Chapter 3 Expressions And Interactivity

In reality, most programs ask for values that will be assigned to variables. This means thevprogram does not have to be modified if the user wants to run it several times with different sets of data.

Your program should always display a prompt before it uses cin to read input. This way, the user will know that he or she must type a value at the keyboard.

In this line, characters are taken from the cin object (which gets them from the keyboard) and are stored in the length variable.

In a statement that uses the cin object, the >> operator always points toward the variable that is receiving the value. This indicates that data is flowing from cin to a variable

cin automatically converts the data read from the keyboard to the data type of the variable used to store it. If the user types 10, it is read as the characters '1' and '0'. cin is smart enough to know this will have to be converted to an int value before it is stored in the length variable. cin is also smart enough to know a value like 10.7 cannot be stored in an integer variable. You must INCLUDE the iostream file in any program that uses cin.

cin >> length >> width;

So, when the user enters the values 5.7, 4, and b, they are stored in the keyboard buffer 3.3

An expression is a programming statement that has a value. Usually, an expression consists of an operator and its operands. Program 3.4

Level 1 - * / % Level 2 = + -

5 − 3 + 2 Both the − and + operators in this expression have the same precedence, and they have left to right ASSOCIAVITY

Parts of a mathematical expression may be grouped with parentheses to force some operations to be performed before others

(3)(12) === 3 * 12 4 xy ==== 4 * x * y

Think of a library function as a "routine" that performs a specific operation. One of the library functions is called pow, and its purpose is to raise a number to a power. Here is an example of how it's used: area = pow(4.0, 2.0); Program 3.6

If an int is multiplied by a float, what data type will the result be? What if a double is divided by an unsigned int? Is there any way of predicting what will happen in these instances? The answer is yes. C++ follows a set of rules when performing mathematical operations on variables of different data types. It's helpful to understand these rules to prevent subtle errors from creeping into your programs.

long double double float unsigned long long unsigned int int when an int and a long are the same size. In that case, an unsigned int outranks long because it can hold a higher value

To demote a value means to convert it to a lower data type. Let's look at the specific rules that govern the evaluation of mathematical expressions.

That's because anytime they are used in a mathematical expression, they are automatically promoted to an int. The only exception to this rule is when an unsigned short holds a value larger than can be held by an int. This can happen on systems where shorts are the same size as ints. In this case, the unsigned short is promoted to unsigned int.

In the following expression, assume that years is an int and interestRate is a float : years * interestRate Before the multiplication takes place, years will be promoted to a float.

In the following statement, assume that area is a long int, while length and width are both int s: area = length * width; Since length and width are both ints, they will not be converted to any other data type. The result of the multiplication, however, will be converted to long so it can be stored in area. Watch out for situations where an expression results in a fractional value being assigned to an integer variable. Here is an example: int x, y = 4; float z = 2.7; x = y * z; In the expression y * z, y will be promoted to float and 10.8 will result from the multiplication. Since x is an integer, however, 10.8 will be truncated and 10 will be stored in x.

Even though 15 divided by 6 is really 2.5, the .5 part of the result is discarded because we are dividing an integer by an integer. It doesn't matter that parts is declared as a double because the fractional part of the result is discarded before the assignment takes place. In order for a division operation to return a floating-point value, at least one of the operands must be of a floating-point data type. For example, the previous code could be written as: double parts; parts = 15.0 / 6;

If b and c are set to values large enough, the multiplication will produce a number too big to be stored in a. To prepare for this, a should have been defined as an int, or a long int. When a variable is assigned a number that is too large for its data type, it overflows. Likewise, assigning a value that is too small for a variable causes it to underflow Program 3.7

• Produces an incorrect result and continues running. • Prints an error message and immediately stops when either floating point overflow or underflow occurs. • Prints an error message and immediately stops when floating point overflow occurs, but stores a 0 in the variable when it underflows. • Gives you a choice of behaviors when overflow or underflow occurs.

where Value is a variable or literal value that you wish to convert and DataType is the data type you wish to convert Value to. Program 3.9

C++ allows you to assign a value to multiple variables at once. If a program has several variables, such as a, b, c, and d, and each variable needs to be assigned a value, such as 12, the following statement may be constructed: a = b = c = d = 12; The value 12 will be assigned to each variable listed in the statement

The expression on the right side of the assignment operator gives the value of number plus 1. The result is then assigned to number, replacing the value that was previously stored there.

In this statement, result is multiplied by the sum of a + 5. When constructing such statements, you must realize the precedence of the combined assignment operators is lower than that of the regular math operators. The statement above is equivalent to result = result * (a + 5);

The way a value is printed is called its formatting. The cout object has a standard way of formatting variables of each data type. Sometimes, however, you need more control over the way data is displayed. Program 3.13

You can control the number of significant digits with which floating-point values are displayed by using the setprecision manipulator.

When the precision of a number is set to a lower value, numbers tend to be printed in scientific notation. Program 3-17

The cout statements will produce the following output. 123.4 456 With most compilers, trailing zeroes are displayed when the setprecision and fixed manipulators are used together.

When cin reads input, it passes over and ignores any leading whitespace characters (spaces, tabs, or line breaks). Once it comes to the first nonblank character and starts reading, it stops reading when it gets to the next whitespace character.

The user chooses an item from the menu by typing a character. The simplest way to read a single character is with cin and the >> operator

• Random numbers are commonly used in games. • Random numbers are useful in simulation programs. In some simulations, the computer must randomly decide how a person, animal, insect, or other living being will behave. Formulas can be constructed in which a random number is used to determine various actions and events that take place in the program. • Random numbers are useful in statistical programs that must randomly select data for analysis. • Random numbers are commonly used in computer security to encrypt sensitive data.

As you look at a statement, you record the contents that each variable will have after the statement executes. This process is often helpful in finding mathematical mistakes and other logic errors.

Object of class ostream that represents the standard error stream oriented to narrow characters (of type char). It corresponds to the C stream stderr.

Object of class ostream that represents the standard output stream oriented to narrow characters (of type char). It corresponds to the C stream stdout.

Object of class ostream that represents the standard logging stream oriented to narrow characters (of type char). It corresponds, along with cerr, to the C stream stderr.