Using scanf for Entry of Numeric Values in VS .NET

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Using scanf for Entry of Numeric Values
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In a peculiar sort of way, the C library function scanf is printf running backward: Instead of outputting formatted data in a character stream, scanf takes a stream of character data from the keyboard and converts it to numeric data stored in a numeric variable Scanf works very well, and it understands a great many formats that I won't be able to explain in this book, especially for the entry of floating-point numbers (Floating-point values are a special problem in assembly work, and I won't be taking them up in this edition of this book) For most simple programs you may write while you're getting your bearings in Linux assembly, you'll be entering simple integers, and scanf is very good at that You pass scanf the name of a numeric variable in which to store the entered value and a formatting code indicating what form that value will take on data entry The scanf function will take the characters typed by the user and convert them to the integer value that the characters represent That is, scanf will take the two ASCII characters "4" and "2" entered successively and convert them to the integer value 42 after the user presses Enter What about a prompt string, instructing the user what to type Well, many newcomers get the idea that you can combine the prompt with the format code in a single string handed to scanf but that won't work It seems like it should hey, after all, you can combine formatting codes with the base string to be displayed using printf And in scanf, you can theoretically use a base string containing formatting codes but the user would then have to type the prompt as well as the numeric data! So, in actual use, the only string used by scanf is a string containing the formatting codes If you want a prompt, you must display the prompt before calling scanf, using printf To keep the prompt and the data entry on the same line, make sure you don't have a newline called out at the end of your prompt string! The scanf function automatically takes character input from standard input You don't have to pass it the file handle stdin, as with fgets (There is a C library routine fscanf to which you do have to pass a file handle, but for integer data entry, there's no hazard in using scanf) Here's how to use the scanf routine: 1 Make sure you have declared extern scanf along with your other external declarations at the top of the [text] section 2 Declare a memory variable of the proper type to hold the numeric data read and converted by scanf
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1 2 My examples here will be for integer data, so you would create such a variable with either the DD directive or the RESD directive Obviously, if you're going to keep several separate values, you'll need to declare one variable per value entered 3 To call scanf for entry of a single value, first push the address of the memory variable that will hold the value (See the following discussion about entry of multiple values in one call) 4 Next, push the address of the format string that specifies what format that data will arrive in For integer values, this is typically the string "%d" 5 Call scanf 6 Clean up the stack The code for a typical call would look like this: push dword intval push dword iformat call scanf add esp,8 ; ; ; ; Push the address of the integer buffer Push the address of the integer format string Call scanf to enter numeric data Clean up the stack
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It's possible to present scanf with a string containing multiple formatting codes, so that the user could enter multiple numeric values with only one call to scanf I've tried this, and it makes for a very peculiar user interface The feature is better used if you're writing a program to read a text file containing rows of integer values expressed as text, and convert them to actual integer variables in memory For simply obtaining numeric values from the user through the keyboard, it's best to accept only one value per call to scanf The following program shows how you would set up prompts alongside a data entry field for accepting both string data and numeric data from the user through the keyboard After accepting the data, the program displays what was entered, using printf ; ; ; ; ; ; ; ; ; ; ; ; Source name Executable name Version Created date Last update Author Description : : : : : : : CHARSINASM CHARSIN 10 11/21/1999 11/30/1999 Jeff Duntemann A data input demo for Linux, using NASM 098
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Build using these commands: nasm -f elf charsinasm gcc charsino -o charsin ; Section containing code ; Standard file variable for input
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