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CSE 20232 Lecture 7 – Loops, Files & More

CSE 20232 Lecture 7 – Loops, Files & More. Loop Termination By Count Sentinel Value End of Input File Operations with streams ifstream, ofstream, open(), close(), >>, << Linux Piping and I/O redirection |, <, >, >>. Later this week …. Command Line Arguments “C” style I/O

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CSE 20232 Lecture 7 – Loops, Files & More

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  1. CSE 20232Lecture 7 – Loops, Files & More • Loop Termination • By Count • Sentinel Value • End of Input • File Operations with streams • ifstream, ofstream, open(), close(), >>, << • Linux Piping and I/O redirection • |, <, >, >>

  2. Later this week … • Command Line Arguments • “C” style I/O • printf(…), scanf(…), fprinf(…), fscanf(…)

  3. Reading (same as last time) • All previously assigned PLUS … • Control Structures • Sections 5.1-5.10 • I/O Streams • Sections 15.3-15.5, 15.7-15.8 • File Streams & Sequential files • Sections 17.3-17.5

  4. Simple avg Program • For today we will use variations of a simple grade averaging program • avgN.cpp - Loop termination by count • User provided # of values and then the values themselves • avgS.cpp – Loop termination by sentinel value • User provides numbers followed by a sentinel value • The sentinel is outside the normal data value range • avgE.cpp – Loop termination by end of input detection • User provides numbers followed by <ctrl-d>

  5. Program “avgN.cpp” // file: avgN.cpp // JHS (2006) – CSE 20232 // Average n non negative integer values (loop terminates by count) #include <iostream> using namespace std; int main () { int n, value, sum(0); // initialize sum to 0 cout << “How many values will be entered? “; cin >> n; for (int i=0; i<n; i++) { cout << “Enter value [“ << i << “] “; cin >> value; sum = sum + value; } if (n == 0) cout << “No values were averaged!” << endl; else cout << “Average of the “ << n << “ values is “ << (sum / n) << endl; return 0; }

  6. Using avgN(user inputs underlined) How many values will be entered? 5 Enter value [0] 10 Enter value [1] 20 Enter value [2] 30 Enter value [3] 40 Enter value [4] 50 Average of the 5 values is 30

  7. Program “avgS.cpp” // file: avgS.cpp // JHS (2006) – CSE 20232 // Average n non negative integer values (loop terminates by sentinel -99) #include <iostream> using namespace std; int main () { int n(0), value, sum(0); // initialize n and sum to 0 cout << “Enter non negative values to be averaged.“ << endl; cout << “Enter value (-99 when done): “; cin >> value; while (value != -99) { n = n + 1; sum = sum + value; cout << “Enter value (-99 when done): “; cin >> value; } if (n == 0) cout << “No values were averaged!” << endl; else cout << “Average of the “ << n << “ values is “ << (sum / n) << endl; return 0; }

  8. Using avgS(user inputs underlined) Enter non negative values to be averaged. Enter value (-99 when done): 10 Enter value (-99 when done): 20 Enter value (-99 when done): 30 Enter value (-99 when done): 40 Enter value (-99 when done): 50 Enter value (-99 when done): -99 Average of the 5 values is 30

  9. Program “avgE.cpp” // file: avgE.cpp // JHS (2006) – CSE 20232 // Average n non negative integer values (loop terminates on end of input) #include <iostream> using namespace std; int main () { int n(0), value, sum(0); // initialize n and sum to 0 cout << “Enter non negative values to be averaged.“ << endl; cout << “Enter value (<ctl-d> when done): “; while (cin >> value) { n = n + 1; sum = sum + value; cout << “Enter value (<ctl-d> when done): “; } cout << endl; // since <ctl-d> does not move us down the screen if (n == 0) cout << “No values were averaged!” << endl; else cout << “Average of the “ << n << “ values is “ << (sum / n) << endl; return 0; }

  10. Using avgE(user inputs underlined) Enter non negative values to be averaged. Enter value (<ctl-d> when done):10 Enter value (<ctl-d> when done):20 Enter value (<ctl-d> when done):30 Enter value (<ctl-d> when done):40 Enter value (<ctl-d> when done):50 Enter value (<ctl-d> when done): Average of the 5 values is 30 NOTE: <ctl-d> was entered at last prompt.

  11. Why use files? • Files save data and results for future use • Files continue to exist when computer is OFF • Files may be shared • User is not required to reenter data every time program is used • Files may be used to accumulate more data over time • Files may be used to take output from one program into another as input

  12. Files and File Streams • The two C++ file stream types used to access and manipulate sequential files are … • ifstream – the input file stream • ofstream – the output file stream • They are defined in the header <fstream>, so • #include <fstream>

  13. Declaring ifstream Objects and Opening Input Files • Declaring an ifstream and opening a file for input in one statement • ifstream infile(“valuesN.txt”); • Declaring an ifstream and opening a file in a later statement • ifstream infile; • infile.open(“valuesN.txt”);

  14. Declaring ifstream Objects and Opening Input Files • Declaring an ifstream, asking the user for a file name, and opening the requested file • string filename; • ifstream infile; • cout << “Enter filename: “; • cin >> filename; • infile.open(filename.c_str()); • note: c_str() function is used above to convert filename string object to the required C-style character array

  15. Reading from an Input File • Once the file is open (and associated with the input file stream) … • Use the stream just like you would use cin, except you will be getting input from the file • infile >> value; // get a value • ch = Infile.get(); // get a character • getline(infile,str); // get a line

  16. Opening an Output File • Declaring an ofstream and opening a file for output in one statement • ofstream outfile(“results.txt”); • Declaring an ofstream and opening a file in a later statement • ofstream outfile; • outfile.open(“results.txt”);

  17. Opening an Output File • Declaring an ofstream, asking the user for a file name, and opening the requested file • string filename; • ofstream outfile; • cout << “Enter filename: “; • cin >> filename; • outfile.open(filename.c_str()); • note: c_str() function is used above to convert filename string object to the required C-style character array

  18. Writing to an Output File • Once the file is open (and associated with the output file stream) … • Use the stream just like you would use cout, except you will be sending output to the file • outfile << value; // send a value • outfile.put(ch); // send a character • outfile << “This & that”; // send a line

  19. Simple avg Program (file input) • Now we will examine variations of a simple grade averaging program that uses a file for input • avgNf.cpp - Loop termination by count • User provided # of values and then the values themselves • avgSf.cpp – Loop termination by sentinel value • User provides numbers followed by a sentinel value • The sentinel is outside the normal data value range • avgEf.cpp – Loop termination by end of input detection • User provides numbers followed by <ctrl-d>

  20. Changes to average program • Obviously … • Files do not need to be prompted for input • And … • Somehow the program needs to find out the name of the file containing the data vales • Embed the name in the program, • Always assume the same file holds the data • Ask the user for the file name • Allow use of multiple data value files • We will do this for now …

  21. Example Data Files valuesN.txtvaluesS.txtvaluesE.txtvaluesERR.txt 10 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 5 5 5 6 6 xxx 6 7 7 6 7 8 8 7 8 9 9 8 9 10 10 9 10 -99 10 _ (counted) (sentinel) (end of file)(error)

  22. Program “avgNf.cpp” // file: avgNf.cpp // JHS (2006) – CSE 20232 // Average n non negative integer values (loop terminates by count) #include <iostream> #include <fstream> #include <string> using namespace std; int main () { ifstream infile; // here is our input file stream string filename; // and here is the name of the file on disk int n(0), value, sum(0); // initialize sum to 0 cout << “What is the name of the data file? “; cin >> filename; infile.open(filename.c_str()); // attempt to open the file if (infile.fail()) // we may or may not succeed { cout << “Could not open file:” << filename << endl; return 1; }

  23. Program “avgNf.cpp” // the file was successfully opened, so lets read from it infile >> n; // read the number of data values to follow for (int i=0; i<n; i++) // for each of the n values { infile >> value; // read the next data value cout << “Value [“ << i << “] is “ << value << endl; sum = sum + value; // add it to the sum } infile.close(); // close the file, we are done if (n == 0) cout << “No values were averaged!” << endl; else cout << “Average of the “ << n << “ values is “ << (sum / n) << endl; return 0; }

  24. Using avgNf(user inputs underlined) What is the name of the data file? valuesN.txt Value [0] is 1 Value [1] is 2 Value [2] is 3 Value [3] is 4 Value [4] is 5 Value [5] is 6 Value [6] is 7 Value [7] is 8 Value [8] is 9 Value [9] is 10 Average of the 10 values is 5

  25. Program “avgSf.cpp” // file: avgSf.cpp // JHS (2006) – CSE 20232 // Average n non negative integer values (loop terminates by sentinel -99) #include <iostream> #include <fstream> #include <string> using namespace std; int main () { ifstream infile; // here is our input file stream string filename; // and here is the name of the file on disk int n(0), value, sum(0); // initialize n and sum to 0 cout << “What is the name of the data file? “; cin >> filename; infile.open(filename.c_str()); // attempt to open the file if (infile.fail()) // we may or may not succeed { cout << “Could not open file:” << filename << endl; return 1; }

  26. Program “avgSf.cpp” // the file was successfully opened, so lets read from it infile >> value; // get first value while (value != -99) // for every value until sentinel { cout << “Value [“ << n << “] is “ << value << endl; n = n + 1; // count it sum = sum + value; // add value to sum infile >> value; // get next value } infile.close(); // close the file, we are done if (n == 0) cout << “No values were averaged!” << endl; else cout << “Average of the “ << n << “ values is “ << (sum / n) << endl; return 0; }

  27. Using avgSf(user inputs underlined) What is the name of the data file? valuesS.txt Value [0] is 1 Value [1] is 2 Value [2] is 3 Value [3] is 4 Value [4] is 5 Value [5] is 6 Value [6] is 7 Value [7] is 8 Value [8] is 9 Value [9] is 10 Average of the 10 values is 5

  28. Program “avgEf.cpp” // file: avgEf.cpp // JHS (2006) – CSE 20232 // Average n non negative integer values (loop terminates on end of input) #include <iostream> #include <fstream> #include <string> using namespace std; int main () { ifstream infile; // here is our input file stream string filename; // and here is the name of the file on disk int n(0), value, sum(0); // initialize n and sum to 0 cout << “What is the name of the data file? “; cin >> filename; infile.open(filename.c_str()); // attempt to open the file if (infile.fail()) // we may or may not succeed { cout << “Could not open file:” << filename << endl; return 1; }

  29. Program “avgEf.cpp” // the file was successfully opened, so lets read from it while (infile >> value) // get next value { cout << “Value [“ << n << “] is “ << value << endl; n = n + 1; // count it sum = sum + value; // add value to sum } infile.close(); // close the file, we are done if (n == 0) cout << “No values were averaged!” << endl; else cout << “Average of the “ << n << “ values is “ << (sum / n) << endl; return 0; }

  30. Using avgEf(user inputs underlined) What is the name of the data file? valuesE.txt Value [0] is 1 Value [1] is 2 Value [2] is 3 Value [3] is 4 Value [4] is 5 Value [5] is 6 Value [6] is 7 Value [7] is 8 Value [8] is 9 Value [9] is 10 Average of the 10 values is 5

  31. Warning about Stream Errors • If an error occurs while using a stream … • The stream is placed in an error state • All subsequent uses of the stream will be skipped as the program continues to run • To fix this and continue … • If a stream fails, always execute its clear() function to clear the error state, before using it again • Examples: • Failing to open a file, you must clear the ifstream before using it to try and open another • Failing to read some data, you must clear the ifstream and skip over the offending bad data before attempting to continue reading more data

  32. Errors in streams • The previous programs will stop accessing file “valuesERR.txt” when the “xxx” line is encountered • Run avgEf on “valuesERR.txt” to see • The next program (avgERRf.cpp) clears the stream, skips over the bad data, and continues

  33. Using avgEf with “valuesERR.txt”(user inputs underlined) What is the name of the data file? valuesERR.txt Value [0] is 1 Value [1] is 2 Value [2] is 3 Value [3] is 4 Value [4] is 5 Average of the 5 values is 3 Note: All data following “xxx” in the 6th line of the file was skipped due to the error state of stream

  34. Program “avgERRf.cpp” // file: avgERRf.cpp // JHS (2006) – CSE 20232 // Average n non negative integer values (loop terminates on end of input) // This copes with error in stream, that can be skipped (bad number) #include <iostream> #include <fstream> #include <string> using namespace std; int main () { ifstream infile; // here is our input file stream string filename; // and here is the name of the file on disk int n(0), value, sum(0); // initialize n and sum to 0 cout << "What is the name of the data file? "; cin >> filename; infile.open(filename.c_str()); // attempt to open the file if (infile.fail()) // we may or may not succeed { cout << "Could not open file:" << filename << endl; return 1; }

  35. Program “avgERRf.cpp” // the file was successfully opened, so lets read from it infile >> value; // get first value while (!infile.eof()) // get next value and ... { if (infile.fail()) { string junk; infile.clear(); // must have been a bad number format, skip it infile >> junk; // skip over junk (bad number) } else { cout << "Value [" << n << "] is " << value << endl; n = n + 1; // count it sum = sum + value; // add value to the sum } infile >> value; // get another value }

  36. Program “avgERRf.cpp” // finally done with reading data from the file infile.close(); // close the file, we are done if (n == 0) cout << "No values were averaged!" << endl; else cout << "Average of the " << n << " values is " << (sum / n) << endl; return 0; }

  37. Using avgERRf & “valuesERR.txt”(user inputs underlined) What is the name of the data file? valuesERR.txt Value [0] is 1 Value [1] is 2 Value [2] is 3 Value [3] is 4 Value [4] is 5 Value [5] is 6 Value [6] is 7 Value [7] is 8 Value [8] is 9 Value [9] is 10 Average of the 10 values is 5 Note: The “xxx” on the 6th line of the file was skipped and the rest of the data was processed.

  38. Summary • Three ways to terminate loops • By count • By sentinel • By end of input • Files are • Accessed using ifstream and ofstream objects • Opened at time of declaration or with open() • Values are extracted from the stream just like they are from cin • Stream access following an error will be skipped unless stream is cleared

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