1. Module 6 Working with Functions

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Why Functions?
Code Reuse
Code Modularity
Hiding Details
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Defining a Function
return-type name (parameters) {
body of code }
Parameters are optional, parentheses are not
void return-type indicates no return value
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Calling a Function
void printMessage( ) { printf(“Programming is fun\n”); } int main( ) printMessage(); return 0;
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5. Parameters, Arguments, Return Values
#include <stdio.h>
int twoNPlus1( int num ) {
num = num * 2;
int result = num + 1;
return result; }
int main( )
int x, y = 33;
x = twoNPlus1(y);
printf(“x = %d\n”, x);
int z = 12 + twoNPlus1(y + 3);
printf(“z = %d\n”, z);
return 0;
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Let’s take a look
CB- TwoNPlus1
Function prototype
Parameter list
Local variables
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7. An Alternative Program
#include <stdio.h>
int twoNPlus1( int num ); // function prototype first
int main( ) {
int x, y = 33;
x = twoNPlus1(y);
printf(“x = %d\n”, x);
int z = 12 + twoNPlus1(x + 3);
printf(“z = %d\n”, z);
return 0; }
int twoNPlus1( int num ) // function definition later
num = num * 2;
int result = num + 1;
return result;
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Let’s take a look
CB-TwoNPlus1-2
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Predicate Functions
A function that returns a Boolean value
bool isEven( int num ) {
return num % 2 == 0; }
___________________
if (isEven( x )== true)
. . .
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Exercises
Pg 162
#3 – Modify 8.8 (squareroot) to accept “epsilon” as a parameter
#7 -- implement and test x_to_the_n(int x, int n)
#8 – implement function for quadratic formula
#9 -- implement LCM using GCD (given)
Give about 30 minutes, just to get started
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11. Functions Calling Functions
main is a function
Functions may call other functions
Top-Down Design
Go over later
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Let’s take a look
CB – hailstone
Let’s look at what happens without prototypes
Compiler assumptions – all functions return an int UNLESS you’ve told the compiler otherwise
Remove isEven prototype
“rebuild” – first a warning about isEven, then an error
Typical if you forget a standard header file
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13. Function Documentation
Brief description of what the function does
Brief description of each parameter
Precondition(s)
What must be true for the function to correctly perform its task?
Postcondition(s)
What will be true after the function completes its task?
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14. Function Documentation
// hailstone // generates the next value in a “hailstone” sequence // Precondition // argument n > 0 // Postcondition // returns n/2 if n is even, 3 * n + 1 if n is odd int hailstone (int n);
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Top-Down Design
Break a complex problem into smaller parts
Break the parts into smaller parts
Repeat until parts are easy to do
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Why Top-Down?
Clarifies the problem
Each part is less complex
Parts may be reusable
More than one person may work on the problem
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Example
Complex Problem
We own a basement waterproofing company
A section of town (zip code 12345) has recently had a flood
We want to send pamphlets to those potential customers
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Top-Level
Get complete customer list from a file
Sort the list according to zip code
Extract customers from zip code and write them to new file
Print an envelope for these customers
Main
Read
Sort
Select
Print
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Top-Down Design
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More Levels?
Should any of these steps be broken down further? Maybe.
How do you know?
Is it easy to write an algorithm for each step? If not, break it down again
When you’re comfortable with the steps write the code for each one
The code for each step is a function
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Let’s take a look
C::B Program8-8
Pg 132
Drawn what that program would look like in a Top-Down design
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22. Done for Functions Part 1
Exercises on the Next Page
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Exercises
FunctionProblems.docx
Ex1 – MathIsFun – 4 different easy math-related functions
Ex2 – NumberTheory – predicate functions (square, perfect, prime)
Ex3 – Combinations – C(n, k), n!
Ex4 – Rectangles (area, perimeter, isSquare (rhombus)
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Passing an Array
Parameters
Do not specify array size in brackets
Pass array size as separate parameter
Arguments
Name of the array is passed to function
Specifies the memory address of start of the array
Passed by reference
Changes to array parameter in the function’s code also changes the array argument in the calling code
Size of array passed by value
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25. Array Parameter Example
int sumArray(int a[], int size){ int sum = 0; for(int k = 0; k < size; k++) sum += a[k]; return sum; } int main( ){ int arraySum; int ages[5] = {6, 4, 12, 11, 22}; int IQs[6] = {100, 93, 99, 120, 89, 103}; arraySum = sumArray(ages, 5); printf(“sum of ages = %d\n”, arraySum); arraySum = sumArray(IQs, 6); printf(“sum of IQs = %d\n”, arraySum); return 0;
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Let’s take a look
C::B - arrays2
Usually have to pass the size too, otherwise the function doesn’t know how big the array is – reusable with arrays of any size
put size in array parameter and run again
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Searching an Array
Examine every element of the array to determine if it’s the one we want -- “Linear” Search
bool linearSearch( int a[ ], int size, int target) {
bool found = false;
int j = 0;
while (j < size && !found) {
if (a[j] == target)
found = true;
++j; }
return found;
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Sorting An Array
Very common operation
Many algorithms – some slow, some fast
Exchange sort
Compare first element with second
If 2nd is smaller, swap them
Compare first element with third
If 3rd is smaller, swap them
Continue comparing first element with all others
When complete, first element will be smallest
Repeat steps above for 2nd element, then 3rd, then 4th and so on. When complete, array will be sorted.
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Binary Search
Fast search for specified target element
Requires sorted array
Algorithm
Look at the middle element
If the middle element is the target, stop
If the target > middle element, look at the middle element of the upper “subarray”
If the target < middle element, look at the middle element of the lower“subarray”
Continue until found or all elements examined
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30. Variable Length Array Parameter
Same as an “ordinary” array
Array parameter does not specify size
Size is separate parameter
int sumElements(int a[ ], int nrElements);
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31. Functions and Variables
Automatic local variables
“come to life” and reinitialized when function starts
“disappear” when function ends
Global variables
Usable anywhere in the file
NOT recommended
Reduces function generality
Prone to errors
OK for constant variables and arrays
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32. Functions and Variables
Automatic local variables
“come to life” and reinitialized when function starts
“disappear” when function ends
Static local variables
Exist for the duration of your program
Value retained between calls to the function
Global variables
Usable anywhere in the file
NOT recommended
Reduces function generality
Prone to errors
OK for constant variables and arrays
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Recursive Problems
A recursive problem is one that is written in terms itself
The solution to the simplest problem is known
Larger problems are redefined in terms of a smaller problem
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Recursive Examples
Sum from 0 to 5
Sum from 0 to 5 = 5 + sum from 0 to 4
Sum from 0 to 4 = 4 + sum from 0 to 3
Sum from 0 to 3 = 3 + sum from 0 to 2
Sum from 0 to 2 = 2 + sum from 0 to 1
Sum from 0 to 1 = 1 + sum from 0 to 0
Sum from 0 to 0 = 0
Sum from 0 to k
Sum from 0 to k = k + sum from 0 to k - 1
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Recursive Examples
The Nth Fibonacci Number
Fib(0) = 1
Fib(1) = 1
Fib(N) = Fib( N – 1 ) + Fib( N – 2 ) if N > 1
The largest element in array a[ ] of size K
a[0] if K = 1
Last element (a[k-1]) or largest of a[0]…a[k-2]
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Recursive Functions
A function that
Can determine the answer of the smallest problem
Finds the answer to the larger problem by calling itself to find the answer to smaller problems
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Binary Search
Requires sorted array
Algorithm
Compare target with middle element
If found, we’re done
If target less than middle element look in first half of the array
Else target is greater than middle element, so look in second half of the array
Got to 1.
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Streams
In C, all I/O is performed using streams
A sequence of bytes that “flows” between your program and an I/O device
A high level abstraction representing a channel to a file or I/O device
Some I/O functions such as printf and scanf use the standard streams (stdout/stdin) by default
opened when your program begins execution
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File I/O
Files must be opened to create a stream before I/O may be performed
Functions that perform I/O with files are similar to those for stdout/stdin, but must specify the stream to which the I/O is performed
Streams are designated as type FILE*
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Opening a Stream
FILE* fopen(char fileName[], char mode[]);
fileName is the path to the file
Windows: C:\folder\filename
Unix: /directory/filename
mode is a 1- or 2-character string
Return value FILE* is the stream used as the argument to other functions
Returns NULL if an error occurs
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Closing a Stream
fclose(FILE* stream);
stream is FILE* returned from fopen( )
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42. Formatted Stream Output
fprintf(FILE* stream, “formatString”, var1, var2,…);
stream is FILE* returned from fopen( )
“formatString” is the same as in printf()
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43. Formatted Stream Input
fscanf(FILE* stream, “formatString”, &var1, &var2,…);
stream is FILE* returned from fopen( )
“formatString” is the same as in scanf()
Returns EOF if error or end-of-file
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Exercises
tracing.docx
trace function calls and provide output
Textbook Chapter 8 – pg 162
#11 – part of Ex0
# Modify 8.12 (ex sort) to have new parameter indicating direction of sort
Ex0 – FunWithArrays – various functions with array parameter
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