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Web Cartoon Maker: a Fun Way to Learn C++ Contents Previous Next


Now that we have created some variables, we would like to store values in them. We do that with an assignment statement.

sFirstName = "Mike"; // assign string "Mike" to sFirstName

cFirstLetter = ’a’; // give cFirstLetter the value ’a’

iHour = 11; // assign the value 11 to iHour

iMinute = 59; // set iMinute to 59

dSeconds = 2.5; // give dSeconds the value 2.5

This example shows four assignments, and the comments show several different ways people sometimes talk about assignment statements. The vocabulary can be confusing here, but the idea is straightforward:

  • When you declare a variable, you create a 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 is 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 five assignment statements:

Note: You can also combine these shapes with other flow symbols to actually diagram your program flow, however that will not be discussed in this book.

I sometimes use different shapes and colors to indicate different variable types. These shapes should help remind you that one of the rules in C++:

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 iHour;

iHour = "Hello."; // WRONG !!

There is one 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 thing as the number 123 . This assignment is illegal:

iMinute = "59"; // WRONG!

Sometimes values can be converted to another type automatically though. For example you can assign all these values to strings and they will be converted automatically. Please keep in mind that this is a WCM C++ extension and you cannot do the same thing in classic C++:

string sConverted = "some text";

sConverted = 5; // sConverted will be "5" after this statement

sConverted = 5.0; // sConverted will be "5.000000" after this statement

sConverted = 'a'; // sConverted will be "a" after this statement

You can see from this example that it is possible to assign a value to variable immediately after a declaration: string sConverted = "some text";

You can also see that it is legal in C++ to make more than one assignment to the same variable. The effect of the second assignment is to replace the old value of the variable with a new value. This kind of multiple assignment is the reason I described variables as a container for values. When you assign a value to a variable, you change the contents of the container, as shown in the figure:

There are also other cases when values can be converted to another type automatically. For example integer values can be converted do double and vice versa:

double dVal = 5; // dVal will be 5.0 after this statement

int iVal = 5.72; // iVal will be 5 after this statement

When you assign a floating-point value to an integer variable the values are rounded down.

Another example is char which can be converted to int

int iNumber = 'a';

The above statement will assign 97 to iNumber . 97 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 unwieldy formalism, but bad for beginning programmers, who are often baffled by the complexity of the rules and the number of exceptions (fun note: it is also bad for programs, such as aircraft flight controls, that must be extremely reliable since the ambiguities may give problems at very inconvenient times. For such applications, more strictly typed languages are often used). In this book I have tried to simplify things by emphasizing the rules and omitting many of the exceptions.

Not So Fun Note: For an example of an “inconvenient time”, see: . Fortunately the pilot walked away unharmed.

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