Scanf C function description

The input format consists of the following four parameters: %[width][modifiers] type. In this case, the "%" sign and the type are mandatory parameters. That is, the minimum format look like this: “%s”, “%d” and so on.

In general, the characters that make up the format string are divided into:

• format specifiers - anything starting with %;
• separating or space characters - they are space, tab(t), newline(n);
• characters other than whitespace.

Function may not be safe.

Use scanf_s() instead of scanf().

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Type, or format specifiers, or conversion characters, or control characters

A scanf C declaration must contain at least a format specifier, which is specified at the end of expressions beginning with "%". It tells the program the type of data to expect when entering, usually from the keyboard. List of all format specifiers in the table below.

Characters in the format string

Star symbol ()

The asterisk () is a flag indicating that the assignment operation should be suppressed. An asterisk is placed immediately after the "%" sign. For example,

That is, if you enter the line "45-20" in the console, the program will do the following:

1. Variable "x" will be assigned the value 45.
2. Variable "y" will be assigned the value 20.
3. And the minus sign (dash) "-" will be ignored thanks to "%c".

Width (or field width)

This is an integer between the "%" sign and the format specifier that specifies the maximum number of characters to read in the current read operation.

There are a few important things to keep in mind:

1. scanf will terminate if it encounters a separator character, even if it didn't count 20 characters.
2. If more than 20 characters are input, only the first 20 characters will be written to str.

Modifierstype (or precision)

These are special flags that modify the type of data expected for input. The flag is specified to the left of the type specifier:

• L or l (small L) When "l" is used with the specifiers d, i, o, u, x, the flag tells the program that long int input is expected. When using "l" with the e or f specifier, the flag tells the program that it should expect a double value. The use of "L" tells the program that a long double is expected. The use of "l" with the "c" and "s" specifiers tells the program that two-byte characters like wchar_t are expected. For example, "%lc", "%ls", "%l[asd]".
• h is a flag indicating the short type.
• hh - indicates that the variable is a pointer to a signed char or unsigned char value. The flag can be used with the specifiers d, i, o, u, x, n.
• ll (two small L's) - indicates that the variable is a pointer to a value of type signed long long int or unsigned long long int. The flag is used with specifiers: d, i, o, u, x, n.
• j - indicates that the variable is a pointer to the intmax_t or uintmax_t type from the stdint.h header file. Used with specifiers: d, i, o, u, x, n.
• z - indicates that the variable is a pointer to the size_t type, the definition of which is in stddef.h. Used with specifiers: d, i, o, u, x, n.
• t - indicates that the variable is a pointer to the ptrdiff_t type. Definition onthis type is in stddef.h. Used with specifiers: d, i, o, u, x, n.

More clearly, the picture with modifiers can be represented as a table. Such a description of scanf C for programmers will be clearer.

Other characters

Any characters encountered in the format will be discarded. At the same time, it should be noted that the presence of whitespace or separator characters (newline, space, tab) in the control string can lead to different behavior of the function. In one version, scanf() will read without saving any number of separators until it encounters a character other than the separator, and in another version, spaces (only they) do not play a role and the expression "%d + %d" is equivalent to "% d+%d".

Examples

Let's look at some examples to help you think and better understand how the function works.

FromThe examples below show how the expected number changes using different symbols.

scanf C - description for beginners

This section will be useful for beginners. Often you need to have at hand not so much a complete description of scanf C as the details of how the function works.

• This feature is somewhat obsolete. There are several different implementations in libraries of different versions. For example, the improved scanf S C function, a description of which can be found on the microsoft website.
• The number of specifiers in the format must match the number of arguments passed to the function.
• Input stream elements must be separated only by separator characters: space, tab, newline. Comma, semicolon, period, etc. - these characters are not separators for the scanf() function.
• If scanf encounters a separator character, input will be stopped. If there is more than one variable to read, then scanf will move on to reading the next variable.
• The slightest inconsistency in the input data format leads to unpredictable results of the program. Well, if the program just ends with an error. But often the program continues to work and does it wrong.
• scanf("%20s …", …); If the input stream exceeds 20 characters, then scanf will read the first 20 characters and either abort or move on to reading the next variable, if one is specified. In this case, the next call to scanf will continue reading the input stream from the point where the work of the previous call to scanf stopped. If when reading the first 20characters, a delimiter character is encountered, scanf will abort or continue reading the next variable, even if it did not read 20 characters for the first variable. In this case, all unread characters will be attached to the next variable.
• If the set of scanned characters starts with a "^", then scanf will read the data until it encounters a delimiter character or a character from the set. For example, "%[^A-E1-5]" will read data from the stream until one of the uppercase English characters from A to E or one of the numbers from 1 to 5 is encountered.
• The scanf C function, as described, returns a number equal to the successful number of writes to variables. If scanf writes 3 variables, then the success result of the function will return the number 3. If scanf could not write any variables, then the result will be 0. And, finally, if scanf could not start at all for some reason, the result will be EOF.
• If the scanf() function ended incorrectly. For example, scanf("%d", &x) - a number was expected, but characters were received as input. The next scanf() call will start at the point in the input stream where the previous function call ended. To overcome this problem, it is necessary to get rid of the problem characters. This can be done, for example, by calling scanf("%s"). That is, the function will read a string of characters and throw it away. In this tricky way, you can continue entering the necessary data.
• In some implementations of scanf(), "-" is not allowed in the scanned character set.
• The "%c" specifier reads each character from the stream. That is, it also reads the separator character. To skip the delimiter character and continue reading the desired character, you can use "%1s".
• When using the "c" specifier, it is permissible to use the width "%10c", but then an array of elements of type char must be passed as a variable to the scanf function.
• “%[a-z]” means “all small letters of the English alphabet”, and “%[z-a]” means just 3 characters: ‘z’, ‘a’, ‘-’. In other words, the "-" character means a range only if it is between two characters that are in the correct order. If "-" is at the end of an expression, at the beginning, or in the wrong order of characters on either side of them, then it is just a hyphen character, not a range.

Conclusion

This concludes the description of scanf C. This is a nice handy feature for working in small programs and when using the procedural programming method. However, the main disadvantage is the number of unpredictable errors that can occur when using scanf. Therefore, the description of scanf C when programming is best kept in front of your eyes. In large professional projects, iostreams are used, due to the fact that they have higher-level capabilities, they are better able to catch and handle errors, and also work with significant amounts of information. It should also be noted that the description of scanf C in Russian is available on many network sources, as well as examples of ituse, due to the age of the function. Therefore, if necessary, you can always find the answer on the thematic forums.