This document discusses functions in C/C++. It covers advantages of functions like modularity and reusability. It explains how to write functions, including return types, passing arguments by value or reference, and function prototypes. Examples demonstrate writing functions to find the maximum of two numbers, square a number, and convert characters to uppercase. The differences between call by value and call by reference are explained using examples. The document also discusses passing structures to functions and returning multiple values or values of different types from functions.

1. Functions

2. Advantages
• Modularizes the Code -> Code is more
readable
• Change in Internal Implementation of
Functions does not propagate further changes
in code
• Increases re-usability of code.

3. Writing Functions
• Return_Type Function_Name ( type_1
variable_1 , type_2 variable_2 , .. )
• In Functions, you can either pass parameters
by value or by reference
• Also, you have to declare a function prototype
if you are about to make a call to a function
which is written below the caller function.

5. Declaring Functions

6. Demo #1
• Write a function which takes two integers as
arguments and returns the value of the
maximum.

7. Demo #2
• Write a function which squares an integer.

8. Approach
• Break the problem into Arguments of Function
and the Return value of the function.
Here the Argument to the function is a single
Integer and the Return value is an Integer as
well.

9. Call By Value
• You CALL your friend to get your mobile number recharged.
You give him a slip which has your number written over it. He
TAKES ANOTHER (FRESH) SLIP AND COPIES THE NUMBER ON
THIS (FRESH) SLIP, goes to the shopkeeper and gets the work
done. But, while returning, he met another friend who also
asked him for mobile recharge, so he COPIES THE NEW
NUMBER ON THE (his own) SLIP NOW. Whose number is
written on your slip now ?
• Same is the case in call by value where if you call a function,
passing a particular argument by value, then the function
TAKES ANOTHER (identical) VARIABLE AND COPIES THE VALUE
OF THE ARGUMENT ON THIS VARIABLE.

10. Any changes made to argument1/2 will not change value1/2
Value1 Value2 Argument1 Argument2
In Memory

11. So, Even if you do:
This will change Memory contents for “val” not variable

12. Call By Reference
• You CALL your friend to get your mobile number
recharged. You give him a slip which has your number
written over it. He TAKES THIS SLIP and goes to the
shopkeeper and gets the work done. But, while
returning, he met another friend who also asked him
for mobile recharge, so he COPIES THE NEW NUMBER
ON THE (Your) SLIP NOW. Whose number is written on
your slip now ?
• Same is the case with functions. If you pass an
argument by Reference then any changes (New
Number copied) made to the argument will be
reflected.

13. Call By Reference
• If you want the function to manipulate the
values of the arguments.
• For ex: Sorting an array , Reversing a string etc.
• Just use the Reference “&” operator.

14. So, If you do:

15. Demo #3
• Write a Function which takes two characters
and prints them in Upper Case but the first
character after the function call must be in
Upper Case while the second character must
not be changed.

16. Approach
• Break the Problem into Arguments of the
function and the return value of the function
• Think about the “call type” for each argument as
in Call by Value or Call by Reference.
Here the Function should have two arguments both
of type “char”. Pass the first argument as reference
(i.e. using the & operator) and the second one by
value/reference. What about return type ? “void”.
Use “return;” in such cases if you want to force the
program to return immediately.

17. Call By Reference
• You can also pass a pointer to the argument if
you want the change to be reflected.
Note: What about call type of pvalue here

18. Call by Value
Arrays are always passed as Reference. ?
• General:
• If function a is calling function b in its’ body then
function b should be written above function a.
What if both functions are calling each other.
Define Function Prototypes at the very beginning
of the File. What is a Function Prototype ?
• Return_type function_name (type_1 arg1 , .. );
• Note the semicolon at the end.

19. Lets’ Tackle Some Real Problem
• Given a string S check if any rotation of S is
palindrome or not.
• Rotation of S is defined as follows:
IF S = c1 c2 c3 c4…. ck ck+1 ck+2 …. cn
then ck+1 ck+2 … cn c1 c2 c3 .. ck
is a rotation of S for all 1<=k<n

20. Approach
• First break the problem into sub-problems:
• What all independent sub-problems need to
be solved?
->Palindrome Checking
-> String Reversal
->Substring Finding
What about Parameters and Return Types and
“call by” of the arguments.
Lets’ see a naïve algorithm for the problem.

21. How to pass struct objects to functions
• Pass Arguments just like any other data type.

22. Demo #4
• Write getter and a setter functions for all the
members of a struct. Assume the struct has
only two attributes string (say name of
student), int (student id)

23. Some Thoughts
• What if we want to return more than one (or
more) values (say minimum and maximum both
simultaneously in an array)?
– Hint: Can we use pointers/array ?
• What if the data types of the values to be
returned are different ?
– Hint: Can we use structs?
• What if our function has no arguments ?
– Hint: Main itself is a function.
– Have you seen it taking arguments ?