Lecture 7 - Sept 22, 2023

Summary

In this lecture, we discuss detecting and handling errors in the input from a file or user.

Last lecture

File I/O.

Today

Handling I/O errors.

Handling I/O errors

Input stream if stored in a buffer.
This buffer is only available when \n is entered.
cin ignores/skips delimiters or whitespaces.
Delimiters are , \t, \n.

Example

Remember the diagram from lecture 6.

Reading happens until a delimiter is seen or when something wrong happens! :::

Example

cin will read 13 into x, but it will not read ● as it is not part of an integer.
cin will look for an int for y, but will find ●.
cin will fail silently , y is unaffected, and the buffer is unaffected.
A fail flag is raised, and all other cin in the program will fail.

What should you do?

Check if the fail flag is raised.
If yes, handle the error.

Example

// file input

#include <fstream>

int main() {
    inputFile("myFile.txt"); // error 1
    int a,b;
    inputFile >> a >> b; // error 2 and 3
    return 0;
}

// standard input

#include <iostream>

int main() {
    int a;
    cin >> a; // error 2 and 3
    return 0;
}

What can go wrong?

Error 1. The file to be opened for input does not exist.

Error 2. The variable cannot be read.

Error 3. Reached the end of a file.

Example

Detecting a failure

// after ifstream inputFile("myFile.txt"); ...

if (inputFile.fail()) {
    cerr << "Cannot open file" << endl;
    return 1;
}

What is cerr? It is an output stream like cout. It is unbuffered unlike cout. This means that the output appears immediately on the console/terminal.

Why return 1? Any non-zero number signals an error.

What to do when a failure with input occurs?

cin.clear() will clear the failure condition so cin.fail() and cin.eof() are back to false.

cin.ignore(int n, char ch) will discard n characters or up to character ch, whichever comes first.

Example

For the following input, I want to ignore numbers before the space and read only the name.

1010 Asmita

cin.ignore(1000, ' ');
cin >> name;

Write a program that reads a number from the user. It is not a number, prompt the user again.

Pacha’s note

I added #include <iostream> and using namespace std here to make the code compile.

#include <iostream>

using namespace std;

int main() {
    int num = 0;
    cout << "Enter a number:" << endl;
    cin >> num;

    // if cin.fail() is true,
    // cin.ignore() will fail too
    while(cin.fail()) {
        // do not swtich the order
        // we must clear first
        cin.clear();
        cin.ignore(1000, '\n');
        cout << "Try again!" << endl;
        cin >> num;
    }

    cout << "The number entered is " << num;
    
    return 0;
}

Write a program that reads numbers from a file and prints their sum. If it reads a non-integer number, ignore it. Numbers are written on separate lines.

Pacha’s note

I added #include <iostream>, #include <fstream> and using namespace std here to make the code compile.

#include <iostream>
#include <fstream>

using namespace std;

int main() {
    ifstream inputFile("myFile.txt");

    int num = 0, sum = 0;

    while(!inFile.eof()) {
        inFile >> num;
        if (inFile.fail()) {
            inFile.clear();
            inFile.ignore(1000, '\n');
        } else {
            sum += num;
        }
    }

    cout << "Reached end of file" << endl;
    cout << "The sum is " << sum << endl;
    return 0;
}