2. Variables and types¶
2.1. More output¶
As we mentioned in the last chapter, you can put as many statements as you
want in main. For example, to output more than one line:
#include <iostream>
using namespace std;
int main()
{
cout << "Hello world." << endl; // output one line
cout << "How are you?" << endl; // output another
return 0;
}
As you can see, it is legal to put comments at the end of a line, as well as on a line by themselves.
The phrases that appear in quotation marks are called strings, because they are made up of a sequence (string) of letters. Actually, strings can contain any combination of letters, numbers, puntuation marks, and other special characters.
Often it is useful to display the output from multiple output statements all on
one line. You can do this by leaving out the first endl:
#include <iostream>
using namespace std;
int main()
{
cout << "The meaning of life, the universe, and everything";
cout << " is of course " << 42 << "." << endl;
return 0;
}
In this case the output appears in a single line as “The meaning of live, the universe, and everything is of course 42.” Notice that there is a space before the word “is” and after the word “course”. These spaces appear in the output, so they affect the behavior of the program.
Spaces that appear outside of quotation marks generally do not affect the behavior of the program. For example, we could write:
#include <iostream>
using namespace std;
int main()
{
cout<<"The meaning of life, the universe, and everything";
cout<<" is of course "<<42<<"."<<endl;
return 0;
}
This program will compile and run just as well as the original.
The breaks at the end of the lines (newlines) do not affect the program’s
behavior either (accept for the #include line), so we could even write:
#include<iostream>
using namespace std;int main(){cout<<"The meaning of life, the universe, and everything";cout<<" is of course "<<42<<"."<<endl;return 0;}
This one works too, although you have probably noticed that the program is getting harder and harder to read.
Only the newline at the end of the #include line and the single spaces
required to separate things like int and main are included in this
version, which contains only two lines (you’ll have to horizontally scroll to
read the second line if you’re viewing this with a web browser).
Newlines and spaces are useful for organizing your program visually, making it easier to read the program and locate syntax errors.
2.2. Values¶
A value is one of the fundamental things - like a letter or a number - that
a program manipulates. The only values we have manipulated so far are the
string values we have been outputting, like “Hello, world.”, and the number
value 42. You (and the compiler) can identify string values because they
are enclosed in quotation marks.
An integer is a whole number like 1 or 17. As we have seen, we can output integer values the same way we do strings, with statements like:
cout << 42 << endl;
A character value is a letter or digit or punctuation mark enclosed in
single quotes, like 'a' or '5'. You can output character values the
same way:
cout << '}' << endl;
This example outputs a single close curly brace on a line by itself.
It is easy to confuse different types of values like "5", '5', and
5, but if you pay attention to the punctuation, it should be clear that
the first is a string, the second a character, and the third an integer. The
reason this distinction is important will become clear soon.
2.3. Variables¶
One of the most powerful features of a programming language is the ability to manipulate variables. A variable is a named location that stores a value.
Just as there are different types of values (integer, character, etc.), there
are different types of variables. When you create a new variable, you have to
declare what type it is. For example, the character type in C++ is called
char. The following statement creates a new variable named fred that
has type char:
char fred;
This kind of statement is called a declaration.
The type of a variable determines what kind of values it can store. A char
variable can contain characters, and it should come as no surprise that int
variables can store integers.
There are several types in C++ that can store string values, but we are going to skip that for now (see Strings and things chapter).
To create an integer variable, the syntax is:
int alice;
where alice is the arbitrary name you made up for the variable. In general,
will want to make up variable names that indicate what you plan to do with the
variable. For example, if you saw these variable declarations:
char first_letter;
char last_letter;
int hour, minute;
you could probably make a good guess at what values will be stored in them.
This example also demonstrates the syntax for declaring multiple variables with
the same type: hour and minute are both integers (int type).
2.4. Assignment¶
Now that we have created some variables, we would like to store values in them. We do that with an assignment statement.
first_letter = 'a'; // give first_letter the value 'a'
hour = 11 // assign the value 11 to hour
minute = 59 // set minute to 59
This example shows three assignments, and the comments show three different ways people talk about assignment statements. The vocabularly can be confusing here, but the idea is straightforward:
When you declare a variable, you create and named storage location.
When you make an assignment to a variable, you give it a value.
A common way to represent variables on paper is to draw a box with the name of the variable on the outside and the value of the variable on the inside. This kind of figure is called a state diagram because it shows what state each of the variables is in (you can think of it as the variable’s “state of mind”). This diagram shows the effect of the three assignment statements:
We sometimes use different shapes to indicate different variable types. These
shapes should help remind you that one of the rules of C++ is that a variable
has to have the same type as the value you assign it. For example, you cannot
store a string in an int variable. The following statement generates a
compiler error:
int hour;
hour = "Hello."; // WRONG!
This rule is sometimes a source of confusion, because there are many ways that you can convert values from one type to another, and C++ sometimes conversts things automatically. But for now you should remember that as a general rule variables and values have the same type. We’ll talk about the special cases later.
Another source of confusion is that some strings look like integers, but they
are not. For example, the string "123", which is made up of the characters
'1', '2', and '3' is not the same as the number 123. This
assignment is illegal:
minute = "59"; // WRONG!
2.5. Outputting variables¶
You can output the value of a variable using the same commands we used to output simple values.
int hour, minute;
char colon;
hour = 11;
minute = 59;
colon = ':';
cout << "The current time is ";
cout << hour;
cout << colon;
cout << minute;
cout << endl;
This program creates two integer variables named hour and minute, and a
character variable named colon. It assigns appropriate values to each of
the variables and then uses a series of output statements to generate the
following:
The current time is 11:59
When we talk about “outputting a variable”, we mean outputting the value of
the variable. To output the name of a variable, you have to put it in quotes.
For example: cout << "hour";
As we have seen, you can include more than one value in a single output statement, which can make the previous program more concise:
int hour, minute;
char colon;
hour = 11;
minute = 59;
colon = ':';
cout << "The current time is " << hour << colon << minute << endl;
On one line, this program outputs a string, two integers, a character, and
the special value endl. Very impressive!
2.6. Keywords¶
A few sections ago, we said that you can make up any name you want for your
variables, but that’s not quite true. There are certain words that are
reserved in C++ because the are used by the compiler to parse the structure
of your program, and if you use them as variable names, it will get confused.
These words, called keywords, include int, char, return, and
many more.
The complete list of keywords is included in the C++ Standard, which is the official language adopted by the International Organization for Standardization (ISO) The official standard needs to be purchased from the ISO, but you can see the list of keywords in the online API Reference Document.
Rather than memorize the list, we suggest that you take advantage of a feature provided in many development environments: code highlighting. As you type, different parts of your program should appear in different colors. For example, keywords might be red, strings blue, and other code black. If you type a variable name and it turns red, watch out! You might get some strange behavior from the compiler.
2.7. Operators¶
Operators are special symbols that are used to represent simple
computations like addition and substraction. Most of the operators in C++ do
exactly what you would expect them to do, because they are common mathematical
symbols. For example, the operator for adding two integers is +.
The following are all legal C++ expressions whose meaning is more or less obvious:
1+1 hour-1 hour*60+minute minute/60
Expressions can contain both variable names and integer values. In each case the name of the variable is replaced with its value before the computation is performed.
Addition, substraction and multiplication all do what you expect, but division gives us a surprise. When we run the following program:
#include <iostream>
using namespace std;
int main()
{
int hour, minute;
hour = 11;
minute = 59;
cout << "Number of minutes since midnight: ";
cout << hour * 60 + minute << endl;
cout << "Fraction of the hour that has passed: ";
cout << minute / 60 << endl;
return 0;
}
we get the following output:
Number of minutes since midnight: 719
Fraction of the hour that has passed: 0
The first line is what we expected, but the second line is not. The value of
the variable minute is 59, and 59 divided by 60 is 0.98333, not 0. The
reason for the discrepancy is that C++ is performing integer division.
When both of the operands are integers (operands are the things operators operate on), the result must also be an integer, and integer division trucates the resulting value at the decimal point. Running this program:
#include <iostream>
using namespace std;
int main()
{
int i1, i2, i3;
i1 = 5;
i2 = 2;
i3 = -5;
cout << "With integer division 5 divided by 2 is " << i1 / i2 << ", ";
cout << "and -5 divied by 2 is " << i3 / i2 << '.' << endl;
return 0;
}
gives us:
With integer division 5 divided by 2 is 2, and -5 divied by 2 is -2.
Truncating here means cutting off the value at the decimal point. It is different from rounding down, since -5/2 rounded down would be -3.
A possible alternative in this case is to calculate the percentage rather than a fraction:
cout << "Percentage of the hour that has passed: " << minute*100/60 << endl;
which results in:
Percentage of the hour that has passed: 98
Again the result is truncated, but at least now the result is approximately correct. In order to get an even more accurate answer, we could use a different type of variable, called a floating-point, that is capable of storing fractional values. We’ll get to that in the next chapter.
2.8. Order of Operations¶
When more than one operator appears in an expression the order of evaluation depends on the rules of precedence. A complete explaination of precendence can get complicated, but just to get you started:
Multiplication and division happen before addition and subtraction. So 2*3-1 yields 5, not 4, and 2/3-1 yields -1, not 1 (remember that with integer division 2/3 is 0).
If the operators have the same precedence, they are evaluated from left to right. So in the expression
minute*100/60, the multiplication happens first, yielding 5900/60, which after the division yields 98. If the operations had gone from right to left, the result would be 59*1 and then 59, which is wrong.Any time you want to override the rules of precedence (or you are not sure what they are) you can use parentheses. Expressions in parentheses are evaluated first, so 2*(3-1) is 4. You can use parentheses to make an expression easier to read, as in (minute * 100) / 60, even though it doesn’t change the result.
2.9. Oporators for characters¶
Interestingly, the same mathematical operations that work on integers also work on characters. For example,
char letter;
letter = 'a' + 1;
cout << letter << endl;
outputs the letter b. Although it is syntactically legal to multiply
characters, it is almost never useful to do it.
Earlier we said you can only assign integer values to integer variables and character values to character variables, but that is not completely true. In some cases C++ converts automatically between types. For example, the following is legal C++:
int number;
number = 'a';
cout << number << endl;
The result is 97, which is the number that is used internally by C++ to represent the letter ‘a’. However, it is generally a good idea to treat characters as characters, and integers as integers, and only convert from one to the other if there is a good reason.
Automatic type conversion is an example of a common problem in designing a programming language, which is that there is a conflict between formalism, which is the requirement that formal languages should have simple rules with few exceptions, and convenience, which is the requirement that programming languages be easy to use in practice.
More often than not, convenience wins, which is usually good for expert programmers, who are spared from rigorous but unwiedy formalism, but bad for beginning programmers, who are often baffled by the complexity of the rules and the number of exceptions. In this book we have tried to simplify things by emphasizing the rules and omitting many of the exceptions.
2.10. Composition¶
So far we have looked at the elements of a programming language - variables, expressions, and statements - in isolation, without talking about how to combine them.
One of the most useful features of programming languages is their ability to take small building blocks and compose them. For example, we know how to multiply integers and we know how to output values; it turns out we can do both at the same time:
cout << 17 * 3;
Actually, we shouldn’t say “at the same time”, since in reality the multiplication has to happen before the output, but the point is that any expression, involving numbers, characters, and variables, can be used inside an output statement. We’ve already seen one example:
cout << hour * 60 + minute << endl;
You can also put arbitrary expressions on the right-hand side of an assignment statement:
int percentage;
percentage = (minute * 100) / 60;
This ability may not seem so impressive now, but we will see other examples where composition makes it possible to express complex computations neatly and concisely.
2.11. Glossary¶
- assignment¶
A statement that assigns a value to a variable.
- composition¶
The ability to combine simple expressions and statements into compound statements and expressions in order to represent complex computations concisely.
- declaration¶
A statement that creates a new variable and defines its type.
- expression¶
A combination of variables, operators and values that represents a single result value. Expressions also have types, as determined by their operators and operands.
- operand¶
One of the values on which an operator operates.
- operator¶
A special symbol that represents a simple computation like addition or multiplication.
- precedence¶
The order in which operations are evaluated.
- statement¶
A line of code that represents a command or action. So far, the statements we have seen are declarations, assignments, and output statements.
- string¶
A sequence of characters. Strings literals in C++ are enclosed in quotes (“”).
- type¶
A set of values. They types we have seen are integers (
intin C++), characters (charin C++), and strings.- value¶
A letter, or number, or other thing that can be stored in a variable.