1 CS 105 Lecture 11 Arrays Version of Wed, Apr 6, 2011, 6:20 pm

2 Exam 2 Exam 2 Next Week 33% of Exam in Lab Must complete a program in lab [50 min]. 67% of Exam in Lecture 50 minutes Know your programming Study Quiz 1 and 2, Exam 1 Know everything through today Files!! Functions!! Loops!! (Array declaration, Array element assignment — less emphasis; maybe 15% of Exam.)

3 Exam 2 Please be sure to study: Ch 5 Exercises 12, 16, 18 (Loops) Ch 8 Exercise 9 (string usage) Ch 3 Exercises 3, 5 (I/O, I/O failure) Ch 3 Pgm'g exercises 1–3 (filestream I/O) Ch 6 Exercises 2, 4, 5 (functions) Ch 7 Exercises 2, 5 (functions) Ch 10 Exercise 2 (arrays) [we'll do this tonight].

4 Function Review Normally parameters are Pass by Value A copy of the value is passed into function. If copy is modified in function, original variable is unchanged. Exception: Passing an array into a function

5 Function Pass by Value #include using namespace std; bool even(int num); main() { int originalNum = 10; bool retVal; retVal = even(originalNum); cout << originalNum; } bool even(int numPassed) { bool valToRet; if (numPassed % 2 == 0) valToRet = true; else valToRet = false; numPassed = 0; return (valToRet); }

6 Display Line Function Write a function that displays a character some number of times to the display. The function is passed the character and the number of times to display it. 5 minutes: GO!

7 displayChar() Function void displayChar(char charPassed, int times) { int i; for(i = 0; i < times; i++) cout << charPassed; }

8 File Review (Question: What does this program do?) #include using namespace std; main() { ofstream outputStream; //ofstream object int num1, num2, num3; cout << “Enter 3 Numbers for datafile: “; cin >> num1 >> num2 >> num3; outputStream.open(“datafile.txt”); outputStream << num1 << “ “ << num2 << “ “ << num3; outputStream.close(); }

9 Arrays Why arrays? To have one name for a family of related values; we can select individual elements depending on runtime conditions. Syntax to declare an array: type variableName[size]; Memory is set aside for size items of type Each variable location in array is accessed by offsetting/indexing into the array with an integer expression. Legitimate offsets/indexes are 0 to size-1

10 Array Example 1 int values[5] = {2, 4, 6, 8, 1}; (Declare an array with initial values) values[0] = values[0] + values[2]; cout << values[0]; (Use specific elements of an array) int i; cin >> i >> values[i]; (Which element to use can vary at runtime)

11 Array Example 2 char lastname[100]; (Declare a character array, uninitialized.) lastname[0] = 'H'; lastname[1] = 'a'; cout << lastname[0] << lastname[1] << endl; (Set elements of a character array and print them.) lastname[2] = '\0'; cout << lastname << endl; (Printing a character array is treated like printing a string; the null character '\0' indicates the end of the “string”) char name1[4] = {'S','a','m','\0'}; char name2[4] = "Sam"; cout << name1 << endl; cout << name2 << endl; (Character arrays can be initialized with strings.)

12 Array Example // Declare an array of integers numArray of size // ARRAY_SIZE (a constant). // const int ARRAY_SIZE = 10; int numArray[ARRAY_SIZE]; // Use a loop to set every element of numArray to its index. // int index; for(index = 0; index < ARRAY_SIZE; index++) { numArray[index] = index; } // Print numArray (all on one line) // for (index = 0; index < ARRAY_SIZE; index++) { cout << numArray[index] << " "; }

13 Array Example const int ARRAY_SIZE = 10; int index, sum = 0, average; int numArray[ARRAY_SIZE]; for(index = 0; index < ARRAY_SIZE; index++) { cout << “Enter Score ” << index << “ : “; cin >> numArray[index]; } for(index = 0; index < ARRAY_SIZE; index++) { sum = sum + numArray[index]; } average = sum / ARRAY_SIZE;

14 Passing an Array to a Function Pgm'g Ex 9.2: Write a C++ function, smallestIndex, that takes as parameters an int array and its size and returns the index of the first occurrence of the smallest element of the array. Also, write a function to test your function. E.g., given int values[6] = { 0, -1, 4, 3, -1, 7 }; we'd like smallestIndex(values,6) to return 2 (Why?) Function prototype: int smallestIndex(int a[], int len); (At runtime, we'll use values as the value of array a.)

15 A driver program calls a routine for testing purposes. "Driver" Program int main() { int values[6] = { 0, -1, 4, 3, -1, 7 }; int indexFound = smallestIndex(values, 6); cout << "Index found: " << indexFound << ", value: " << values[indexFound] << endl; return 0; }

16 Think about smallestIndex Basic idea: Use a loop to traverse array a. Keep track of the smallest value we've seen and its index. Whenever we find a new smallest value, we update our variables that hold the smallest value and its index. Initially, the smallest value is a[0] and the index of the smallest value is 0. We have to look at elements a[1], a[2], …, a[len-1].

17 Pseudocode for smallestIndex smallestValue = a[0] smallestIndex = 0 for i = 1, 2, …, len-1 if a[i] is smaller than smallestValue update smallestValue = a[i] and smallestIndex = i; otherwise we're done looking at a[i] When loop ends, return smallestIndex.

18 Code for smallestIndex // smallestIndex(a, len) searches a[0], a[1],..., // a[len-1] to find the smallest value; it returns // the index of the smallest value. If the smallest // value occurs more than once, we return the index // of the leftmost occurrence. // int smallestIndex(int a[], int len) { int smallestValue, smallestIndex; smallestValue = a[0]; smallestIndex = 0; int i; for (i = 1; i < len ; i++) { if (a[i] < smallestValue) { smallestValue = a[i]; smallestIndex = i; } return smallestIndex; }