c++ 标准输入输出流基础
有三个重要的对象
cin cout cerr
以及一些manipulator有的带参数，有的不带参数。
/*
* This program demonstrates various options for floating-point output
* by displaying three different constants (pi, the speed of light in meters/second,
* and the fine-structure constant). These constants
* are chosen because they illustrate a range of exponent scales.
*/
#include
#include
#include
using namespace std;
const double PI = 3.1415926535897;
const double SPEED_OF_LIGHT = 2.99792458e+8;
const double FINE_STRUCTURE = 7.2573525e-3;
void printPrecisionTable();
int main() {
cout << uppercase << right;
cout << "Default format:" << endl << endl;
printPrecisionTable();
cout << endl << "Fixed format:" << fixed << endl << endl;
printPrecisionTable();
cout << endl << "Scientific format:" << scientific << endl << endl;
printPrecisionTable();
return 0;
}
void printPrecisionTable() {
cout << "prec | pi | speed of light | fine-structure" << endl;
cout << "-------------------------------------------------------" << endl;
for (int prec = 0; prec <= 6; prec +=2) {
cout << setw(4) << prec << " |";
cout << " " << setw(12) << setprecision(prec) << PI << " |";
cout << " " << setw(16) << setprecision(prec) << SPEED_OF_LIGHT << " |";
cout << " " << setw(14) << setprecision(prec) << FINE_STRUCTURE << endl;
}
} Default format:
prec | pi | speed of light | fine-structure
-------------------------------------------------------
0 | 3 | 3E+08 | 0.007
2 | 3.1 | 3E+08 | 0.0073
4 | 3.142 | 2.998E+08 | 0.007257
6 | 3.14159 | 2.99792E+08 | 0.00725735
Fixed format:
prec | pi | speed of light | fine-structure
-------------------------------------------------------
0 | 3 | 299792458 | 0
2 | 3.14 | 299792458.00 | 0.01
4 | 3.1416 | 299792458.0000 | 0.0073
6 | 3.141593 | 299792458.000000 | 0.007257
Scientific format:
prec | pi | speed of light | fine-structure
-------------------------------------------------------
0 | 3E+00 | 3E+08 | 7E-03
2 | 3.14E+00 | 3.00E+08 | 7.26E-03
4 | 3.1416E+00 | 2.9979E+08 | 7.2574E-03
6 | 3.141593E+00 | 2.997925E+08 | 7.257352E-03
file操作
Reading or writing a file in C++ requires the following steps.
1. Declare a stream variable to refer to the file.
ifstream infile;
ofstream outfile;
2. Open the file.
infile.open("Jabberwocky.txt");If however, the name of the file is stored in a string variable named filename, you will need to open the file like this:
infile.open(filename.c_str());
3. Transfer the data.
4. Close the file.
infile.close();
单个字符操作
Input stream in the C++ library support reading a single charactor using a method called get, which exists in two forms.
char ch;
infile.get(ch);
The general pattern for reading all the characters in a file looks like this:
char ch;
while (infile.get(ch)) {
// perform some operation on the character.
}另外一种写法
while (true) {
int ch = infile.get();
if (ch == EOF) break;
}还有一种写法
int ch;
while ((ch = infile.get()) != EOF) {
// Perform some operation on the character.
}For output streams, the put method takes a char values as its argument and writes that character to the stream.
文本需要放到生成可执行程序相同的目录下，它是读取整个字符串，并没有解析的功能。
/*
* This program displays the contents of a file chosen by the user.
*/
#include
#include
#include
#include "console.h"
using namespace std;
string promptUserForFile(ifstream &infile, string prompt = "");
int main() {
ifstream infile;
promptUserForFile(infile, "Input file: ");
char ch;
while (infile.get(ch)) {
cout.put(ch);
}
infile.close();
return 0;
}
string promptUserForFile(ifstream &infile, string prompt) {
while (true) {
cout << prompt;
string filename;
getline(cin, filename);
infile.open(filename.c_str());
if (!infile.fail()) return filename;
infile.clear();
cout << "Unable to open that file. Try again." << endl;
if (prompt == "") prompt = "Input file: ";
}
} Input file: test.txt
Die gyre and gimble in the wabe;
All misy were the borogoves,
And the mome raths outgrabe.
The open call, for example, need to use c_str to convert the C++ string stored in filename to the old-style C string that the stream library requires. Similarly, the call to clear inside the while loop is necessary to ensure that the failure status indicator in the stream is reset before the user enters a new file name.
infile.unget();The effect of this call is to "push" the most recent character back into the input stream so that it is returned on the text call to get.
Calling
getline(infile, str);copies the next line of the file into the variable str, up to but not including the newline character that signals the end of the line.
string line;
while (getline(infile, line)) {
cout << line << endl;
}从文件中获取数值
int value;
while (infile >> value) {
total += value;
}The crux of the problem is that the extraction operator in the expression
infile >> value;
will set the failure indicator in either of two case.
1. Reaching the end of the file, at which point there are no more value to read.
2. Trying to read data from the file that cannot be convert to an integer.
可以进行错误检查
if (!infile.eof()) {
error("Data error in file");
}The error doesn't provide the user with much guidance as to the source of the data error, but at least it's better than nothing.
字符串流 string streams
The existence of the class makes it possible to implement the stringToInteger method described in the last section as follows:
int stringToInteger(string str) {
istringstream stream(str);
int value;
stream >> value >> ws;
if (stream.fail() || !stream.eof()) {
error("stringToInteger: Illegal integer format");
}
return value;
}The following function, for example, converts an integer into a string of decimal digits:
string integerToString(int n) [
ostringstream stream;
stream << n;
return stream.str();
}The most effective way to ensure that user input is valid is to read an entire line as a string and then convert that string to an integer.
int getInteger(sting prompt) {
int value;
string line;
while (true) {
cout << prompt;
getline(cin, line);
istringstream stream(line);
stream >> value >> ws;
if (!stream.fail() && stream.eof()) break;
cout << "Illegal integer format. Try again." << endl;
}
return value;
}Class hierarchies
类继承
封装和继承
Of these, the most important is that classes provide a framework for encapsulation, which is the process of combining the data representation and the associated operations into a coherent whole that reveals as few details as possible about the underlying structure.
Classes in an object-oriented language from a hierarchy in which each class automatically acquires the characteristics of the classes that precede it in the hierarchy.
The original system contained only the plant and animal kingdoms.
Each kingdom is then further broken down into the hierarchical categories of phylum, class, order, family, genus, and species.
The classes in the stream libraries form hierarchies that are in many ways similar to the biological introduced in the preceding section.
Thus, if the istream class exports a particular method, that method is automatically available to any ifstream or istringstream object.
This enhanced diagram adopts parts of a standard methodology for illustrating class hierarchies called the Universal Modeling Language, or UML for short.
Any ifstream object has access to the following methods:
The open and close methods from the ifstream class itself.
The get and upget methods and the >> operator from the istream class.
The clear, fail, and eof methods from the ios class.
void copyStream(istream &is, ostream &os) {
char ch;
while (is.get()) {
os.put(ch);
}
}
The advantage of the new coding is that you can use this version of copyStream with any stream types.
介绍stanford引用的两个库，
simpio.h 和 filelib.h 两个库。
the simpio.h interface you have already seen and a filelib.h interface for the method that are more closely related to files.
With the expansion of the web, programmers use online reference materials more often than printed ones.
In any event, reading .h file is a programming skill that you will need to cultivate if you want to become proficient in C++.
The
Review Question.
问答题