scanf Function with Buffered Input

5 Key excerpts on "scanf Function with Buffered Input"

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  Learning C with Fractals

  • Roger T. Stevens(Author)
  • 2014(Publication Date)
  • Academic Press

    (Publisher)

  If used to read from a disk file, the file must be opened in the byte mode. The code is int integer; integer = getw(stdin); The scanf Function The scanf function will read a series of input fields from stdin, one character at a time. The scanf function provides a lot of flexibility in formatting and defining numbers and strings that are to be read in from the keyboard. (It automatically inputs from stdin.) The scanf function can handle a variable number of parameters. The first parameter is always a format string, which is enclosed in double quotation marks. The remaining parameters are address of arguments whose values will be read in the form specified in the format string. This is unlike printf where the argument values, rather than their addresses, are passed to the function. You, as programmer, need to make sure that your format string specifies the format of each variable in your parameter list and that you have a variable in the list for each format specifier. If the number of format specifiers and the number of variables do not match, you can get some really strange results from the scanf function. Unlike the printf function, where anything in the format string that is not a format specifier is printed out, the scanf function format string ignores all format specifiers, so it should contain nothing else. Specifying the Input Format The entire format specifier used to determine the format of an input is of the form 118 Keyboard Input % [*] [width] [FIN] [h 111L] type Table 11-1 indicates the meaning of these symbols. Note that the ones in brackets are optional and may be used or not as the occasion demands. The straight lines separating characters indicate that any one of the characters may be used at that point. For each variable in the variable list of your scanf statement, you must have a format specifier in the format string. As a minimum, the format specifier must consist of a % character followed by a type character.

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  Basic Data Structures and Program Statements

  • Xingni Zhou, Qiguang Miao, Lei Feng(Authors)
  • 2020(Publication Date)
  • De Gruyter

    (Publisher)

  Interested readers may update the above program and test again. 4.3.3 Formatted Input function The function signature and functionality of the formatted input function are given in Figure 4.23. The function name is scanf . There are several items inside parenthe-ses, of which the format control sequence is similar to the one used in printf . It is worth noting that the address argument is passed to the function by prefixing vari-able names with “ & ” sign. The “ & ” sign is the address-of operator. It is not needed if the variable is already an address. Example 4.11 Example of formatted input function: log into Office of Registrar website Mr. Brown wanted to log into the Office of Registrar website to upload the grades of students. A username and a password are necessary to log in, where the username (ID) is the payroll number and the password should be a sequence of digits and characters with a length no more than 20, as per university policies. The login program is given in Figure 4.24. [Analysis] On line 8, the first scanf stores the username retrieved from keyboard input into the variable id . id is defined as an integer variable, so the type specifier should be %d . Signature scanf( format control sequence, address 1…, address n ); Functionality Read data from keyboard in the format specified by the format control sequence and store them into corresponding variables Formatted input function Figure 4.23: Formatted input function. 154 4 Input/output

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  Learn C Programming

  A beginner's guide to learning C programming the easy and disciplined way

  • Jeff Szuhay(Author)
  • 2020(Publication Date)
  • Packt Publishing

    (Publisher)

  If you want to prove that the buffer is flushed when the program exits, remove the very last \n character from the very last printf() call, save the file, then compile and rerun the program. You should see identical behavior.

  Understanding the standard input stream

  The standard input stream is a pointer to a complex FILE structure named stdin. This stream reads any characters typed from the keyboard to be formatted by the scanf() function.

  Like the output stream, by default, the input stream is also buffered. For input, this means that characters are sent to the buffer. The buffer is not flushed until either the buffer is full or until CR is encountered in the input stream. As we enter characters, program control is maintained by the console and is not returned to our program until the buffer is flushed. The processing of the input characters then appears to our program as if they were received one at a time.

  In reality, however, it is a bit more complicated than this. The console has two modes of processing—cooked mode and raw mode .

  Cooked mode uses buffered input and is the default mode unless it is explicitly changed. It also means that we can alter our input in the buffer until we type in the CR key. Backspace and delete work as expected in this mode so that we can edit our input in the buffer before the buffer is flushed to our program. The console is managing the buffer in this mode. Even for single character input, we must enter CR for it to leave the buffer and be passed to our program.

  In raw mode, each individual character is received by the console, and control is immediately returned to our program to process that single character. The character buffer is a single character only. No input processing is done by the console, so no editing is possible in this mode. We will see how to put the Terminal in raw mode and perform single-character processing after we introduce the scanf() function.

  We will latersee how these two modes operate in the console.

  Revisiting the console output with printf() and fprintf()

  The function prototype for printf() and fprintf() are as follows: int prinft( const char* format , ... );int fprintf( FILE* stream , const char* format , ... );

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  Learn C Programming

  • Jeff Szuhay(Author)
  • 2022(Publication Date)
  • Packt Publishing

    (Publisher)

  Chapter 21 : Exploring Formatted Input

  Using console command-line arguments to get string input into our running programs is often handy, but not very useful if we want to read lots of values of any data type while our program is running. To do that, we need to explore how to use formatted input. Formatted input is the opposite end of the pipe, so to speak, to formatted output. Just as we can use formatted output with print() , various value formats can be easily read to the console by using scanf() . Both of these functions do the heavy lifting of converting values into desired output strings or, conversely, converting input character strings into desired values.

  To understand input formatting, we will need to first understand the concept of streams. We will then use the technique of experimentation—or, more specifically, trial and observation —to discover and verify our understanding of how C input streams operate. We will later expand the concept of streams to include files and file processing in Chapter 22 , Working with Files , and Chapter 23 , Using File Input and File Output . 

  As we explore formatted console input, we will also look at a few important side examples to learn about using unformatted input and output (I/O ), converting unformatted strings into integers or floats, and, finally, creating internal strings from values and reading values from an internal string.

  The following topics will be covered in this chapter:

  • Understanding I/O streams
  • Revisiting formatted console output with printf()
  • Exploring formatted console input with scanf()
  • Using scanf()  to read numerical values from the console
  • Using scanf()  to read string and character values from the console
  • Controlling the scanf()  input field width
  • Exploring internal data conversion
  • Using sscanf()  and sprintf()  to convert values into and from strings
  • Exploring unformatted I/O
  • Getting string I/O from the console using gets()  and puts()
  • Converting strings into numbers with atoi()  and atof()

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  Computer Programming with C++

  • Kunal Pimparkhede(Author)
  • 2017(Publication Date)
  • Cambridge University Press

    (Publisher)

  56 ✦ Computer Programming with C++ This will obviously not be an issue if this scanf() statement is the very first input statement in the program but it is always better to keep a habit to put a SPACE before each of the format specifiers to avoid scanning of incorrect inputs. Notes Always ensure that there is a SPACE before %c in the control string while reading character input using scanf() . All the other format specifiers such as %d,%f,and %ld (including %s which is for strings) skip the new line and spaces at input and hence it is not necessary to put a space before them in the control string. But it is always a better programming practice to separate the format specifiers within the control string of scanf() function by space as it improves the overall readability of the statement. 2.9.1 Delimited input using scanf() It is possible to use any other delimiter apart from space in the control string of scanf() . For example, consider three variables a, b, and c declared as below int a,b,c; The statement, scanf(%d$%d$%d$,&a,&b,&c); has a symbol $ in the control string which acts as an separator between two format specifiers. Hence, it is the responsibility of the user to input data which is $ separated . This means that after entering each input value, user must enter a $ symbol as shown below 10$20$30$ After typing this input when the user hits an ENTER key, the scanf() statement will store 10 in variable a, 20 in variable b, and 30 in variable c . If user fails to enter $ and inputs data in any other format, the scanf() will crash. Remember when we put $ as a separator in format specifiers, it actually informs scanf() to skip $ in the input string and hence variables a, b, and c are correctly initialized to 10, 20, and 30, respectively. This feature is rarely used in programs because there is a risk that scanf() will crash if the user does not input data exactly in the same format as needed by scanf() .

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