Functions

1
Functions

• Exercise 5

2
Functions

• a group of declarations and statements that is
  assigned a name
• effectively, a named statement block
• usually has a value
• a sub-program
• when we write our program we always define a
  function named main
• inside main we can call other functions
• which can themselves use other functions, and so
  on

3
Example - Square

• include ltstdio.hgt
• double square(double a)
• return aa
• int main(void)
• double num
• printf("enter a number\n")
• scanf("lf",num)
• printf("square of g is g\n",num,square(num))
  
• return 0

This is a function defined outside main
Here is where we call the function square
4
Why use functions?

• they can break your problem down into smaller
  sub-tasks
• easier to solve complex problems
• generalize a repeated set of instructions
• we dont have to keep writing the same thing over
  and over
• printf and scanf are good examples
• they make a program much easier to read and
  maintain

5
Characteristics of Functions

• return-type name(arg_type1 arg_name1, arg_type2
  arg_name2, )
• function body
• return value

int main(void)
double square(double a) return aa
6
Return Statement

• Return causes the execution of the function to
  terminate and returns a value to the calling
  function
• The type of the value returned must be the same
  as the return-type defined for the function (or a
  lower type)

7
Exercise

• Write a program that gets a positive integer from
  the user and prints all the prime numbers from 2
  up to that integer.
• (Use a function that returns 1 if its parameter
  is prime, 0 otherwise)

8
Solution

• is_prime_func.c

9
The Great Void

• Sometimes theres no reason for a function to
  return a value
• In these cases, the function return type should
  be void
• If the return keyword is used within such a
  function it exits the function immediately. No
  value needs be specified

10
The Great Void

• Calling return in a function returning void is
  not obligatory
• If the function receives no parameters, the
  parameter list should be replaced by void (or
  just nothing)

11
Example

• void ShowHelp(void)
• printf("This function explains what this program
  does\n")
• printf("Yadayadayada")
• / ... /
• int main(void)
• char choice
• printf("Please enter your selection ")
• scanf("c", choice)
• if (choiceh)
• ShowHelp()
• else if / Program continues /

12
Pass-by-value

• Function arguments are passed to the function by
  copying their values rather than giving the
  function direct access to the actual variables
• A change to the value of an argument in a
  function body will not change the value of
  variables in the calling function
• Example add_one.c

13
add_one step by step

• int add_one(int b)
• bb1
• return b
• int main(void)
• int a34,b1
• a add_one(b)
• printf("a d, b d\n", a, b)
• return 0

main() memory state
a
b
34
1
14
add_one step by step

• int add_one(int b)
• bb1
• return b
• int main(void)
• int a34,b1
• a add_one(b)
• printf("a d, b d\n", a, b)
• return 0

main() memory state
a
b
34
1
15
add_one step by step

• int add_one(int b)
• bb1
• return b
• int main(void)
• int a34,b1
• a add_one(b)
• printf("a d, b d\n", a, b)
• return 0

main() memory state
a
b
34
1
add_one memory state
b
1
16
add_one step by step

• int add_one(int b)
• bb1
• return b
• int main(void)
• int a34,b1
• a add_one(b)
• printf("a d, b d\n", a, b)
• return 0

main() memory state
a
b
34
1
add_one memory state
b
2
17
add_one step by step

• int add_one(int b)
• bb1
• return b
• int main(void)
• int a34,b1
• a add_one(b)
• printf("a d, b d\n", a, b)
• return 0

main() memory state
a
b
34
1
add_one memory state
b
2
18
add_one step by step

• int add_one(int b)
• bb1
• return b
• int main(void)
• int a34,b1
• a add_one(b)
• printf("a d, b d\n", a, b)
• return 0

main() memory state
a
b
2
1
19
add_one step by step

• int add_one(int b)
• bb1
• return b
• int main(void)
• int a34,b1
• a add_one(b)
• printf("a d, b d\n", a, b)
• return 0

main() memory state
a
b
2
1
20
Riddle me this
int main(void) int n printf("enter a
number\n") scanf("d",n)
printf("d!d\n", n,
factorial(n)) / What will this print? /
printf("n d\n", n) return 0

• include ltstdio.hgt
• int factorial(int n)
• int fact 1
• while (ngt1)
• fact n
• n--
• return fact

21
Scope of variables

• A variable declared within a function is
  unrelated to variables declared elsewhere, even
  if they have the same name
• A function cannot access variables that are
  declared in other functions
• Example scope.c

22
Wrong way to do it

• int add_one(int b)
• ab1
• int main(void)
• int a34,b1
• add_one(b)
• printf("a d, b d\n", a, b)
• return 0

23
Function Declaration

• Most software projects in C are composed of more
  than one file
• We want to be able to define the function in one
  file, and to use it in all files

24
Function Declaration

• For this reason, the function must be declared in
  every file in which its called, before its
  called for the first time
• the declaration contains

return_type Function_name(argument types)
25
Function Declaration

• include ltstdio.hgt
• int factorial(int a) / Function Declaration! /
• int main(void)
• int num
• printf("enter a number\n")
• scanf("d", num)
• printf("d ! d\n", num, factorial(num))
• return 0
• int factorial(int a)
• int i, b 1
• for(i 1 I lt a i)
• b bi

26
Function Declaration

• stdio.h actually contains a large set of function
  declarations
• The include directive tells the compiler to
  insert these declarations into the file, so that
  these functions could be called

27
The math library

• A collection of mathematical functions
• Need to include the header file math.h (include
  ltmath.hgt)
• Use functions of the library, e.g.
• double s,p
• s sqrt(p)
• Declared in math.h
• double sqrt (double x)

28
The math library

• sin(x), cos(x), tan(x)
• x is given in radians
• asin(x), acos(x), atan(x)
• log(x)
• sqrt(x)
• pow(x,y) raise x to the yth power.
• ceil(x), floor(x) and more

29
Exercise

• Write a function that uses the formula
• in order to approximate ?. The function should
  accept
• an argument n which determines the number of
  terms in
• the formula. It should return the approximation
  of ?.
• Write a program that gets an integer n from the
  user,
• and approximate ? using n terms of the above
  formula.

30
Solution

• pi.c

31
Exercise

• Modify the previous function that approximates
• ?. The function should accept an argument
• specifying the desired accuracy, and keep
• adding terms until the contribution of the next
• term drops below this level.
• Write a program that gets a (small) double
• epsilon from the user, and approximates ?
• within this function.

32
Solution

• pi_eps.c

33
The debugger

• Some programs may compile correctly, yet not
  produce the desirable results.
• These programs are valid and correct C programs,
  yet not the programs we meant to write!
• The debugger can be used to follow the program
  step by step and may help detecting bugs in an
  already compiled program.

34
The debuggers common features

• Setting breakpoints (a point where the execution
  stops) bring the cursor to desired line and
  press the palm icon or F9. A dark red dot appears
  near the line.
• Executing a debugged run Build-gtstart debug-gtgo
  or F5. The program will run and stop at the first
  breakpoint.

35
The debuggers common features (cont.)

• Stopping at a specific line Bringing the cursor
  to the line and press ctrlF10, or Build-gtstart
  debug-gtgo to cursor. The program will stop at
  that point.
• Stepping to the next line F10.
• Entering a function F11.
• Seeing variable values quickwatch and/or debug
  window at the bottom.
• The yellow arrow indicates our whereabouts at any
  given moment.