Scanf
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- The correct title of this article is scanf. The initial letter is shown capitalized due to technical restrictions.
scanf is a function that reads data with specified format from a given string stream source, originated from C programming language, and is present in many other programming languages.
The scanf function prototype is:
int scanf (char *format, ...);
The function returns the total number of items successfully read, EOF when the input stream reached its end, or a negative number if an error occurs.
So far as is traceable, "scanf" stands for "scan format".
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[edit] Usage
The scanf function is especially important for C, which reads input for numbers and other datatypes directly from a console or command prompt.
The following shows code in C that reads a variable number of unformatted decimal integers from the console and prints out each of them on a separate line:
#include <stdio.h>
int main(void) {
int n;
while (scanf("%d", &n) > 0)
printf ("%d\n", n);
return 0;
}
After being processed by the program above, a messy list of integers such as
456 123 789 456 12
456 1
2378
will appear neatly as:
456 123 789 456 12 456 1 2378
No matter what the datatype the programmer wants the program to read, the arguments (such as &n above) must appear in form of pointers pointing to memory locations instead of a variable value. Otherwise, the function will not perform correctly.
As scanf is designated to read input only from the console, many programming languages with interfaces, such as PHP, have derivatives such as sscanf and fscanf but not scanf itself.
[edit] Derivatives of scanf
Depending on the actual source of input, programmers can utilize different derivatives of scanf. Two common examples are sscanf and fscanf.
[edit] sscanf
The sscanf derivative reads input from a user-specified string source instead of the console (known as standard input, stdin). It has prototypes as follows:
(C or C++)
int sscanf (const char *source, const char *format, ...);
(PHP)
int sscanf (const string source, const string format [, mixed args...])
Unlike scanf and fscanf, sscanf does not remove the read items away from the input flush. Therefore, every time the function is executed, the string source is read again from the very beginning..
[edit] fscanf
The fscanf derivative reads input from a file instead of the standard input. The prototypes are as follows:
(C or C++)
int fscanf (FILE *file, const char *format, ...);
(PHP)
int fscanf (resource file, const string format [, mixed args...])
The fscanf derivative works like the original scanf function - parts of the input, once read, will not be read again until the file is closed and reopened.
[edit] vscanf, vsscanf, and vfscanf
int vscanf (const char *format, va_list args); int vsscanf (const char *source, const char *format, va_list args); int vfscanf (FILE *file, const char *format, va_list args);
These are like the same functions without the v prefix, except they take their arguments from a va_list. (See stdarg.h.) These variants may be used in variable-argument functions.
[edit] Format string specifications
The formatting placeholders in scanf are more or less the same as that in printf, its reverse function. There are rarely constants in a format string, mainly because a program is usually not designed to read known data.
Some of the most commonly used placeholders follow:
- '%d', '%i' : Scan an integer as a signed decimal number.
- '%u' : Scan for decimal unsigned int, unsigned short, and unsigned char
- '%f' : Scan a floating-point number in normal (fixed-point) notation.
- '%g', '%G' : Scan a floating-point number in either normal or exponential notation. '%g' uses lower-case letters and '%G' uses upper-case.
- '%x', '%X' : Scan an integer as a hexadecimal number. '%x' uses lower-case letters and '%X' uses upper-case.
- '%s' : Scan a character string. The scan terminates at whitespace. Without a length modifier, this conversion specifier is dangerous because it can lead to buffer overflows.
- '%c' : Scan a character (char).
- '(space)': Space scans for whitespace characters.
The above can be used in compound with numeric modifiers and the 'l', 'L' modifiers which stand for "long" in between the percent symbol and the letter. There can also be numeric values between the percent symbol and the letters, preceding the "long" modifiers if any, that specifies the number of characters to be scanned. An optional asterisk (*) right after the percent symbol denotes that the datum read by this format specifier is not to be stored in a variable. No argument behind the format string should be included for this dropped variable.
It must be noted that the "ll" and "ff" modifiers in printf are not present in scanf, causing differences between modes of input and output.
An example of a format string is
"%7d%s %c%lf"
The above format string scans the first seven characters as a decimal integer, then reads the remaining as a string until a space, new line or tab is found, then scans the first non-whitespace character following and a double-precision floating-point number afterwards.
[edit] Error handling
scanf is usually used in situations when the program cannot guarantee that the input is in the expected format. Therefore a robust program must check whether the scanf call succeeded and take appropriate action. If the input was not in the correct format, the erroneous data will still be on the input stream and must be read and discarded before new input can be read. An alternative method of reading input, which avoids this, is to use fgets and then examine the string read in. The last step can be done by sscanf, for example.
