Basic Input/Output

Until now, the example programs of previous sections provided very little interaction with the user, if any at all. Using the standard input and output library, we will be able to interact with the user by printing messages on the screen and getting the user's input from the keyboard.

C++ uses a convenient abstraction called streams to perform input and output operations in sequential media such as the screen or the keyboard. A stream is an object where a program can either insert or extract characters to/from it. We do not really need to care about many specifications about the physical media associated with the stream - we only need to know it will accept or provide characters sequentially.

The standard C++ library includes the header file iostream, where the standard input and output stream objects are declared.

Standard Output (cout)

By default, the standard output of a program is the screen, and the C++ stream object defined to access it is cout.

cout is used in conjunction with the insertion operator, which is written as << (two "less than" signs).

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cout << "Output sentence"; // prints Output sentence on screen cout << 120; // prints number 120 on screen cout << x; // prints the content of x on screen

The << operator inserts the data that follows it into the stream preceding it. In the examples above it inserted the constant stringOutput sentence, the numerical constant 120 and variable x into the standard output stream cout. Notice that the sentence in the first instruction is enclosed between double quotes (") because it is a constant string of characters. Whenever we want to use constant strings of characters we must enclose them between double quotes (") so that they can be clearly distinguished from variable names. For example, these two sentences have very different results:

1 2	cout << "Hello"; // prints Hello cout << Hello; // prints the content of Hello variable

The insertion operator (<<) may be used more than once in a single statement:

cout << "Hello, " << "I am " << "a C++ statement";

This last statement would print the message Hello, I am a C++ statement on the screen. The utility of repeating the insertion operator (<<) is demonstrated when we want to print out a combination of variables and constants or more than one variable:

cout << "Hello, I am " << age << " years old and my zipcode is " << zipcode;

If we assume the age variable to contain the value 24 and the zipcode variable to contain 90064 the output of the previous statement would be: 

Hello, I am 24 years old and my zipcode is 90064

It is important to notice that cout does not add a line break after its output unless we explicitly indicate it, therefore, the following statements:

1 2	cout << "This is a sentence."; cout << "This is another sentence.";

will be shown on the screen one following the other without any line break between them:


This is a sentence.This is another sentence. 

even though we had written them in two different insertions into cout. In order to perform a line break on the output we must explicitly insert a new-line character into cout. In C++ a new-line character can be specified as \n (backslash, n):

1 2	cout << "First sentence.\n "; cout << "Second sentence.\nThird sentence.";

This produces the following output: 


First sentence.
Second sentence.
Third sentence.

Additionally, to add a new-line, you may also use the endl manipulator. For example: 

1 2	cout << "First sentence." << endl; cout << "Second sentence." << endl;

would print out: 


First sentence.
Second sentence. 

The endl manipulator produces a newline character, exactly as the insertion of '\n' does, but it also has an additional behavior when it is used with buffered streams: the buffer is flushed. Anyway, cout will be an unbuffered stream in most cases, so you can generally use both the \n escape character and the endl manipulator in order to specify a new line without any difference in its behavior.

Standard Input (cin).

The standard input device is usually the keyboard. Handling the standard input in C++ is done by applying the overloaded operator of extraction (>>) on the cin stream. The operator must be followed by the variable that will store the data that is going to be extracted from the stream. For example:

1 2	int age; cin >> age;

The first statement declares a variable of type int called age, and the second one waits for an input from cin (the keyboard) in order to store it in this integer variable.

cin can only process the input from the keyboard once the RETURN key has been pressed. Therefore, even if you request a single character, the extraction from cin will not process the input until the user presses RETURN after the character has been introduced.

You must always consider the type of the variable that you are using as a container with cin extractions. If you request an integer you will get an integer, if you request a character you will get a character and if you request a string of characters you will get a string of characters. 

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// i/o example #include <iostream> using namespace std; int main () { int i; cout << "Please enter an integer value: "; cin >> i; cout << "The value you entered is " << i; cout << " and its double is " << i*2 << ".\n"; return 0; }

Please enter an integer value: 702 The value you entered is 702 and its double is 1404.

The user of a program may be one of the factors that generate errors even in the simplest programs that use cin (like the one we have just seen). Since if you request an integer value and the user introduces a name (which generally is a string of characters), the result may cause your program to misoperate since it is not what we were expecting from the user. So when you use the data input provided by cin extractions you will have to trust that the user of your program will be cooperative and that he/she will not introduce his/her name or something similar when an integer value is requested. A little ahead, when we see the stringstreamclass we will see a possible solution for the errors that can be caused by this type of user input.

You can also use cin to request more than one datum input from the user: 

cin >> a >> b;

is equivalent to:

1 2	cin >> a; cin >> b;

In both cases the user must give two data, one for variable a and another one for variable b that may be separated by any valid blank separator: a space, a tab character or a newline.

cin and strings

We can use cin to get strings with the extraction operator (>>) as we do with fundamental data type variables:

cin >> mystring;

However, as it has been said, cin extraction stops reading as soon as if finds any blank space character, so in this case we will be able to get just one word for each extraction. This behavior may or may not be what we want; for example if we want to get a sentence from the user, this extraction operation would not be useful.

In order to get entire lines, we can use the function getline, which is the more recommendable way to get user input with cin:

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// cin with strings #include <iostream> #include <string> using namespace std; int main () { string mystr; cout << "What's your name? "; getline (cin, mystr); cout << "Hello " << mystr << ".\n"; cout << "What is your favorite team? "; getline (cin, mystr); cout << "I like " << mystr << " too!\n"; return 0; }

What's your name? Juan Souli� Hello Juan Souli�. What is your favorite team? The Isotopes I like The Isotopes too!

Notice how in both calls to getline we used the same string identifier (mystr). What the program does in the second call is simply to replace the previous content by the new one that is introduced.

stringstream

The standard header file <sstream> defines a class called stringstream that allows a string-based object to be treated as a stream. This way we can perform extraction or insertion operations from/to strings, which is especially useful to convert strings to numerical values and vice versa. For example, if we want to extract an integer from a string we can write:

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string mystr ("1204"); int myint; stringstream(mystr) >> myint;

This declares a string object with a value of "1204", and an int object. Then we use stringstream's constructor to construct an object of this type from the string object. Because we can use stringstream objects as if they were streams, we can extract an integer from it as we would have done on cin by applying the extractor operator (>>) on it followed by a variable of type int.

After this piece of code, the variable myint will contain the numerical value 1204.

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// stringstreams #include <iostream> #include <string> #include <sstream> using namespace std; int main () { string mystr; float price=0; int quantity=0; cout << "Enter price: "; getline (cin,mystr); stringstream(mystr) >> price; cout << "Enter quantity: "; getline (cin,mystr); stringstream(mystr) >> quantity; cout << "Total price: " << price*quantity << endl; return 0; }

Enter price: 22.25 Enter quantity: 7 Total price: 155.75

In this example, we acquire numeric values from the standard input indirectly. Instead of extracting numeric values directly from the standard input, we get lines from the standard input (cin) into a string object (mystr), and then we extract the integer values from this string into a variable of type int (quantity).

Using this method, instead of direct extractions of integer values, we have more control over what happens with the input of numeric values from the user, since we are separating the process of obtaining input from the user (we now simply ask for lines) with the interpretation of that input. Therefore, this method is usually preferred to get numerical values from the user in all programs that are intensive in user input

Operators

Once we know of the existence of variables and constants, we can begin to operate with them. For that purpose, C++ integrates operators. Unlike other languages whose operators are mainly keywords, operators in C++ are mostly made of signs that are not part of the alphabet but are available in all keyboards. This makes C++ code shorter and more international, since it relies less on English words, but requires a little of learning effort in the beginning.

You do not have to memorize all the content of this page. Most details are only provided to serve as a later reference in case you need it.

Assignment (=)

The assignment operator assigns a value to a variable.

a = 5;

This statement assigns the integer value 5 to the variable a. The part at the left of the assignment operator (=) is known as thelvalue (left value) and the right one as the rvalue (right value). The lvalue has to be a variable whereas the rvalue can be either a constant, a variable, the result of an operation or any combination of these.
The most important rule when assigning is the right-to-left rule: The assignment operation always takes place from right to left, and never the other way:

a = b;

This statement assigns to variable a (the lvalue) the value contained in variable b (the rvalue). The value that was stored until this moment in a is not considered at all in this operation, and in fact that value is lost.

Consider also that we are only assigning the value of b to a at the moment of the assignment operation. Therefore a later change of b will not affect the new value of a.

For example, let us have a look at the following code - I have included the evolution of the content stored in the variables as comments:

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// assignment operator #include <iostream> using namespace std; int main () { int a, b; // a:?, b:? a = 10; // a:10, b:? b = 4; // a:10, b:4 a = b; // a:4, b:4 b = 7; // a:4, b:7 cout << "a:"; cout << a; cout << " b:"; cout << b; return 0; }

a:4 b:7

This code will give us as result that the value contained in a is 4 and the one contained in b is 7. Notice how a was not affected by the final modification of b, even though we declared a = b earlier (that is because of the right-to-left rule).

A property that C++ has over other programming languages is that the assignment operation can be used as the rvalue (or part of an rvalue) for another assignment operation. For example:

a = 2 + (b = 5);

is equivalent to:

1 2	b = 5; a = 2 + b;

that means: first assign 5 to variable b and then assign to a the value 2 plus the result of the previous assignment of b (i.e. 5), leaving a with a final value of 7.

The following expression is also valid in C++:

a = b = c = 5;

It assigns 5 to the all the three variables: a, b and c.

Arithmetic operators ( +, -, *, /, % )

The five arithmetical operations supported by the C++ language are:

+	addition
-	subtraction
*	multiplication
/	division
%	modulo

Operations of addition, subtraction, multiplication and division literally correspond with their respective mathematical operators. The only one that you might not be so used to see is modulo; whose operator is the percentage sign (%). Modulo is the operation that gives the remainder of a division of two values. For example, if we write:

a = 11 % 3;

the variable a will contain the value 2, since 2 is the remainder from dividing 11 between 3.

Compound assignment (+=, -=, *=, /=, %=, >>=, <<=, &=, ^=, |=)

When we want to modify the value of a variable by performing an operation on the value currently stored in that variable we can use compound assignment operators:

expression	is equivalent to
value += increase;	value = value + increase;
a -= 5;	a = a - 5;
a /= b;	a = a / b;
price *= units + 1;	price = price * (units + 1);

and the same for all other operators. For example:

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// compound assignment operators #include <iostream> using namespace std; int main () { int a, b=3; a = b; a+=2; // equivalent to a=a+2 cout << a; return 0; }

5

Increase and decrease (++, --)

Shortening even more some expressions, the increase operator (++) and the decrease operator (--) increase or reduce by one the value stored in a variable. They are equivalent to +=1 and to -=1, respectively. Thus:

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c++; c+=1; c=c+1;

are all equivalent in its functionality: the three of them increase by one the value of c.

In the early C compilers, the three previous expressions probably produced different executable code depending on which one was used. Nowadays, this type of code optimization is generally done automatically by the compiler, thus the three expressions should produce exactly the same executable code.

A characteristic of this operator is that it can be used both as a prefix and as a suffix. That means that it can be written either before the variable identifier (++a) or after it (a++). Although in simple expressions like a++ or ++a both have exactly the same meaning, in other expressions in which the result of the increase or decrease operation is evaluated as a value in an outer expression they may have an important difference in their meaning: In the case that the increase operator is used as a prefix (++a) the value is increased before the result of the expression is evaluated and therefore the increased value is considered in the outer expression; in case that it is used as a suffix (a++) the value stored in a is increased after being evaluated and therefore the value stored before the increase operation is evaluated in the outer expression. Notice the difference:

Example 1	Example 2
B=3; A=++B; // A contains 4, B contains 4	B=3; A=B++; // A contains 3, B contains 4

In Example 1, B is increased before its value is copied to A. While in Example 2, the value of B is copied to A and then B is increased.

Relational and equality operators ( ==, !=, >, <, >=, <= )

In order to evaluate a comparison between two expressions we can use the relational and equality operators. The result of a relational operation is a Boolean value that can only be true or false, according to its Boolean result.

We may want to compare two expressions, for example, to know if they are equal or if one is greater than the other is. Here is a list of the relational and equality operators that can be used in C++:

==	Equal to
!=	Not equal to
>	Greater than
<	Less than
>=	Greater than or equal to
<=	Less than or equal to

Here there are some examples:

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(7 == 5) // evaluates to false. (5 > 4) // evaluates to true. (3 != 2) // evaluates to true. (6 >= 6) // evaluates to true. (5 < 5) // evaluates to false.

Of course, instead of using only numeric constants, we can use any valid expression, including variables. Suppose that a=2, b=3and c=6,

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(a == 5) // evaluates to false since a is not equal to 5. (a*b >= c) // evaluates to true since (2*3 >= 6) is true. (b+4 > a*c) // evaluates to false since (3+4 > 2*6) is false. ((b=2) == a) // evaluates to true.

Be careful! The operator = (one equal sign) is not the same as the operator == (two equal signs), the first one is an assignment operator (assigns the value at its right to the variable at its left) and the other one (==) is the equality operator that compares whether both expressions in the two sides of it are equal to each other. Thus, in the last expression ((b=2) == a), we first assigned the value 2 to b and then we compared it to a, that also stores the value 2, so the result of the operation is true.

Logical operators ( !, &&, || )

The Operator ! is the C++ operator to perform the Boolean operation NOT, it has only one operand, located at its right, and the only thing that it does is to inverse the value of it, producing false if its operand is true and true if its operand is false. Basically, it returns the opposite Boolean value of evaluating its operand. For example:

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!(5 == 5) // evaluates to false because the expression at its right (5 == 5) is true. !(6 <= 4) // evaluates to true because (6 <= 4) would be false. !true // evaluates to false !false // evaluates to true.

The logical operators && and || are used when evaluating two expressions to obtain a single relational result. The operator &&corresponds with Boolean logical operation AND. This operation results true if both its two operands are true, and false otherwise. The following panel shows the result of operator && evaluating the expression a && b:

&& OPERATOR

a	b	a && b
true	true	true
true	false	false
false	true	false
false	false	false

The operator || corresponds with Boolean logical operation OR. This operation results true if either one of its two operands is true, thus being false only when both operands are false themselves. Here are the possible results of a || b:

|| OPERATOR

a	b	a || b
true	true	true
true	false	true
false	true	true
false	false	false

For example:

1 2	( (5 == 5) && (3 > 6) ) // evaluates to false ( true && false ). ( (5 == 5) || (3 > 6) ) // evaluates to true ( true || false ).

Conditional operator ( ? )

The conditional operator evaluates an expression returning a value if that expression is true and a different one if the expression is evaluated as false. Its format is:


condition ? result1 : result2

If condition is true the expression will return result1, if it is not it will return result2.

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7==5 ? 4 : 3 // returns 3, since 7 is not equal to 5. 7==5+2 ? 4 : 3 // returns 4, since 7 is equal to 5+2. 5>3 ? a : b // returns the value of a, since 5 is greater than 3. a>b ? a : b // returns whichever is greater, a or b.

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// conditional operator #include <iostream> using namespace std; int main () { int a,b,c; a=2; b=7; c = (a>b) ? a : b; cout << c; return 0; }

7

In this example a was 2 and b was 7, so the expression being evaluated (a>b) was not true, thus the first value specified after the question mark was discarded in favor of the second value (the one after the colon) which was b, with a value of 7.

Comma operator ( , )

The comma operator (,) is used to separate two or more expressions that are included where only one expression is expected. When the set of expressions has to be evaluated for a value, only the rightmost expression is considered.

For example, the following code:

a = (b=3, b+2);

Would first assign the value 3 to b, and then assign b+2 to variable a. So, at the end, variable a would contain the value 5 while variable b would contain value 3.

Bitwise Operators ( &, |, ^, ~, <<, >> )

Bitwise operators modify variables considering the bit patterns that represent the values they store.

operator	asm equivalent	description
&	AND	Bitwise AND
|	OR	Bitwise Inclusive OR
^	XOR	Bitwise Exclusive OR
~	NOT	Unary complement (bit inversion)
<<	SHL	Shift Left
>>	SHR	Shift Right

Explicit type casting operator

Type casting operators allow you to convert a datum of a given type to another. There are several ways to do this in C++. The simplest one, which has been inherited from the C language, is to precede the expression to be converted by the new type enclosed between parentheses (()):

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int i; float f = 3.14; i = (int) f;

The previous code converts the float number 3.14 to an integer value (3), the remainder is lost. Here, the typecasting operator was (int). Another way to do the same thing in C++ is using the functional notation: preceding the expression to be converted by the type and enclosing the expression between parentheses:

i = int ( f );

Both ways of type casting are valid in C++.

sizeof()

This operator accepts one parameter, which can be either a type or a variable itself and returns the size in bytes of that type or object:

a = sizeof (char);

This will assign the value 1 to a because char is a one-byte long type.
The value returned by sizeof is a constant, so it is always determined before program execution.

Other operators

Later in these tutorials, we will see a few more operators, like the ones referring to pointers or the specifics for object-oriented programming. Each one is treated in its respective section.

Precedence of operators

When writing complex expressions with several operands, we may have some doubts about which operand is evaluated first and which later. For example, in this expression:

a = 5 + 7 % 2

we may doubt if it really means:

1 2	a = 5 + (7 % 2) // with a result of 6, or a = (5 + 7) % 2 // with a result of 0

The correct answer is the first of the two expressions, with a result of 6. There is an established order with the priority of each operator, and not only the arithmetic ones (those whose preference come from mathematics) but for all the operators which can appear in C++. From greatest to lowest priority, the priority order is as follows:

Level	Operator	Description	Grouping
1	::	scope	Left-to-right
2	() [] . -> ++ -- dynamic_cast static_cast reinterpret_cast const_cast typeid	postfix	Left-to-right
3	++ -- ~ ! sizeof new delete	unary (prefix)	Right-to-left
	* &	indirection and reference (pointers)
	+ -	unary sign operator
4	(type)	type casting	Right-to-left
5	.* ->*	pointer-to-member	Left-to-right
6	* / %	multiplicative	Left-to-right
7	+ -	additive	Left-to-right
8	<< >>	shift	Left-to-right
9	< > <= >=	relational	Left-to-right
10	== !=	equality	Left-to-right
11	&	bitwise AND	Left-to-right
12	^	bitwise XOR	Left-to-right
13	|	bitwise OR	Left-to-right
14	&&	logical AND	Left-to-right
15	||	logical OR	Left-to-right
16	?:	conditional	Right-to-left
17	= *= /= %= += -= >>= <<= &= ^= |=	assignment	Right-to-left
18	,	comma	Left-to-right

Grouping defines the precedence order in which operators are evaluated in the case that there are several operators of the same level in an expression.

All these precedence levels for operators can be manipulated or become more legible by removing possible ambiguities using parentheses signs ( and ), as in this example:

a = 5 + 7 % 2;

might be written either as:

a = 5 + (7 % 2);

or

a = (5 + 7) % 2;

depending on the operation that we want to perform.

So if you want to write complicated expressions and you are not completely sure of the precedence levels, always include parentheses. It will also become a code easier to read.