1. 1 CS 105 Lecture 9 Files Version of Mon, Mar 28, 2011, 3:13 pm

2. 2 Files Data in main memory is “volatile” File: Place for “permanent” data storage C: drive, A: drive, flash drive, etc. In C++, files can be read different ways Stream input (or output) lets us read (or write) a sequence of data. We also use streams (cin, cout) to model keyboard input and text output.

3. 3 In C++, the programmer can declare output streams and link them to output files. Output File Streams #include using namespace std;.... int num1, num2, num3; cout << "Enter 3 numbers for datafile: "; cin >> num1 >> num2 >> num3; ofstream outputStream; outputStream.open("datafile.txt"); outputStream << num1 << " "; outputStream << num2 << " "; outputStream << num3 << endl; outputStream.close();

4. 4 In C++, the programmer can declare input streams and link them to input files. Input File Streams #include using namespace std;.... int num1, num2, num3; ifstream inputStream; inputStream.open("datafile.txt"); inputStream >> num1 >> num2 >> num3; inputStream.close();

5. 5 To Read or Write from a File Declare the file stream object. I.e., give it a name. Associate a file name with the file stream object by opening the file. Read or write to the file using > operators. Close the file.

6. 6 Manipulators Used to format output neatly: Left justification (columns of names) Right justification (columns of numbers) Minimum field length (padding) Decimal places For more information, Malik pgs. 135-145 There's also an older style of formatting that uses "printf" and "scanf" that has been copied into Java.

7. 7 Example of Output Manipulation #include using namespace std; int main() { ifstream input; int loops, ival, i; float fval; string name; cout << showpoint << scientific; cout << setprecision(2); input.open("mydata.txt"); input >> loops; for (i=0; i < loops; i++) { input >> ival; input >> fval; input >> name; cout << right; cout << setw(4) << ival << " "; cout << setw(12) << fval << " "; cout << left; cout << setw(10) << name << endl; } return(0); } Output 8 9.30e+000 Jon 6 1.43e+001 Bill 0 3.57e+010 Mary -23 -4.55e+000 Smith -3 -4.00e+003 xyz

8. 8 Example with File Input #include using namespace std; int main() { ifstream input; int loops, ival, i; float fval; string name; cout << showpoint << scientific << setprecision(2); input.open("mydata.txt"); input >> loops; for (i=0; i < loops; i++) { input >> ival; input >> fval; input >> name; cout << right; cout << setw(4) << ival << " "; cout << setw(12) << fval << " "; cout << left; cout << setw(10) << name << endl; } return(0); } mydata.txt file: 5 8 9.3 Jon 6 14.335 Bill 0 35.67e9 Mary -23 -4.55 Smith -3 -4e3 xyz Output: 8 9.30e+00 Jon 6 1.43e+01 Bill 0 3.57e+10 Mary -23 -4.55e+00 Smith -3 -4.00e+03 xyz