C + + formatted output, C + + output format control

C + + formatted output, C + + output format control in the output data, for the sake of simplicity, often do not specify the format of the output, by the system according to the type of data to take the default format, but sometimes you want the data to output in the specified format, such as the requirement to output an integer in 16 or eight binary form, Only two decimal places are reserved for the small number of outputs. There are two ways to achieve this. One is the method of using the controls that we have already described (see: C + + input cout and output cin), and the 2nd type is to use the relevant member functions of the Stream object. Separately described below. Use controls to control the format of output format control it is not mentioned here, just for illustrative purposes, see: C + + input cout and output cin.

[Example 13.2] control the output format with a control character.

2. #include <iostream>
4. #include <iomanip>//Don't forget to include this header file
6. Using namespace std;
8. int main()
10. {
12. int a;
14. cout<<"input A:";
16. CIN>>a;
18. cout<<"Dec:"<<dec<<a<<endl; //output integers in decimal form
20. cout<<"hex:"<<<a<<endl; //Output integer A in 16 binary form
22. cout<<"Oct:"<<setbase(8) <<a<<endl; //Output integer A in eight binary form
24. char *pt="China"; //pt pointing to the string "China"
26. cout<<setw(ten) <<pt<<endl; //Specify field width as, output string
28. cout<<setfill(' * ') <<setw(ten) <<pt<<endl; //Specify field width, output string, blank to ' * ' Fill
30. double Pi=22.0/7.0; //Calculate PI value
32. //Output as exponential, 8 decimal places
34. cout<<setiosflags(iOS:: Scientific) <<setprecision(8);
36. cout<<"pi="<<pi<<endl; //Output PI value
38. cout<<"pi="<<setprecision(4) <<pi<<endl; //change to decimal place
40. cout<<"pi="<<setiosflags(iOS:: fixed) <<pi<<endl ; //change to decimal form output
42. return 0;
44. }

The results of the operation are as follows:
Input a:34 (Enter the value of a)
dec:34 (decimal form)
HEX:22 (16 in binary form)
Oct:42 (in octal form)
China (field width is)
China (field width is, blank is filled with ' * ')
pi=3.14285714e+00 (exponential output, 8 decimal places)
pi=3.1429e+00 (exponential output, 4 decimal places)
pi=3.143 (fractional output, precision still) control the output format with the member function of the stream object in addition to controlling the output format with a control, you can control the output format by calling member functions in the Stream object cout that control the output format. The commonly used member functions for controlling output formats are shown in table 13.4.

Table 13.4 Flow member functions for output format control

Stream member functions	control with the same function	function
Precision (N)	Setprecision (N)	Set the precision of a real number to n bits
Width (n)	SETW (N)	Set field width to n bits
Fill (c)	Setfill (c)	Set the fill-in character C
SETF ()	Setiosflags ()	Set the output format state, in parentheses should give the format state, the content is the same as the control setiosflags in parentheses, as shown in table 13.5
UNSETF ()	Resetioflags ()	Terminates the output format state that has been set, and the contents should be specified in parentheses

The flow member function SETF and the parameters in the control setiosflags parentheses represent the format state, which is specified by the format flag. The format flag is defined as an enumeration value in class iOS. Therefore, when referencing these format flags, precede the class name iOS and the domain operator "::". The format flags are shown in table 13.5.

table 13.5 Formatting flags for formatting states

Format Flags	function
Ios::left	Output data aligned to the left in a wide range of fields
Ios::right	The output data is aligned to the right within the wide range of the domain
Ios::internal	The sign bit of a number is left-aligned within the width of the field, the value is right-aligned, and the middle is filled with padding characters
iOS::d EC	Set the cardinality of an integer to 10
Ios::oct	Set the cardinality of an integer to 8
Ios::hex	Set the cardinality of an integer to 16
Ios::showbase	The cardinality of the Force output integer (octal number starts with 0, hexadecimal number starts with 0x)
Ios::showpoint	Force the DOT and mantissa of the output floating point 0
Ios::uppercase	In uppercase notation in scientific notation format e and in hexadecimal output letters
Ios::showpos	Show "+" for positive numbers
Ios::scientific	Floating-point numbers are exported in scientific notation format
Ios::fixed	Floating-point numbers output in fixed-point format (fractional form)
Ios::unitbuf	Flush all streams after each output
Ios::stdio	Clear stdout after each output, stderr

[Example 13.3] output data with the Flow control member function.

2. #include <iostream>
4. Using namespace std;
6. int main( )
8. {
10. int a=
12. cout. SETF(iOS:: Showbase); Show cardinality symbol (0x or)
14. cout<<"Dec:"<<a<<endl; //output A in decimal form by default
16. cout. UNSETF(iOS::d ec); //Terminating decimal format settings
18. cout. SETF(iOS:: Hex); //Set the status to hexadecimal output
20. cout<<"hex:"<<a<<endl; //Output A in 16 binary form
22. cout. UNSETF(iOS:: Hex); //terminating hexadecimal formatting
24. cout. SETF(iOS:: Oct); //Set the status to output in octal
26. cout<<"Oct:"<<a<<endl; //Output A in eight binary form
28. cout. Unseft(iOS:: Oct);
30. char *pt="China"; //pt pointing to the string "China"
32. cout. Width(ten); //Specify a domain width of
34. cout<<pt<<endl; //output string
36. cout. Width(ten); //Specify a domain width of
38. cout. Fill(' * '); //Specify blanks to fill with ' * '
40. cout<<pt<<endl; //output string
42. double Pi=22.0/7.0; //Output PI value
44. cout. SETF(iOS:: Scientific); //designation with scientific notation output
46. cout<<"pi="; //Output "pi="
48. cout. Width(+); //Specify a domain width of
50. cout<<pi<<endl; //Output PI value
52. cout. UNSETF(iOS:: Scientific); //terminating scientific notation status
54. cout. SETF(iOS:: fixed); //Specify output in fixed-point form
56. cout. Width(N); //Specify a domain width of
58. cout. SETF(iOS:: Showpos); //Positive output "+" number
60. cout. SETF(iOS:: internal); //digits appear on the left
62. cout. Precision(6); //Reserved decimal places
64. cout<<pi<<endl; //Output pi, note the position of the number "+"
66. return 0;
68. }

The operating conditions are as follows:
dec:21 (decimal form)
hex:0x15 (16 binary form, beginning with x)
oct:025 (octal form, with opening)
China (field width is)
China (field width is, blank is filled with ' * ')
pi=**3.142857e+00 (exponential output, domain wide, default decimal)
+***3.142857 (decimal output, precision, leftmost output number "+")

a few notes on the program:
1) The member function width (n) and the control character SETW (n) are only valid for the first output item thereafter. Such as:
cout. Width (6);
cout <<20 <<3.14<<endl;
Output is 203.14

At the first output entry 20 o'clock, the field width is 6, so there are 4 spaces before 20, and width (6) has no effect at output 3.14, which is output by the system's default field width (output by the actual length of the data). If it is required to output data in the same field as the specified width n output, you cannot call width (n) only once, and you must call width (N&GT) before outputting each item, which is done in the program above.

2) in table 13.5, the output format status is divided into 5 groups, each group can only choose one (such as Dec, Hex and Oct only one, they are mutually exclusive). After setting the output format state with the member function SETF and the control setiosflags, if you want to change to another state of the same group, you should call the member function UNSETF (corresponding to the member function self) or resetiosflags (corresponding to the control character Setiosflags) , the state of the original setting is terminated first. Then you can see this in the program by setting other states. Program at the beginning although not with the member function self and the control setiosflags set the DEC output format state, but the system is specified by default Dec, so to change to hex or Oct, you should also first use the UNSETF function to terminate the original settings. If you delete lines 7th and 10th in the program, although the hex and Oct formats are set with the member function setf in line 8th and 11th, the hex and Oct settings do not work because the DEC format is not terminated, and the system still outputs in decimal form.

Similarly, the call to the UNSETF function on line 8th of the program is also indispensable.

3) When you use the SETF function to format the state, you can include two or more format flags, since these formatting flags are defined as enumeration values in the iOS class, each with a binary representation, so you can use the bitwise OR operator "|" Combine multiple format flags. If the 5th and 6th lines can be replaced by the following line:
COUT.SETF (ios::internal I ios::showpos); Contains two status flags, with "|" Combination

4) can be seen: control of the output format can be used either as a control (example 13.2), or with the cout flow of the relevant member functions (such as example 13.3), the role of the two are the same. The control character is defined in the header file Iomanip, so you must include the Iomanip header file when you use the control. The member functions of the cout stream are defined in the header file iostream, so only the header file iostream is included, not iomanip. Many program personnel feel that it is easy to use a control character and can use multiple controls consecutively in a cout output statement. Reproduced from the network

C + + formatted output, C + + output format control