CMSC 202

1
CMSC 202

• Lesson 5
• Functions I

2
Warmup

• Use setw() to print the following (in tabular
  format)
• Fred Flintstone
• Barney Rubble
• Bugs Bunny
• Daffy Duck

3
Announcements

• CMSC Help Center is now open!
• Location ITE 201E
• Announcement on BB with link for hours
• Project 1 is out
• Design due this Sunday
• Homework 1 should be graded by Tuesday (1 week
  after due)

4
C Functions

• Predefined Functions
• cmath library
• sqrt(x) takes the sqrt of x
• pow(x, n) computes xn
• cstdlib library
• abs(x) integer absolute value
• exit(x) end program
• assert library
• assert(z) ends program if z is false
• Tip pay attention to the parameter types for
  these! The libraries are VERY particular

5
Functions

• You are familiar with functions that
• Return a value
• Do not return a value (void)
• Have parameters (primitives, structs, arrays)
• Have no parameters
• Call other functions

6
Functions

• New stuff
• Scope (partial review)
• Parameters
• Call by value
• Call by reference
• Constant
• Default
• Function return types
• Function overloading

7
Functions - Design

• Abstraction
• Hiding the details
• Do not need to know how a function works to use
  it
• Pre-conditions
• Comment describing constraints on parameter
  values to ensure proper function execution
• Post-conditions
• Comment describing state of the program after
  function execution
• VERY IMPORTANT!!!

8
Pre/Post-condition Example

• //------------------------------------------------
  ------
• // Function ShowInterest
• // PreCondition
• // balance is a nonnegative savings account
  balance
• // rate is the interest rate expressed as a
  percent
• // such as 5 for 5
• // PostCondition
• // the amount of interest for the given balance
  at the
• // given rate is displayed to cout.
• // if the parameters are invalid, "No Interest"
  is displayed
• //------------------------------------------------
  -------
• void ShowInterest( double balance, double rate )
• if (balance gt 0 rate gt0)
• // code to calculate and display interest
• else
• cout ltlt "No Interest\n"

9
Preconditions

• How to write a good precondition?
• Describe assumptions/limitations of each
  parameter
• Ex denominator cannot be equal to zero
• Describe assumptions/limitations of the program
  state
• Ex global array students must have at least
  one student in it
• What must the function do?
• Test every precondition!!!
• Ex if (denominator 0) cerr ltlt Error
  Denom 0 ltlt endl
• Exif (NbrOfStudents lt 1) cerr ltlt Error
  NbrOfStud lt 1 ltlt endl

10
Preconditions

• How to deal with unmet preconditions?
• Handle the error by returning a safe value or
  printing an error
• Prefer NOT to print errors from functions!
• Return a status value
• Throw an exception (later)
• Last resort Abort the program (exit or assert)

11
Postconditions

• How to write a good postcondition?
• Describe all possible message from the function
• Ex Error message is printed if preconditions are
  violated
• Describe all possible return values
• Ex Return value is 0 if an error is encountered,
  otherwise, a positive value representing the
  current rate calculated is returned
• What must the function do?
• Functionality must match postcondition claims!

12
Scope

• Area of a program where a variable is defined
• Denoted with
• Can be nested
• Types of Scope
• Global Scope
• Outside of all functions
• Entire file can use global variables
• Local Scope
• Within a pair of
• Only other code within the can use these
• If name conflict, overrides the outer-scope
• Getting back to global?
• scope resolution operator
• Accesses global variable if name conflict

13
Scope Issues

• int a 1
• int doubleVal(int a)
• return 2 a
• int main()
• a doubleVal(a 7)
• char a a
• a a 1 // a 1 gt b
• a a 1 // 16 1 gt 17
• return 0

14
Scope Rules

• Variables can be declared anywhere
• Declare them when you need them
• Global variables should always be constant!
• Not all constants should be global
• For loops initializer list is scoped inside the
  for loop
• for (int i 0 ) // i is INSIDE the loop
• Try to avoid reusing variable names

15
Function Parameters

• Argument
• Value/variable passed IN to a function
• Parameter (or Formal Parameter)
• Variable name INSIDE the function
• Call-by-Value
• Changes to the parameter do not affect the
  argument
• Syntax
• retType funcName( type varName, )

16
Call by Value Example

• void mystery(int b)
• b
• cout ltlt b ltlt endl
• int main()
• int a 7
• mystery(a)
• cout ltlt a ltlt endl
• return 0

b 7
Allocate
Copy value
Allocate
a 7
17
Call by Reference

• Call by Reference
• Changes to the parameter change the argument
• Function declares that it will change argument
• Share memory
• Essentially a pointer
• Syntax
• retType funcName( type varName, )

Look familiar? Works backwards
18
Call by Reference Example

• void mystery(int b)
• b
• cout ltlt b ltlt endl
• int main()
• int a 7
• mystery(a)
• cout ltlt a ltlt endl
• return 0

Use this space
b
Allocate
a 7
19
Value versus Reference?

• Why choose value or reference?
• Value
• Data going in, nothing coming out
• Only one piece of data coming out (return it!)
• Reference
• Need to modify a value
• Need to return more than one piece of data
• Passed an array (by default are by reference, no
  needed)

20
Call-by-Reference Issue!

• What happens in the following?
• void mystery(int b)
• b
• cout ltlt b ltlt endl
• int main()
• mystery(6)
• return 0

21
Practice

• What is printed in the following code block?
• void mystery(int a, int b)
• a
• b
• cout ltlt a ltlt ltlt b ltlt endl
• int main()
• int a 1
• int b 1
• mystery(a, b)
• mystery(b, a)
• mystery(a, a)
• cout ltlt a ltlt ltlt b ltlt endl

22
Challenge

• Write a function named getInfo
• Accepts 3 parameters
• First name
• Last name
• Age
• Ask the user to give you these 3 pieces of
  information
• Write a main function that will use getInfo to
  retrieve this information
• Think about
• Will you use value or reference?