Programming Tutorials

cin.ignore() in C++

By: Grenfel in C++ Tutorials on 2007-09-17  

At times you want to ignore the remaining characters on a line until you hit either end of line (EOL) or end of file (EOF). The member function ignore() serves this purpose. ignore() takes two parameters, the maximum number of characters to ignore and the termination character. If you write ignore(80,'\n'), up to 80 characters will be thrown away until a newline character is found. The newline is then thrown away and the ignore() statement ends. Listing below illustrates the use of ignore().

Using ignore().

1:     // ignore()
2:     #include <iostream.h>
4:     int main()
5:     {
6:        char stringOne[255];
7:        char stringTwo[255];
9:        cout << "Enter string one:";
10:       cin.get(stringOne,255);
11:       cout << "String one" << stringOne << endl;
13:       cout << "Enter string two: ";
14:       cin.getline(stringTwo,255);
15:       cout << "String two: " << stringTwo << endl;
17:       cout << "\n\nNow try again...\n";
19:       cout << "Enter string one: ";
20:       cin.get(stringOne,255);
21:       cout << "String one: " << stringOne<< endl;
23:       cin.ignore(255,'\n');
25:       cout << "Enter string two: ";
26:       cin.getline(stringTwo,255);
27:       cout << "String Two: " << stringTwo<< endl;
28:     return 0;
29: }

Output: Enter string one:once upon a time
String oneonce upon a time
Enter string two: String two:

Now try again...
Enter string one: once upon a time
String one: once upon a time
Enter string two: there was a
String Two: there was a

Analysis: On lines 6 and 7, two character arrays are created. On line 9, the user is prompted for input and types once upon a time, followed by Enter. On line 10, get() is used to read this string. get() fills stringOne and terminates on the newline, but leaves the newline character in the input buffer.

On line 13, the user is prompted again, but the getline() on line 14 reads the newline that is already in the buffer and terminates immediately, before the user can enter any input.

On line 19, the user is prompted again and puts in the same first line of input. This time, however, on line 23, ignore() is used to "eat" the newline character. Thus, when the getline() call on line 26 is reached, the input buffer is empty, and the user can input the next line of the story.

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