Customizing I/O. (Chapter 11)

Август 8, 2022

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Customizing I/O. (Chapter 11)

Содержание

2. Overview Input and output Numeric output Integer Floating point File modes Binary I/O Positioning String streams
3. Kinds of I/O Individual values See Chapters 4, 10 Streams See Chapters 10-11 Graphics and GUI
4. Observation As programmers we prefer regularity and simplicity But, our job is to meet people’s expectations
5. Output formats Integer values 1234 (decimal) 2322 (octal) 4d2 (hexadecimal) Floating point values 1234.57 (general) 1.2345678e+03
6. Numerical Base Output You can change “base” Base 10 == decimal; digits: 0 1 2 3
7. “Sticky” Manipulators You can change “base” Base 10 == decimal; digits: 0 1 2 3 4
8. Other Manipulators You can change “base” Base 10 == decimal; digits: 0 1 2 3 4
9. Floating-point Manipulators You can change floating-point output format defaultfloat – iostream chooses best format using n
10. Precision Manipulator Precision (the default is 6) defaultfloat – precision is the number of digits scientific
11. Output field width A width is the number of characters to be used for the next
12. Observation This kind of detail is what you need textbooks, manuals, references, online support, etc. for
13. A file At the fundamental level, a file is a sequence of bytes numbered from 0
14. File open modes By default, an ifstream opens its file for reading By default, an ofstream
15. Text vs. binary files In binary files, we use sizes to delimit values In text files,
16. Text vs. binary Use text when you can You can read it (without a fancy program)
17. Binary files int main() // use binary input and output { cout string iname; cin >>
18. Binary files vector v; // read from binary file: for (int i; ifs.read(as_bytes(i),sizeof(int)); ) // note:
19. Positioning in a filestream fstream fs {name}; // open for input and output // … fs.seekg(5);
20. Positioning Whenever you can Use simple streaming Streams/streaming is a very powerful metaphor Write most of
21. String streams A stringstream reads/writes from/to a string rather than a file or a keyboard/screen double
22. String streams See textbook for ostringstream String streams are very useful for formatting into a fixed-sized
23. Type vs. line Read a string string name; cin >> name; // input: Dennis Ritchie cout
24. Characters You can also read individual characters for (char ch; cin>>ch; ) { // read into
25. Character classification functions If you use character input, you often need one or more of these
26. Line-oriented input Prefer >> to getline() i.e. avoid line-oriented input when you can People often use
27. C++14 Binary literals 0b1010100100000011 Digit separators 0b1010'1001'0000'0011 Can also be used for for decimal, octal, and
29. Скачать презентацию

Слайд 2

Overview
Input and output
Numeric output
Integer
Floating point
File modes
Binary I/O
Positioning
String streams
Line-oriented input
Character input
Character classification
Stroustrup/Programming/2015

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Kinds of I/O
Individual values
See Chapters 4, 10
Streams
See Chapters 10-11
Graphics and GUI
See

Chapters 12-16
Text
Type driven, formatted
Line oriented
Individual characters
Numeric
Integer
Floating point
User-defined types

Stroustrup/Programming/2015

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Observation
As programmers we prefer regularity and simplicity
But, our job is to

meet people’s expectations
People are very fussy/particular/picky about the way their output looks
They often have good reasons to be
Convention/tradition rules
What does 110 mean?
What does 123,456 mean?
What does (123) mean?
The world (of output formats) is weirder than you could possibly imagine

Stroustrup/Programming/2015

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Output formats
Integer values
1234 (decimal)
2322 (octal)
4d2 (hexadecimal)
Floating point values
1234.57 (general)
1.2345678e+03 (scientific)
1234.567890 (fixed)
Precision (for floating-point values)
1234.57 (precision 6)
1234.6 (precision 5)
Fields
|12| (default for

| followed by 12 followed by |)
| 12| (12 in a field of 4 characters)

Stroustrup/Programming/2015

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Numerical Base Output
You can change “base”
Base 10 == decimal; digits: 0

1 2 3 4 5 6 7 8 9
Base 8 == octal; digits: 0 1 2 3 4 5 6 7
Base 16 == hexadecimal; digits: 0 1 2 3 4 5 6 7 8 9 a b c d e f
// simple test:
cout << dec << 1234 << "\t(decimal)\n"
<< hex << 1234 << "\t(hexadecimal)\n"
<< oct << 1234 << "\t(octal)\n";
// The '\t' character is “tab” (short for “tabulation character”)
// results:
1234 (decimal)
4d2 (hexadecimal)
2322 (octal)

Stroustrup/Programming/2015

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“Sticky” Manipulators
You can change “base”
Base 10 == decimal; digits: 0

1 2 3 4 5 6 7 8 9
Base 8 == octal; digits: 0 1 2 3 4 5 6 7
Base 16 == hexadecimal; digits: 0 1 2 3 4 5 6 7 8 9 a b c d e f
// simple test:
cout << 1234 << '\t'
<< hex << 1234 << '\t'
<< oct << 1234 << '\n';
cout << 1234 << '\n'; // the octal base is still in effect
// results:
1234 4d2 2322
2322

Stroustrup/Programming/2015

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Other Manipulators
You can change “base”
Base 10 == decimal; digits: 0 1

2 3 4 5 6 7 8 9
Base 8 == octal; digits: 0 1 2 3 4 5 6 7
Base 16 == hexadecimal; digits: 0 1 2 3 4 5 6 7 8 9 a b c d e f
// simple test:
cout << 1234 << '\t'
<< hex << 1234 << '\t'
<< oct << 1234 << endl; // '\n'
cout << showbase << dec; // show bases
cout << 1234 << '\t'
<< hex << 1234 << '\t'
<< oct << 1234 << '\n';
// results:
1234 4d2 2322
1234 0x4d2 02322

Stroustrup/Programming/2015

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Floating-point Manipulators
You can change floating-point output format
defaultfloat – iostream chooses best

format using n digits (this is the default)
scientific – one digit before the decimal point plus exponent; n digits after .
fixed – no exponent; n digits after the decimal point
// simple test:
cout << 1234.56789 << "\t\t(defaultfloat)\n" // \t\t to line up columns
<< fixed << 1234.56789 << "\t(fixed)\n"
<< scientific << 1234.56789 << "\t(scientific)\n";
// results:
1234.57 (defaultfloat)
1234.567890 (fixed)
1.234568e+03 (scientific)

Stroustrup/Programming/2015

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Precision Manipulator
Precision (the default is 6)
defaultfloat – precision is the number

of digits
scientific – precision is the number of digits after the . (dot)
fixed – precision is the number of digits after the . (dot)
// example:
cout << 1234.56789 << '\t' << fixed << 1234.56789 << '\t'
<< scientific << 1234.56789 << '\n';
cout << general << setprecision(5)
<< 1234.56789 << '\t' << fixed << 1234.56789 << '\t'
<< scientific << 1234.56789 << '\n';
cout << general << setprecision(8)
<< 1234.56789 << '\t' << fixed << 1234.56789 << '\t'
<< scientific << 1234.56789 << '\n';
// results (note the rounding):
1234.57 1234.567890 1.234568e+03
1234.6 1234.56789 1.23457e+03
1234.5679 1234.56789000 1.23456789e+03

Stroustrup/Programming/2015

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Output field width
A width is the number of characters to be

Stroustrup/Programming/2015

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Observation
This kind of detail is what you need textbooks, manuals, references,

online support, etc. for
You always forget some of the details when you need them

Stroustrup/Programming/2015

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A file
At the fundamental level, a file is a sequence of

bytes numbered from 0 upwards
Other notions can be supplied by programs that interpret a “file format”
For example, the 6 bytes "123.45" might be interpreted as the floating-point number 123.45

Stroustrup/Programming/2015

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File open modes
By default, an ifstream opens its file for reading

By default, an ofstream opens its file for writing.
Alternatives:
ios_base::app // append (i.e., output adds to the end of the file)
ios_base::ate // “at end” (open and seek to end)
ios_base::binary // binary mode – beware of system specific behavior
ios_base::in // for reading
ios_base::out // for writing
ios_base::trunc // truncate file to 0-length
A file mode is optionally specified after the name of the file:
ofstream of1 {name1}; // defaults to ios_base::out
ifstream if1 {name2}; // defaults to ios_base::in
ofstream ofs {name, ios_base::app}; // append rather than overwrite
fstream fs {"myfile", ios_base::in|ios_base::out}; // both in and out

Stroustrup/Programming/2015

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Text vs. binary files
In binary files, we use sizes to delimit

values
In text files, we use separation/termination characters

Stroustrup/Programming/2015

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Text vs. binary
Use text when you can
You can read it (without

a fancy program)
You can debug your programs more easily
Text is portable across different systems
Most information can be represented reasonably as text
Use binary when you must
E.g. image files, sound files

Stroustrup/Programming/2015

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Binary files
int main() // use binary input and output
{
cout << "Please enter

input file name\n";
string iname;
cin >> iname;
ifstream ifs {iname,ios_base::binary}; // note: binary
if (!ifs) error("can't open input file ", iname);
cout << "Please enter output file name\n";
string oname;
cin >> oname;
ofstream ofs {oname,ios_base::binary}; // note: binary
if (!ofs) error("can't open output file ", oname);
// “binary” tells the stream not to try anything clever with the bytes

Stroustrup/Programming/2015

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Binary files
vector v;
// read from binary file:
for (int i; ifs.read(as_bytes(i),sizeof(int)); ) //

note: reading bytes
v.push_back(i);
// … do something with v …
// write to binary file:
for(int i=0; i ofs.write(as_bytes(v[i]),sizeof(int)); // note: writing bytes
return 0;
}
// For now, treat as_bytes() as a primitive
// Warning! Beware transferring between different systems

Stroustrup/Programming/2015

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Positioning in a filestream
fstream fs {name}; // open for input and output
//

…
fs.seekg(5); // move reading position (‘g’ for ‘get’) to 5 (the 6th character)
char ch;
fs>>ch; // read the x and increment the reading position to 6
cout << "sixth character is " << ch << '(' << int(ch) << ")\n";
fs.seekp(1); // move writing position (‘p’ for ‘put’) to 1 (the 2nd character)
fs<<'y'; // write and increment writing position to 2

Stroustrup/Programming/2015

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Positioning
Whenever you can
Use simple streaming
Streams/streaming is a very powerful metaphor
Write most

of your code in terms of “plain” istream and ostream
Positioning is far more error-prone
Handling of the end of file position is system dependent and basically unchecked

Stroustrup/Programming/2015

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String streams
A stringstream reads/writes from/to a string
rather than a file or

a keyboard/screen
double str_to_double(string s)
// if possible, convert characters in s to floating-point value
{
istringstream is {s}; // make a stream so that we can read from s
double d;
is >> d;
if (!is) error("double format error: “,s);
return d;
}
double d1 = str_to_double("12.4"); // testing
double d2 = str_to_double("1.34e-3");
double d3 = str_to_double("twelve point three"); // will call error()

Stroustrup/Programming/2015

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String streams
See textbook for ostringstream
String streams are very useful for
formatting into

a fixed-sized space (think GUI)
for extracting typed objects out of a string

Stroustrup/Programming/2015

Слайд 23

Type vs. line
Read a string
string name;
cin >> name; // input: Dennis Ritchie
cout

<< name << '\n'; // output: Dennis
Read a line
string name;
getline(cin,name); // input: Dennis Ritchie
cout << name << '\n'; // output: Dennis Ritchie
// now what?
// maybe:
istringstream ss(name);
ss>>first_name;
ss>>second_name;

Stroustrup/Programming/2015

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Characters
You can also read individual characters
for (char ch; cin>>ch; ) { //

read into ch, skip whitespace characters
if (isalpha(ch)) {
// do something
}
}
for (char ch; cin.get(ch); ) { // read into ch, don’t skip whitespace characters
if (isspace(ch)) {
// do something
}
else if (isalpha(ch)) {
// do something else
}
}

Stroustrup/Programming/2015

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Character classification functions
If you use character input, you often need one

or more of these (from header ):
isspace(c) // is c whitespace? (' ', '\t', '\n', etc.)
isalpha(c) // is c a letter? ('a'..'z', 'A'..'Z') note: not '_'
isdigit(c) // is c a decimal digit? ('0'..'9')
isupper(c) // is c an upper case letter?
islower(c) // is c a lower case letter?
isalnum(c) // is c a letter or a decimal digit?
etc.

Stroustrup/Programming/2015

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Line-oriented input
Prefer >> to getline()
i.e. avoid line-oriented input when you can
People

often use getline() because they see no alternative
But it easily gets messy
When trying to use getline(), you often end up
using >> to parse the line from a stringstream
using get() to read individual characters

Stroustrup/Programming/2015

Слайд 27

C++14
Binary literals
0b1010100100000011
Digit separators
0b1010'1001'0000'0011
Can also be used for for decimal, octal, and

hexadecimal numbers
User-Defined Literals (UDLs) in the standard library
Time: 2h+10m+12s+123ms+3456ns
Complex: 2+4i

Stroustrup/Programming/2015

Customizing I/O. (Chapter 11)

Содержание

OverviewInput and outputNumeric outputIntegerFloating pointFile modesBinary I/OPositioningString streamsLine-oriented inputCharacter inputCharacter classificationStroustrup/Programming/2015

Kinds of I/OIndividual valuesSee Chapters 4, 10StreamsSee Chapters 10-11Graphics and GUISee

ObservationAs programmers we prefer regularity and simplicityBut, our job is to

Output formatsInteger values1234 (decimal)2322 (octal)4d2 (hexadecimal)Floating point values1234.57 (general)1.2345678e+03 (scientific)1234.567890 (fixed)Precision (for floating-point values)1234.57 (precision 6)1234.6 (precision 5)Fields|12| (default for

Numerical Base OutputYou can change “base”Base 10 == decimal; digits: 0

“Sticky” Manipulators You can change “base”Base 10 == decimal; digits: 0

Other ManipulatorsYou can change “base”Base 10 == decimal; digits: 0 1

Floating-point ManipulatorsYou can change floating-point output formatdefaultfloat – iostream chooses best

Precision ManipulatorPrecision (the default is 6)defaultfloat – precision is the number

Output field widthA width is the number of characters to be

ObservationThis kind of detail is what you need textbooks, manuals, references,

A fileAt the fundamental level, a file is a sequence of

File open modesBy default, an ifstream opens its file for reading

Text vs. binary filesIn binary files, we use sizes to delimit

Text vs. binaryUse text when you canYou can read it (without

Binary filesint main() // use binary input and output{ cout << "Please enter

Binary files vector v; // read from binary file: for (int i; ifs.read(as_bytes(i),sizeof(int)); ) //

Positioning in a filestreamfstream fs {name}; // open for input and output//

PositioningWhenever you canUse simple streamingStreams/streaming is a very powerful metaphorWrite most

String streamsA stringstream reads/writes from/to a stringrather than a file or

String streamsSee textbook for ostringstreamString streams are very useful forformatting into

Type vs. lineRead a stringstring name;cin >> name; // input: Dennis Ritchiecout

CharactersYou can also read individual charactersfor (char ch; cin>>ch; ) { //

Character classification functionsIf you use character input, you often need one

Line-oriented inputPrefer >> to getline()i.e. avoid line-oriented input when you canPeople

C++14Binary literals0b1010100100000011Digit separators0b1010'1001'0000'0011Can also be used for for decimal, octal, and

Похожие презентации

Overview
Input and output
Numeric output
Integer
Floating point
File modes
Binary I/O
Positioning
String streams
Line-oriented input
Character input
Character classification
Stroustrup/Programming/2015

Kinds of I/O
Individual values
See Chapters 4, 10
Streams
See Chapters 10-11
Graphics and GUI
See

Observation
As programmers we prefer regularity and simplicity
But, our job is to

Output formats
Integer values
1234 (decimal)
2322 (octal)
4d2 (hexadecimal)
Floating point values
1234.57 (general)
1.2345678e+03 (scientific)
1234.567890 (fixed)
Precision (for floating-point values)
1234.57 (precision 6)
1234.6 (precision 5)
Fields
|12| (default for

Numerical Base Output
You can change “base”
Base 10 == decimal; digits: 0

“Sticky” Manipulators
You can change “base”
Base 10 == decimal; digits: 0

Other Manipulators
You can change “base”
Base 10 == decimal; digits: 0 1

Floating-point Manipulators
You can change floating-point output format
defaultfloat – iostream chooses best

Precision Manipulator
Precision (the default is 6)
defaultfloat – precision is the number

Output field width
A width is the number of characters to be

Observation
This kind of detail is what you need textbooks, manuals, references,

A file
At the fundamental level, a file is a sequence of

File open modes
By default, an ifstream opens its file for reading

Text vs. binary files
In binary files, we use sizes to delimit

Text vs. binary
Use text when you can
You can read it (without

Binary files
int main() // use binary input and output
{
cout << "Please enter

Binary files
vector v;
// read from binary file:
for (int i; ifs.read(as_bytes(i),sizeof(int)); ) //

Positioning in a filestream
fstream fs {name}; // open for input and output
//

Positioning
Whenever you can
Use simple streaming
Streams/streaming is a very powerful metaphor
Write most

String streams
A stringstream reads/writes from/to a string
rather than a file or

String streams
See textbook for ostringstream
String streams are very useful for
formatting into

Type vs. line
Read a string
string name;
cin >> name; // input: Dennis Ritchie
cout

Characters
You can also read individual characters
for (char ch; cin>>ch; ) { //

Character classification functions
If you use character input, you often need one

Line-oriented input
Prefer >> to getline()
i.e. avoid line-oriented input when you can
People

C++14
Binary literals
0b1010100100000011
Digit separators
0b1010'1001'0000'0011
Can also be used for for decimal, octal, and