of the first parameter
passed to printf offer many possibilities. It follows a
description of some examples:
Specify the minimum field width: The C language
allows you to add an integer between the percent sign (%) and the
letter in a format specifier. This ensures that the output reaches
the minimum width. For example, %10f ensures that the
output is at least 10 character spaces wide. This is especially
useful when printing out a column of values. For example, with the
next code:
int num = 12;
int num2 = 12345;
printf("%d\n",num2);
printf("%5d\n",num);the following output is printed:
12345 12
Align output: By default, all output is
right-justified when using the minimum field width. You can change
this and force output to be left- justified. To do so, you need to
prefix the minimum field specifier with the minus sign (-). For
example, %-12d specifies the minimum field width as 12,
and justifies the output from the left edge of the field.
Specify precision: You can put a period . and an integer right after the minimum field width specifier to have a precision specifier. You can use the precision specifier to determine the number of decimal places for floating-point numbers, or to specify the maximum field width (or length) for integers or strings.
Check with these questions if you understood this document
We want to print the string
char string[30]; with the
printf function:
| |  | |
8.3.3.
The scanf function
|  | 8.4. Input functions to read strings in a secure way |