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Contents
• Header Files
• Scope of Identifiers
• Functions
- Call by Value
- Call by Reference
Header Files
You have already been using a header file from day-zero. You know that we used to
write at the top before the start of the
main() function <iostream.h>,
with ‘.h’ as an
extension, you might have got the idea that it is a header file.
Now we will see why a Header file is used.
In the previous lecture, we discussed a little bit about Function Prototypes. One
thing is
Declaration and other is Definition. Declaration can also be called as 'Prototype'.
Normally, if we have lot of functions and want to use them in some other function
or
program, then we are left with only one way i.e. to list the prototypes of all of
them
before the body of the function or program and then use them inside the function
or
program. But for frequent functions inside a program, this technique increases the
complexity (of a program). This problem can be overcome by putting all these function
prototypes in one file and writing a simple line of code for including the file
in the
program. This code line will indicate that this is the file, suppose 'area.h' containing
all
the prototypes of the used functions and see the prototypes from that file. This
is the basic
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concept of a header file.
So what we can do is:
- Make our own header file which is usually a simple text file with '.h'
extension ('.h'
extension is not mandatory but it is a rule of good programming practice).
- Write function prototypes inside that file. (Recall that prototype is just
a simple line
of code containing return value, function name and an argument list of data types
with semi-colon at the end.)
- That file can be included in your own program by using the ‘#include’ directive
and
that would be similar to explicitly writing that list of function prototypes.
Function prototypes are not the only thing that can be put into a header file. If
you
remember that we wrote a program for calculating Area of a Circle in our previous
lectures. We used the value of 'pi' inside that and we have written the value of
'pi' as
3.1415926. This kind of facts are considered as Universal Constants or Constants
within
our domain of operations . It would be nice, if we can assign meaningful names to
them.
There are two benefits of doing this. See, We could have declared a variable of
type
double inside the program and given a name like 'pi':
double pi = 3.1415926;
Then everywhere in the subsequent calculations we can use 'pi'.
But it is better to pre-define the value of the constant in a header file ( one
set for all) and
simply including that header file, the constant ‘pi’, is defined. Now, this meaningful
name ‘pi’ can be used in all calculations instead of writing the horrendous number
3.1415926 again and again.
There are some preprocessor directives which we are going to cover later. At the
moment, we will discuss about ‘#define’ only. We define the constants using this
preprocessor directive as:
#define pi 3.1415926
The above line does a funny thing as it is not creating a variable. Rather it associates
a
name with a value which can be used inside the program exactly like a variable.
(Why it
is not a variable?, because you can’t use it on the left hand side of any assignment.).
Basically, it is a short hand, what actually happens. You defined the value of the
‘pi’ with
‘#define’ directive and then started using ‘pi’ symbol in your program. Now we will
see
what a compiler does when it is handed over the program after the writing process.
Wherever it finds the symbol ‘pi’, replaces the symbol with the value 3.1415926
and
finally compiles the program.
Thus, in compilation process the symbols or constants are replaced with actual values
of
them. But for us as human beings, it is quite readable to see the symbol ‘pi’. Additionally,
if we use meaningful names for variables and see a line ‘2 * pi * radius’, it becomes
obvious that circumference of a circle is being calculated. Note that in the above
statement, ‘2 * pi * radius’; 2 is used as a number as we did not define any constant
for it.
We have defined ‘pi’ and ‘radius’ but defining 2 would be over killing.
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Scope of Identifiers
An 'Identifier' means any name that the user creates in his/her program. These names
can
be of variables, functions and labels. Here the scope of an identifier means its
visibility.
We will focus Scope of Variables in our discussion.
Suppose we write the function:
void func1()
{
int i;
. . . //Some other lines of code
int j = i+2; //Perfectly alright
. . .
}
Now this variable ‘i’ can be used in any statement inside the function func1().
But
consider this variable being used in a different function like:
void func2()
{
int k = i + 4; //Compilation error
. . .
}
The variable ‘i’ belongs to func1() and is not visible outside that. In other words,
‘i’ is
local to func1().
To understand the concept of scope further, we have to see what are Code
Blocks? A
code block begins with ‘{‘ and ends with ‘}’.Therefore, the body of a function is
essentially a code block. Nonetheless, inside a function there can be another block
of
code like 'for loop' and 'while loop' can have their own blocks of code respectively.
Therefore, there can be a hierarchy of code blocks.
A variable declared inside a code block becomes the local variable for that for
that block.
It is not visible outside that block. See the code below:
void func()
{
int outer; //Function level scope
. . .
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{
int inner; //Code block level scope
inner = outer; //No problem
. . .
}
inner ++; //Compilation error
}
Please note that variable ‘outer’ is declared at function level scope and variable
‘inner’ is
declared at block level scope.
The ‘inner’ variable declared inside the inner code block is not visible outside
it . In other
words, it is at inner code block scope level. If we want to access that variable
outside its
code block, a compilation error may occur.
What will happen if we use the same names of variables at both function level scope
and
inner block level scope? Consider the following code:
Line
1. void increment()
2. {
3. int num; //Function level scope
4. . . .
5. {
6. int num; //Bad practice, not recommended
7. . . .
8. num ++; //inner num is incremented
9. . . .
10. }
11. }
Note that there is no compilation error if the variable of the same name ‘num’ is
declared
at line 6 inside the inner code block (at block level scope). Although , there is
no error in
naming the variables this way, yet this is not recommended as this can create confusion
and decrease readability. It is better to use different names for these variables.
Which variable is being used at line 8? The answer is the ‘num’
variable declared for
inner code block (at block level scope). Why is so? It is just due to the fact that
the outer
variable ‘num’ (at function level scope) is hidden in the inner code block as there
is a
local variable of the same name. So the local variable ‘num’ inside the inner code
block
over-rides the variable ‘num’ in the outer code block.
Remember, the re-use of a variable is perfectly alright as we saw in the code snippet
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above while using ‘outer’ variable inside the inner code block. But re-declaring
a variable
of the same name like we did for variable ‘num’ in the inner code block, is a bad
practice.
Now, is there a way that we declare a variable only once and then use it inside
all
functions. We have already done a similar task when we wrote a function prototype
outside the body of all the functions. The same thing applies to declaration of
variables.
You declare variables outside of a function body (so that variable declarations
are not
part of any function) and they become visible and accessible inside all functions
of that
file. Notice that we have just used a new word ‘file’.
A file or a source code file with extension ‘.c’ or ‘.cpp’ can have many functions
inside.
A file will contain one main()
function maximum and rest of the functions as many as
required. If you want a variable to be accessible from within all functions, you
declare the
variable outside the body of any function like the following code snippet has declared
such a variable ‘size’ below.
#include <iostream.h>
. . .
// Declare your global variables here
int size;
. . .
int main( … )
{
. . .
}
Now, this ‘size’ is visible in all functions including
main(). We call this as
'file scope
variable' or a 'global variable'. There are certain benefits of using global variables.
For
example, you want to access the variable ‘size’ from anywhere in your program but
it
does have some pitfalls. You may inadvertently change the value of the variable
‘size’
considering it a local variable of the function and cause your program to behave
differently or affect your program logic.
Hence, you should try to minimize the use of global variables and try to use the
local
variables as far as possible. This philosophy leads us to the concept of Encapsulation
and
Data Hiding that encourages the declaration and use of data locally.
In essence, we should take care of three levels of scopes associated with identifiers:
global scope, function level scope and block level scope.
Let's take a look of very simple example of global scope:
#include <iostream.h>
//Declare your global variables here
int i;
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void main()
{
i = 10;
cout << “\n” << “In main(), the value of i is: “ << i;
f();
cout << “\n” << “Back in main(), the value of i is: “ << i;
}
void f()
{
cout << “\n” << ”In f(), the value of i is: “ << i;
i = 20;
}
Note the keyword ‘void’ here, which is used to indicate that this function does
not return
anything.
The output of the program is:
In main(), the value of i is: 10
In f(), the value of i is: 10
Back in main(), the value of i is: 20
Being a global variable, ‘i’ is accessible to all functions. Function f() has changed
its
value by assigning a new value i.e. 20.
If the programmer of function f() has changed the value of ‘i’ accidentally taking
it a
local variable, your program’s logic will be affected.
Function Calling
We have already discussed that the default function calling mechanism of C is a
'Call by
Value'. What does that mean? It means that when we call a function and pass some
arguments (variables) to it, we are passing a copy of the arguments (variables)
instead of
original variables. The copy reaches to the function that uses it in whatever way
it wants
and returns it back to the calling function. The passed copy of the variable is
used and
original variable is not touched. This can be understood by the following example.
Suppose you have a letter that has some mistakes in it. For rectification, you depute
somebody to make a copy of that letter, leave the original with you and make corrections
in that copy. You will get the corrected copy of the letter and have the unchanged
original
one too. You have given the copy of the original letter i.e. the call by value part.
But if you give the original letter to that person to make corrections in it, then
that person
will come back to you with the changes in the original letter itself instead of
its copy.
This is call by reference.
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The default of C is 'Call by Value'. It is better to use it as it saves us from
unwanted side
effects. Relatively, 'Call by Reference' is a bit complex but it may be required
sometimes
when we want the actual variable to be changed by the function being called.
Let's consider another example to comprehend 'Call by Value' and how it works. Suppose
we write a main()
function and another small function
f(int) to call it from
main(). This
function f( ) accepts
an integer, doubles it and returns it back to the
main() function. Our
program would look like this:
#include <iostream.h>
void f(int); //Prototype of the function
void main()
{
int i;
i = 10;
cout << “\n” << ” In main(), the value of i is: “ << i;
f(i);
cout << “\n” << ” Back in main(), the value of i is: “ << i;
}
void f (int i)
{
i *= 2;
cout << “\n” << “ In f(), the value of i is: “ << i;
}
The output of this program is as under:
In main(), the value of i is: 10
In f(), the value of i is: 20
Back in main(), the value of i is: 10
As the output shows the value of the variable ‘i’ inside function
main() did not change,
it
proves the point that the call was made by value.
If there are some values we want to pass on to the function for further processing,
it will
be better to make a copy of those values , put it somewhere else and ask the function
to
take that copy to use for its processing. The original one with us will be secure.
Let's take another example of call by value, which is bit more relevant. Suppose
we want
to write a function that does the square of a number. In this case, the number can
be a
double precision number as seen below:
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#include <iostream.h>
double square (double);
void main()
{
double num;
num = 123.456;
cout << “\n” << “ The square of “ << num << “ is “ << square(num);
cout << “\n” << “ The current value of num is “ << num;
}
double square (double x)
{
return x*x;
}
'C' does not have built-in mathematical operators to perform square, square root,
log and
trigonometric functions. The C language compiler comes along a complete
for
that. All the prototypes of those functions are inside ‘<math.h>’. In order to use
any of
the functions declared inside ‘<math.h>’, the following line will be added.
#include <math.h>
Remember, these functions are not built-in ones but is supplied with the
Ccompiler.
It may be of interest to you that all the functions inside ‘<math.h>’ are called
by value. Whatever variable you will pass in as an argument to these functions,
nothing
will happen to the original value of the variable. Rather a copy is passed to the
function
and a result is returned back, based on the calculation on that copy.
Now, we will see why Call by Reference is used.
We would like to use 'call by reference' while using a function to change the value
of the
original variable. Let's consider the
square(double) function
again, this time we want the
original variable ‘x’ to be squared. For this purpose, we passed a variable to the
square()
function and as a result, on the contrary to the ‘Call by Value’, it affected the
calling
functions original variable. So these kinds of functions are ‘Call by Reference’
functions.
Let us see, what actually happens inside Call by Reference?
As apparent from the name ‘By Reference’, we are not passing the value itself but
some
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form of reference or address. To understand this, you can think in terms of variables
which are names of memory locations. We always access a variable by its name (which
in fact is accessing a memory location), a variable name acts as an address of the
memory
location of the variable.
If we want the called function to change the value of a variable of the calling
function, we
must pass the address of that variable to the called function. Thus, by passing
the address
of the variable to the called function, we convey to the function that the number
you
should change is lying inside this passed memory location, square it and put the
result
again inside that memory location. When the calling function gets the control back
after
calling the called function, it gets the changed value back in the same memory location.
In summary, while using the call by reference method, we can’t pass the value. We
have
to pass the memory address of the value. This introduces a new mechanism which is
achieved by using ‘&’ (ampersand) operator in C language. This ‘&’ operator is used
to
get the address of a variable. Let's look at a function, which actually is a modification
of
our previous square()
function.
#include <iostream.h>
void square(double);
void main()
{
double x;
x = 123.456;
cout << “\n” << “ In main(), before calling square(), x = “ << x;
square(&x); //Passing address of the variable x
cout << “\n” << “ In main(), after calling square(), x = “ << x;
}
void square(double* x) //read as: x is a pointer of type double
{
*x = *x * *x; //Notice that there is no space in *x
}
Here *x means whatever the x points to and &x means address of the variable x. We
will
discuss Pointers in detail later.
We are calling function square(double*)
with the statement
square(&x) that is actually
passing the address of the variable x , not its value. In other words, we have told
a box
number to the function square(double*)
and asked it to take the value inside that box,
multiply it with itself and put the result back in the same box. This is the mechanism
of
‘Call by Reference’.
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Notice that there is no return statement of
square(double*) as we are
putting the changed
value (that could be returned) inside the same memory location that was passed by
the
calling function.
The output of the program will be as under:
In main(), before calling square(), x = 123.456
In main(), after calling square(), x = 15241.4
By and large, we try to avoid a call by reference. Why? Mainly due to the side-effects,
its
use may cause. As mentioned above, it will be risky to tell the address of some
variables
to the called function. Also, see the code above for some special arrangements for
call by
reference in C language. Only when extremely needed, like the size of the data to
be
passed as value is huge or original variable is required to be changed, you should
go for
call by reference, otherwise stick to the call by value convention.
Now in terms of call by reference, we see that there are some places in ‘C’ where
the call
by reference function happens automatically. We will discuss this later in detail.
For the
moment, as a hint, consider array passing in ‘C’.
Recursive Function
This is the special type of function which can call itself. What kind of function
it would
be? There are many problems and specific areas where you can see the repetitive
behavior (pattern) or you can find a thing, which can be modeled in such a way that
it
repeats itself.
Let us take simple example of x10, how will we calculate it? There
are many ways of
doing it. But from a simple perspective, we can say that by definition x10
= x * x9. So
what is x9? It is x9 = x * x8 and so on.
We can see the pattern in it:
xn = x * xn-1
To compute it, we can always write a program to take the power of some number. How
to
do it? The power function itself is making recursive call to itself. As a recursive
function
writer, you should know where to stop the recursive call (base case). Like in this
case,
you can stop when the power of x i.e. n is 1 or 0.
Similarly, you can see lot of similar problems like Factorials. A factorial of a
positive
integer ‘n’ is defined as:
n! = (n) * (n-1) * (n-2) * ….. * 2 * 1
Note that
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n! = (n) * (n-1)!
and (n-1)! = (n-1) * (n-2)!
This is a clearly a recursive behavior. While writing a factorial function, we can
stop
recursive calling when n is 2 or 1.
long fact(long n)
{
if (n <= 1)
return 1;
else
return n * fact(n-1);
}
Note that there are two parts (branches) of the function: one is the base case (
which
indicates when the function will terminate) and other is recursively calling part.
All the problems can be solved using the iterative functions and constructs we have
studied until now. So the question is: do we need to use recursive functions? Yes,
it adds
little elegance to the code of the function but there is a huge price to pay for
this. Its use
may lead to the problems of having memory overhead. There may also be stacking
overhead as lots of function calls are made. A lot of functions can be written without
recursion (iteratively) and more efficiently.
So as a programmer, you have an option to go for elegant code or efficient code,
sometimes there is a trade-off. As a general rule, when you have to make a choice
out of
elegance and efficiency, where the price or resources is not an issue, go for elegance
but
if the price is high enough then go for efficiency.
‘C’ language facilitates us for recursive functions like lot of other languages
but not all
computer languages support recursive functions. Also, all the problems can not be
solved
by recursion but only those, which can be separated out for base case, not iterative
ones.
Tips
- Header file is a nice mechanism to put function prototypes and define constants
(global constants) in a single file. That file can be included simply with a single
line
of code.
- There are three levels of scopes to be taken care of, associated with identifiers:
global
scope, function level scope and block level scope.
- For Function calling mechanism, go for ‘Call by Value’ unless there is
a need of ‘Call
by Reference’.
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- Apply the recursive function where there is a repetitive pattern, elegance
is required
and there is no resource problem. |
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