Ch. 10 Input/Output

Ch. 10 Input/Output Oregon State University Timothy Budd Ch 10. Input/Output

Introduction • Two competing I/O systems in C++. • Standard I/O Library,stdio • Stream I/O Library, iostream • Never use both Library in the same program. Ch 10. Input/Output

The stdio Library • Inherited from the C. • Widely known and available. • Based on stable and well-exercised libraries. • Not as extendable or adaptable as the newer Stream I/O Library. • For developing new programs, the Stream I/O Library is preferred. Ch 10. Input/Output

Examples of stdio # include <cstdio> int c = getchar(); // read a single input character putchar('x'); // print a single character char * text = "please enter your name:";puts(text); // print the text stringchar buffer[120];gets(buffer); // read a line of input FILE * fp = fopen("mydata.dat", "r");if (fp == NULL)puts("file cannot be opened"); fputc('z', fp); // write character to fpint c = fgetc(fp); // read character from fpchar * msg = "unrecoverable program error";fputs(msg, stderr); // write message to standard error output Ch 10. Input/Output

Formatted Output • The printf facility works only for formatted primitive values, such as integers and floats. • Not easily extendable to user-defined data types. • Always verify that formatting commands match the argument types. Ch 10. Input/Output

Examples of Formatted Output %d integer decimal value %o integer printed as octal %x integer printed as hexadecimal %c integer printed as character %u unsigned integer decimal value %f floating point value %g floating point value %s null terminated string value %% percent sign Ch 10. Input/Output

int i = 3; int j = 7; double d = i / (double) j; printf("the value of %d over %d is %g", i, j, d); char * fileName = ...; if (fopen(fileName, "r") == null) fprintf(stderr,"Cannot open file %s\n", fileName); char buffer[180]; sprintf(buffer, "the value is %d\n", sum); double d = 23.5; printf("the value is %d\n", d); // error -- float printed as int scanf("%d %f", &i, &f); // read an int, then a float Ch 10. Input/Output

The Stream I/O Facility • Whenever possible use the Stream I/O Library rather than the Standard I/O Library. • Use the ability to overload function names in C++. • Better possibilities for extendability as well as improved error detection. Ch 10. Input/Output

Figure 10.2 Various Overloading Versions of the << Operator ostream & operator << (ostream & out, const int value) { // print signed integer values on a stream unsigned int usvalue; if (value < 0) { // print leading minus sign out << '-'; usvalue = - value; }else usvalue = value; // print non-negative number out << usvalue; return out; } Ch 10. Input/Output

inline char digitCharacter(unsigned int value) { // convert non-negative integer digit into printable digit // assume value is less than nine return value + '0'; } ostream & operator << (ostream & out, const unsigned int value) { // print unsigned integer values on a stream if (value < 10) out << digitCharacter(value); else { out << (value / 10); // recursive call out << digitCharacter(value % 10); // print single char } return out; } Ch 10. Input/Output

Examples of Stream I/O # include <iostream> cout << "n " << n << " m " << m << " average " << (n+m)/2.0 << '\n'; • Easy to provide formatting capabilities for a new data type. ostream & operator << (ostream & out, const rational & value) { // print representation of rational number on an output stream out << value.numerator() << '/' << value.denominator(); return out; } rational frac(3,4); cout << "fraction of " << 3 << " and " << 4 << " is “ << frac << endl; Ch 10. Input/Output

Another Examples • A manipulator is used to change features of the I/O system. ostream & endl (ostream & out) {// write the end of line characterout << '\n';// then flush the bufferout.fflush();// then return the bufferreturn out; } ostream & operator << (ostream & out, ostream & (*fun)(ostream &)) {// simply execute functionreturn fun (out); } Ch 10. Input/Output

Stream Input • Stream input always ignores white space. cin >> intval; while (cin >> intval) { // process intval ...}// reach this point on end of input... • Visualize >> and << as arrows • Input operator, >> x : points data into x • Output operator, << x : copies data out of x Ch 10. Input/Output

String Streams • A string stream writes to or reads from a string. • Must include the sstream header file. • ostringstream creates an output stream. • Values are buffered in an internal string and can be accessed with the member function str. • Useful for formatting complex output; not only perform catenation, but also automatically convert the right argument from numerous primitive data types into character values. • Can be used as the string catenation operator + commonly used in Java Ch 10. Input/Output

# include <sstream> int n = ...; int m = ...; ostringstream formatter; formatter << "the average of " << n << " and " << m << " is " << ((n + m)/2.0); string s = formatter.str(); string text = "Isn't this a wonderful feature"; istringstream breaker(text); string word; while (breaker >> word)cout << word << endl; // print each word on a separate line Ch 10. Input/Output

File Streams • A file stream is a stream that reads from or writes to a file. • Must include the header file # include <fstream> • fstream header file includes the iostream header, so not necessary to include both. • The class ifstream and ofstream are used to creat streams that are attached to input and output files. Ch 10. Input/Output

A conversion operator changes a file stream into a boolean value, whereby the value indicates the success or failure of the file opening operation. char fileName = "outfile.dat"; ofstream ofd(fileName); // create file for output if (! ofd) { cerr << " cannot open file " << fileName } else { ofd << "first line in file" ... } • File operations in C++ throw far fewer exceptions than do in Java. Ch 10. Input/Output

An Example Program # include <iostream> # include <string> int main() {int wordCount = 0;double totalWordLen = 0;double totalSenCount = 0;string word;while (cin >> word) { int wordLen = word.length(); wordCount++; if (word[wordLen-1] == '.') { totalWordLen += (wordLen-1); totalSenCount++; } else { totalWordLen += wordLen; }} Ch 10. Input/Output

cout << "total number of words " << wordCount << endl;if (wordCount > 0)cout << "average word length " << totalWordLen / wordCount << endl;cout << "total number of sentences " << totalSenCount << endl; if (totalSenCount > 0) cout << "average sentence length " << wordCount / totalSenCount << endl;return 0; } Ch 10. Input/Output

Ch. 10 Input/Output