[printf function and scanf function] (learning note 5 -- standard i/o function)

One 、printf function

printf Functions are often called “ Format print function ”, Its first parameter is called “ Formatted string ”, stay “ Formatted string ” Can be used in “ Place holder ” Insert some other types of data into the text string for printout .“ Place holder ” It's all about “%” At the beginning , When the program prints out , these “ Place holder ” Will be replaced by real data .

printf Function :

Place holder	Show content
%c	character
%f	Floating point numbers
%s	character string
%x	Hexadecimal number
%d	Integers
%o	Octal number
%u	Unsigned integer
%%	% character

printf Modifier of function ：

Modifier	significance
digit( Numbers )	Specifies the output width of the data , In characters . If the specified width is not enough , Subject to the actual width
.digit( Numbers )	Specify the output precision of the data , For floating-point numbers, the number of significant decimal places , For a string, it refers to the number of output characters , For integers, the minimum number of bits of output （ Not enough digits , High compensation 0）
-	Specifies that the data alignment is left aligned . The default data output is right aligned
+	For signed data , Positive 、 Minus sign
#	Display the prefix of the data , Display prefix for octal numbers 0, Display prefix for hexadecimal numbers 0x or 0X

These modifiers are used with placeholders , Not to be used alone .

Two 、scanf function

In the use of scanf When using functions, pay attention to , You need to add a before the parameter variable name “&” Symbol , Indicates the memory address of the variable .

char ch;
int n;
float flt;
scanf("%c%d%f",&ch,&n,&flt);

Users should pay attention to , Blank characters should be left between each data （ For example, the space character ）.
scanf Function when reading character data , The first character entered by the user （ Include white space characters ） Read in , And save to the character variable . The so-called white space characters include spaces 、 Invisible characters such as horizontal tabs and line breaks .
scanf Function when reading non character data , The leading white space characters in the user input are automatically skipped , Read from the first legal character , Do not stop reading until white space or illegal characters are encountered , Then these characters are converted into corresponding data and saved in variables . What are legal characters ？ for example , If you read a decimal integer , Legal characters mean 0 To 9 These numeric characters ; If you read an octal number , Legal characters mean 0 To 7 These numeric characters ; If you read a hexadecimal number , Legal characters mean 0 To 9 These numeric characters and A To F、a To f These characters ; If you read a floating point number , Then the legal characters include 0 To 9 Except for these numeric characters , It also includes a decimal point “.” The characters of .
scanf Try not to include characters other than placeholders in the format string of the function , Because the user must input strictly according to the format of the formatted string , Otherwise, it will easily lead to errors .