1
Fundamental
s
to
Computing I
Notes
Hala Osman Eldigair
by
2
Chapter
C++ Basics
2:
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2.1 Variables and Assignments
Variables
› Variables are named storage locations that hold value or data.
_ A C++ variable is guaranteed to always hold value in it,
_ The number or other type of data held in a variable is called a
value.
Identifiers
› Variable names are called identifiers.
_ To make your program easy to understand, you should always
use meaningful names for variables.
_ An identifier must start with either a letter or the underscore
symbol, and all the rest of the characters must be letters, digits,
or the underscore symbol.
x x1 x_1 _abc ABC123z7 sum RATE count data2 Big_Bonus
_ All of the previously mentioned names are legal and would be
accepted by the compiler, but the first five are poor choices for
identifiers, since they are not descriptive of the identifier’s use.
None of the following are legal identifiers and all would be
rejected by the compiler:
12 3X %change data-1 myfirst.c PROG.CPP
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_ The first three are not allowed because they do not start with a
letter or an underscore. The remaining three are not identifiers
because they contain symbols other than letters, digits, and the
underscore symbol
_ C++ is a case-sensitive language; that is, it distinguishes
between uppercase and lowercase letters in the spelling of
identifiers. Hence the following are three distinct identifiers and
could be used to name three distinct variables:
rate RATE Rate
_ There is a special class of identifiers, called keywords or
reserved words, that have a predefined meaning in C++ and that
you cannot use as names for variables or anything else. Such as
int or double.
› The compiler assigns a memory location to each variable name in the
program. The value of the variable, in a coded form consisting of 0s
and 1s, is kept in the memory location assigned to that variable.
Variable Declarations
› Every variable in a C++ program must be declared. When you declare
a variable, you are telling the compiler what kind of data you will be
storing in the variable.
int number_of_bars;
double one_weight, total_weight;
Type_Name Variable_Name_1, Variable_Name_2, ...;
_ When there is more than one variable in a declaration, the
variables are separated by commas. Also note that each
declaration ends with a semicolon.
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_ Int is short for integer. This means that the value must be a whole
number.
_ Double can hold numbers with a fractional part, such as 1.75 or
-0.55.
_ Char is short for characters. They are used to store a single
character. The character must be surrounded by single quotes ,
like ‘A’.
_ The kind of data that is held in a variable is called its type and
the name for the type, such as int or double, is called a type
name.
Assignment Statements
_ The most direct way to change the value of variable is to use an
assignment statement. An assignment statement is an order to
the computer saying, “set the value of this variable to what I have
written down.” The following line from the program is an example
of an assignment statement:
total_weight = one_weight * number_of_bars;
_ An assignment statement always consists of a variable on the
left-hand side of the equal sign and an expression on the righthand side. An assignment statement ends with a semicolon.
_