Variables Arithmetic IO
Variables and types
It is very important when writing computer programs that one is able to save data in temporary
locations, move data in and out of these locations and change the data over time. How we do this in
C++ is with variables.
Consider the mathematical formula
(Pythagoras' formula). This formula can find
the length of the hypotenuse of
any
right triangle. If we were to write a program that implemented
this formula, then we would have a program that can find the length of the hypotenuse of any right
triangle. This is clearly useful (though perhaps not as its own program).
The reason the letters
,
, and
are used in the formula is because they
stand in
for whatever
values some particular right triangle has. Our program would ask for these values (specifically, the
values for
and
) from the user (or read them from a file, or whatever). Our program must also
save these values in variables.
However, unlike mathematics, C++ variables are
typed
. This means that every variable in C++ is a
variable of a certain type. We have the following types available to us:
int
for integers (..., 2, 1, 0, 1, 2, ...)
double
for "floating point" values (i.e. decimal values like 0.5 or
)
string
for "strings" (i.e. text)
bool
for true/false values
and some others...
Variables in C++ are typed for two reasons: (1) programs can be executed more efficiently if the
type is known beforehand and need not be determined based on what kinds of values the user
provides (technically, this advantage is only gained because C++ is
statically
typed, which means
the types of variables never change while the program is executing); (2) the C++ compiler is able to
ensure that you are using variables the right way by checking if you are using values of the right
type. For example, trying to save a string inside an integer type results in an error.
Overflow and underflow
Computers are finite machines, so they cannot store arbitrarily large values or arbitrarily small
values (i.e. teenytiny fractions). Most programs do not need extremely large or small values, so
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very rarely is the fact that computers are finite an issue. However, C++ types also have their limits,
and some of these limits need to be considered when writing certain kinds of programs.
For any general category of types such as integers, decimal numbers, etc. C++ has multiple specific
types that you can choose from to use in your programs. For example,
int
is not the only C++ type
that stores integer values. There is also
char
,
short
,
long
,
long long
, and possibly more
depending on the compiler. As well,
double
is not the only type that can store decimal values.
There is also
float
and
long double
. A quick comparison of these types will help you decide
which is appropriate for each variable in your program.
Integer types
char
can store values between 127 and 128 (signed) or between 0 and 255 (unsigned)
short
can store values between 32767 and 32767 (signed) or between 0 and 65535
(unsigned)
int
can store values between 2147483647 and 2147483647 (signed) or between 0 and
4294967295
long
is the same as
int
(on CSE server)
long long
can store values between 9223372036854775807 and 9223372036854775807
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 Winter '12
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