Iterative Constructs Mechanisms for deciding under what conditions an action should be repeated JPC and JWD © 2002 McGraw-Hill, Inc.

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Presentation transcript:

Iterative Constructs Mechanisms for deciding under what conditions an action should be repeated JPC and JWD © 2002 McGraw-Hill, Inc.

Averaging

Determining Average Magnitude Suppose we want to calculate the average apparent brightness of a list of five star magnitude values Can we do it Yes, it would be easy Suppose we want to calculate the average apparent brightness of a list of 8,479 stars visible from earth Yes, but it would be gruesome without the use of iteration Suppose we want to calculate the average apparent brightness of a list of five star magnitude values Can we do it? Yes, it would be easy

C++ Iterative Constructs Three constructs while statement for statement do-while statement

While Syntax

While Semantics

Computing an Average int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl;

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl;

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl;

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum value --

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum value 1

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 1 value 1

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 1 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 1 value 1

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 1 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 1 value 1

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 1 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 1 value --

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 1 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 1 value 5

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 1 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 6 1 value 5

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 2 1 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 6 value 5

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 2 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 6 6 value 5

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 2 2 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 6 value --

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 2 2 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 6 value 3

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 2 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 9 6 value 3

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 3 2 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 9 value 3

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 3 3 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 9 value 3

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 3 3 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 9 value --

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 3 3 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 9 value 1

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 3 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 10 9 value 1

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 4 3 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 10 value 1

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 4 3 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 10 value 1

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 4 3 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 10 average 2.5

Execution Trace Suppose input contains: 1 5 3 1 6 listSize 4 numberProcessed 4 3 int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; sum 10 average 2.5

Stays in stream until extracted Suppose input contains: 1 5 3 1 6 Execution Trace Stays in stream until extracted int listSize = 4; int numberProcessed = 0; double sum = 0; while (numberProcessed < listSize) { double value; cin >> value; sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl;

Power of Two Table const int TableSize = 20; int i = 0; long Entry = 1; cout << "i" << "\t\t" << "2 ** i" << endl; while (i < TableSize) { cout << i << "\t\t" << Entry << endl; Entry = 2 * Entry; ++i; }

Better Way of Averaging The value of the input operation corresponds to true only if a successful extraction was made int numberProcessed = 0; double sum = 0; double value; while ( cin >> value ) { sum += value; ++numberProcessed; } double average = sum / numberProcessed ; cout << "Average: " << average << endl; What if list is empty?

Even Better Way of Averaging int numberProcessed = 0; double sum = 0; double value; while ( cin >> value ) { sum += value; ++numberProcessed; } if ( numberProcessed > 0 ) { double average = sum / numberProcessed ; cout << "Average: " << average << endl; else { cout << "No list to average" << endl;

The For Statement Syntax for (ForInit ; ForExpression; PostExpression) Action Example for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; }

Execution Trace i for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl;

Execution Trace i for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl;

Execution Trace i for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0

Execution Trace i for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0

Execution Trace i 1 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0

Execution Trace i 1 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl;

Execution Trace i 1 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0 i is 1

Execution Trace i 1 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0 i is 1

Execution Trace i 2 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0 i is 1

Execution Trace i 2 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0 i is 1

Execution Trace i 2 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0 i is 1 i is 2

Execution Trace i 2 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0 i is 1 i is 2

Execution Trace i 3 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0 i is 1 i is 2

Execution Trace i 3 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0 i is 1 i is 2

Execution Trace i 3 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; } cout << "all done" << endl; i is 0 i is 1 i is 2 all done

Table Revisiting const int TableSize = 20; long Entry = 1; cout << "i" << "\t\t" << "2**i" << endl; for (int i = 0; i <= TableSize; ++i) { cout << i << "\t\t" << Entry << endl; Entry *= 2; }

Table Revisiting const int TableSize = 20; long Entry = 1; cout << "i" << "\t\t" << "2**i" << endl; for (int i = 0; i < TableSize; ++i) { cout << i << "\t\t" << Entry << endl; Entry = 2 * Entry; } cout << "i is" << i << endl; // illegal The scope of i is limited to the loop!

Displaying a Diagonal SimpleWindow W("One diagonal", 5.5, 2.25); W.Open(); for (int j = 1; j <= 3; ++j) { float x = j * 0.75 + 0.25; float y = j * 0.75 - 0.25; float Side = 0.4; RectangleShape S(W, x, y, Blue, Side, Side); S.Draw(); }

Sample Display

Displaying Three Diagonals SimpleWindow W("Three diagonals", 6.5, 2.25); W.Open(); for (int i = 1; i <= 3; ++i) { for (int j = 1; j <= 3; ++j) { float x = i - 1 + j * 0.75 + 0.25; float y = j * 0.75 - 0.25; float Side = 0.4; RectangleShape S(W, x, y, Blue, Side, Side); S.Draw(); } The scope of i includes the inner loop. The scope of j is just the inner loop.

Sample Display

int Counter1 = 0; int Counter2 = 0; int Counter3 = 0; int Counter4 = 0; int Counter5 = 0; ++Counter1; for (int i = 1; i <= 10; ++i) { ++Counter2; for (int j = 1; j <= 20; ++j) { ++Counter3; } ++Counter4; } ++Counter5; cout << Counter1 << " " << Counter2 << " " << Counter3 << " " << Counter4 << " " << Counter5 << endl;

For Into While Observation The for statement is equivalent to { ForInit; while (ForExpression) { Action; PostExpression; }

Only extracts nonblank characters Counting Characters int NumberOfNonBlanks = 0; int NumberOfUpperCase = 0; char c; while (cin >> c) { ++NumberOfNonBlanks; if ((c >= 'A') && (c <= 'Z')) { ++NumberOfUpperCase; } cout << "Nonblank characters: " << NumberOfNonBlanks << endl << "Uppercase characters: " << NumberOfUpperCase << endl; Only extracts nonblank characters

Counting All Characters char c; int NumberOfCharacters = 0; int NumberOfLines = 0; while ( cin.get(c) ) { ++NumberOfCharacters; if (c == '\n') { ++NumberOfLines } } cout << "Characters: " << NumberOfCharacters << endl << "Lines: " << NumberOfLines << endl; Extracts all characters

File Processing #include <iostream> #include <fstream> using namespace std; int main() { ifstream fin("mydata.txt"); int ValuesProcessed = 0; float ValueSum = 0; float Value; while ( fin >> Value ) { ValueSum += Value; ++ValuesProcessed; } if (ValuesProcessed > 0) { ofstream fout("average.txt"); float Average = ValueSum / ValuesProcessed; fout << "Average: " << Average << endl; return 0; else { cerr << "No list to average" << endl; return 1;

Iteration Do’s Key Points Make sure there is a statement that will eventually terminate the iteration criterion The loop must stop! Make sure that initialization of loop counters or iterators is properly performed Have a clear purpose for the loop Document the purpose of the loop Document how the body of the loop advances the purpose of the loop

The Do-While Statement Syntax do Action while (Expression) Semantics Execute Action If Expression is true then execute Action again Repeat this process until Expression evaluates to false Action is either a single statement or a group of statements within braces Action true Expression false

Waiting for a Proper Reply char Reply; do { cout << "Decision (y, n): "; if (cin >> Reply) Reply = tolower(Reply); else Reply = 'n'; } while ((Reply != 'y') && (Reply != 'n'));