CS Class 13 Today  File I/O (reading from and writing to files)  Reading until EOF  Formatting output  Nested Loops Announcements  Programming project #4: will be posted by Thursday is due by midnight Oct. 23  Exam 2 is Oct. 28 during class  Read pages , 317 – 331 for next time

Input Overview Up until now  all reads from “standard input” using cin >> …  all writes to “standard output” using cout << … Want to expand this  Read from any input  Write to any output C++ input/output uses a “stream”  Flow of characters cin : data flows into pgm cout : data flows from pgm  Can also define streams of input/output with files

Reading/Writing to files Need an identifier to use when reading from (or writing to) a file Can’t use cin and cout, they go to standard input/output ofstream outFile; ifstream inFile; Need to associate this variable with an actual file outFile.open(“results.txt”); inFile.open(“data.txt”); Same general syntax when reading/writing outFile << “answer is :” << x << endl; inFile >> x >> y; Close this file once you are done outFile.close( ); inFile.close();

Examples #include using namespace std; int main( ) { int value, temp = 1; ifstream Alabama; Alabama.open("data.txt"); while (temp < 10) { Alabama >> value; cout << "Read = " << value << endl; temp = temp + 1; } Alabama.close( ); return 0; } #include using namespace std; int main( ) { int temp = 1; ofstream bigAL; bigAL.open("results.txt"); while (temp < 10) { bigAL << temp << endl; temp = temp + 1; } bigAL.close( ); return 0; }

In-Class Exercise Write a C++ program that will read the ten numbers shown below from the file data.txt and write the even numbers to the file even.txt and the write the odd numbers to the file odd.txt You can create the input file data.txt by either 1) using Notepad 2) File / Add New Item / Make sure your input data file is in the same directory as your C++ project.

Summary: File-based I/O For each file used, need a variable that links the C++ program with a specific external file C++ Program #include #include int main( ) { ifstream inFile; ofstream outFile; inFile.open(“data.txt”); outFile.open(“results.txt”); inFile >> … ; outFile << … ; data.txt results.txt

Reading input Up until now Either Knew exactly how many input values there were  Count-controlled loops Or Knew what the last item in the data looked like  Sentinel-controlled loops What if you don’t know when the data ends (and what the last item is) ? Read until EOF Terminology: read until “end-of-file” (EOF) Two solutions  let the “read” fail  check for end-of-file How does the program see the “EOF”?  Reading from file system detects the end of the file  Reading from cin input a CTRL-d or CTRL-z

Method #2: Test for EOF #include using namespace std; int main( ) { int a; cin >> a; while ( ! cin.eof( ) ) { cout > a; } cout << "done" << endl; return 0; } // signal EOF via CTRL-z Read until you hit EOF Method #1 : Let read fail #include using namespace std; int main( ) { int a; while ( cin >> a ) { cout << a << endl; } cout << "done" << endl; return 0; } // signal EOF via CTRL-d

Class Exercise Write a C++ program that reads integers from standard input until EOF, printing the total and the average value once the user enters EOF. Your average value should be a double. Modify your program to read from the file data.txt and output to the screen.

Formatting Output Program output is “presented” to the user. Want professional look  Alignment  justification All output streams can be formatted.  cout  File output streams Specify flags for presentation  Field width  Precision  Left justify  Right justify  Scientific notation

Formatting example e Comments  Width only for next output item  Will print entire number if field is too small int num0 = 16; int num1 = 1024; cout.width(5); cout << num0 << endl; cout << num1 << endl; cout.width(8); cout.setf(ios::right); cout << num0 << endl; cout << num1 << endl; cout.width(2); cout << num1 << endl; double num2 = ; cout.setf(ios::scientific); cout << num2 << endl; cout.setf(ios::fixed); cout << num2 << endl; cout.precision(2); cout << num2 << endl;

Rules for Formatting Just specify the flags needed to give you the desired format Can “unset” these flags later on (if needed) cout.unsetf(ios::showpos); Lots of flags exist. Examples  Print integers right- justified and always show the “+” sign cout.setf(ios::right); cout.setf(ios::showpos);  Print doubles using scientific notation cout.setf(ios::scientific);  Print next integer in a field of width 8 cout.width(8);

Basic formatting alternatives Integer Values  Show “+” w/positive num setf(ios::showpos)  Specifies width (only for the next value output) width(N)  left-justify output setf(ios::left)  right-justify output setf(ios::right) Real Values  Fixed decimal notation setf(ios::fixed)  Scientific notation setf(ios::scientific)  Show decimal point setf(ios::showpoint)  Show “+” w/positive num setf(ios::showpos)  Set decimal places precision(N)

Class Exercises – Trace then run int main( ) { int a = 5, loop = 3; while (loop <= 8) { cout.width(loop); cout.setf(ios::right); if (loop%2 == 0) cout.setf(ios::showpos); else cout.unsetf(ios::showpos); cout << a << endl; loop = loop + 1; a = a * 2; } return 0; } int main( ) { double z= ; int loop = 3; while (loop < 8) { cout.precision(loop); if (loop == 6) cout.setf(ios::left); cout.width(15-loop); cout << z << endl; loop = loop + 1; } return 0; }

Class Exercises Write a program that generates the output below: Write a program that generates the output below:

Nesting - Review Remember: If and while statements can be nested inside each other Can nest as deeply as needed by algorithm Can nest in any order

Example: multiplication table #include using namespace std; int main( ) { for (int a=1; a<6; a++) { for (int b=1; b<6; b++) { cout.width(4); cout << a*b; } cout << endl; } return 0; }

Example: powers, up to N N #include using namespace std; int main( ) { for (int a=1; a<6; a++) { for (int b=1; b<=a; b++) { cout.width(5); cout << pow(a,b); } cout << endl; } return 0; }

Write C++ programs that use nested for loops to display one of the following designs: XXXXXX XXXXXXXXXX XXXXX XXXXX XXXXXXXX X X XXXX XXXXXX X X XXX XXXX X X XX XX X X Class Exercise

End of Class 13