C++ Input and Output


title: Input and Output

Input and Output with Streams

To print things to the console, or read from it, you use cout and cin, which are so-called streams. This metaphor is used because you use streams like you would use a sink, or a tap: you either flush data into a sink (cout), or get data out of a tap (cin).

Output with cout

The “Hello World” program uses cout to print “Hello World!” to the console:

#include<iostream> using namespace std; int main() { cout << "Hello world!" << endl; }

The first two lines at the top are necessary for you to use cout and other streams. #include<iostream> makes the stream objects available, and using namespace std; lets you type cout directly instead of having to type std::cout, that is, having to specify that we want to use cout from the std namespace.

cout stands for “Console Output”, and is a so-called output stream that represents the console. When you want to print something to the console, you can put it into cout; imagine it as a hole that leads to the terminal. To put things into this hole, one at a time, you use the << operator, a.k.a. the insertion operator1. The operator can be chained, that is, you can put several things in one after the other. The following will print “The cake is a lie.”:

cout << "The cake " << "is " << "a " << "lie." << endl;

endl stands for “End Line” and is another item that comes from <iostream>. When you put endl into cout, it will print a newline character (“\n”) to the console, and also flush cout, which means that it will force cout to print everything you have put into it right now. If you don’t put endl into cout, cout might keep the data you’ve put into it but wait for more data before actually printing it all. This is called buffering and is very good for performance, but if you’ve already given it everything it’s supposed to print, you want cout to print it immediately. Therefore it is very good practice to end with endl in places where it makes sense.

Almost everything can be put into a stream: strings, numbers, variables, expressions, etc. Here area some examples of valid stream insertions:

// Notice we can use the number 42 and not the string "42". cout << "The meaning of life is " << 42 << endl;` // Output: The meaning of life is 42
string name = "Tim"; cout << "Except for you, " << name << endl;`// Output: Except for you, Tim
string name = "Tim"; cout << name; cout << " is a great guy!" << endl;` // Output: Tim is a great guy!
int a = 3; cout << a*2 + 18/a << endl;`// Output: 12

A note about whitespace

C++ always puts you in control, and only does exactly the things you tell it to do. This can sometimes be surprising, as in the following example:

string name = "Sarah"; cout << "Good morning" << name << "how are you today? << endl;

You might expect it to print “Good morning Sarah how are you today?”, but actually, the output would be “Good morningSarahhow are you today?”. The reason for this bug is that you did not write spaces in the strings surrounding name, and so, since you didn’t specify any spaces, cout didn’t print any. The correct version would be: cout << "Good morning " << name << " how are you today? << endl;.

Line breaks don’t happen by themselves, either. You might think this would print a recipe on four lines:

cout << "To make bread, you need:"; cout << "* One egg"; cout << "* One water"; cout << "* Two wheat";

but the output is actually all on one line: “To make bread, you need:* One egg* One water* Two wheat”. This is because there are no newline characters at the end of the lines, so naturally, C++ assumes we don’t want it to print newline characters.

You could fix this by adding endls after every line, because as discussed earlier, endl inserts a newline character into the output stream. However, it also forces the buffer to be flushed, which loses us a little performance since we could have printed all the lines in one go. Therefore the best would be to add actual newline characters at the end of the lines, and only use endl at the end:

cout << "To make bread, you need:\n"; cout << "* One egg\n"; cout << "* One water\n"; cout << "* Two wheat" << endl;

If you’re just printing a small recipe, the time you save is miniscule and not worth the hassle, but if you’re printing millions of items, the difference could be very noticeable.

Input with cin

To read from the console, you use the input stream cin in the same way as you would cout, but instead of putting things into cin, you “take them out”. The following program reads two numbers from the user and adds them together:

#include<iostream> using namespace std; int main() { int a, b; // Variables to hold the two numbers. cout << "Please enter the first number:" << endl; cin >> a; cout << "Please enter the second number:" << endl; cin >> b; cout << "The sum of your numbers is: " << a + b << endl; return 0; }

cin stands for “Console Input” and is an input stream that represents input from the console. In the expression cin >> a;, data is read from cin and saved into the variable a, using the operator >>, the extraction operator2. The extraction operator reads exactly as much data as required to write into the variable we specify, and skips any whitespace, so if the user types “6” that will just be read as the value 6.

It’s worth noting that cin will stop the whole program to wait for the user to type in their value. The program will not continue until the user has pressed enter, and there is some data to be written into the variable. If the user just presses enter without typing anything, cin will keep waiting for a value.

The extraction operator << can be chained too. Here is the same program as last time, but written in a more concise manner:

#include<iostream> using namespace std; int main() { int a, b; // Variables to hold the two numbers. cout << "Please enter two numbers:" << endl; cin >> a >> b; cout << "The sum of your numbers is: " << a + b << endl; }

When chained, the extraction operator will first read data from cin to fill a, and then read data to fill b.

C’s standard printf and scanf statements can also be used with c++ by importing ” header file.

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