Control Structures 1 The if, for, and while Statements §5.1, §5.4, & §5.5 (Several examples here; also Lab #4)

Basic Kinds of Control The behavior of main() (and other functions) is determined by the statements within it. Statements fall into one of three categories called control structures: Statements that simply execute in ___________. Statements that _________ one of several alternatives. Statements that _________ another statement. 2 EVERY PROGRAM CAN BE WRITTEN USING THESE 3 CONTROL STRUCTURES. See flowcharts in §5.4 & 5.5

Sequential execution The design of computers in which program instructions are stored in memory along with the data processed by the program is known as the ____________________ architecture. A special register, called the _______________________ (IC), stores the address of the memory location where the next instruction to be executed is found. It is initialized with the address of the first instruction and execution proceeds as follows: 3 Fetch the instruction whose address is in the IC Decode it to find the opcode and the operands Execute the operation ______ (unless the IC is changed in some other way) Named after mathematician/ physicist John von Neumann (see Ch.1, p. 8)

The last step (IC++) indicates that the default manner of executing statements is ______________________. In C++, a list of statements enclosed in curly braces { Statement 1 Statement 2... Statement N } 4 is known as a (________________________________ or _________). It is a statement that produces sequential execution of the statements enclosed by the { and }. Note: A compound statement is a single statement and can thus be used any place a statement is needed.  STYLE TIP Indent& align the statements they enclose.

A variable declared in a block is called a. It exists (has memory allocated to it) only from its declaration to the end of the block. We say that its extends from its declaration to the end of the block. A variable declared in a block is called a ________ ___________. It exists (has memory allocated to it) only from its declaration to the end of the block. We say that its ____________ extends from its declaration to the end of the block. For example, in the code For example, in the code... cin >> item; while (item != -999) { int i = 0;... cin >> item; i++; } cout << "Value of i = " << i; the last line won't compile because local variable i is out of scope. The "lifetime" of i ends at the }. 5 Scope This is one of several scope rules. Others will be described later.

In contrast to sequential execution, there are situations in which a problem’s solution requires that a statement be executed selectively, based on a ___________ (a __________ expression): The C++ ________________ is a statement that causes selective execution, allowing a program to choose. The C++ ________________ is a statement that causes selective execution, allowing a program to choose exactly one of two statements and execute it. 6 if (Condition) Statement 1 else Statement 2 optional Note again: An if statement is a single state- ment, even if it extends over many lines. Selective Execution (§ 5.4)

The Simple if If Condition is true, Statement is ____________ ; otherwise Statement is _________. Condition Statement T F 7 The C++ if statement has several different forms. The first form has no or Statement 2, and is called the simple if: The C++ if statement has several different forms. The first form has no else or Statement 2, and is called the simple if: if (Condition) Statement

Examples: 8 // Display date in mm/dd/yyyy format if (month < 10) cout << 0; cout << month << '/'; if (day < 10) cout << 0; cout << day << '/' << year << endl; // if value is negative, make it positive if (value < 0) value = -value;

9 // Stop program if a is 0 // (in quadratic equation solver) if (a == 0) { cerr << "*** Coefficient of x^2 cannot be 0 ***\n"; ____________; // Stop execution & return 1 to OS } Examples showing Statement can be a compound statement: // if a < b, interchange a and b (integers) if (a < b) { int temp = a; a = b; b = temp; } cout is buffered; cerr is not. Common Alternative Style: if (a < b){ int temp = a; a = b; b = temp; } Lab 4: Exer. 4.3

The Two-Branch if In the second form of if, the else and Statement 2 are present: if (Condition) Statement 1 else Statement 2 If Condition is true, Statement 1 is __________ and Statement 2 is _________; otherwise Statement 1 is ___________ and Statement 2 is ________________. Condition Statement 1 T F Statement 2 10 The ternary operator ? : is essentially an inline if of this form. STYLE TIP Align if and else and indent & align the statements in each part..

Example: 11 // Continuation of quadratic equation solver // Legal quadratic equation, so find roots (if any) double discrim = b*b - 4*a*c; if (discrim >= 0) { double root1 = (-b + sqrt(discrim)) / (2*a), root2 = (-b - sqrt(discrim)) / (2*a); cout << "Solutions: " << root1 << ", " << root2 << endl; } else cout << "No real roots\n"; Example:

The Multi-branch if The final form of the if statement is: 12 if (Cond 1 ) Stmt 1 else if (Cond 2 ) Stmt 2... else if (Cond N ) Stmt N else Stmt N+1 of the statements will be selected and executed, namely, the one corresponding to the first that is true. ________________ of the statements Stmt i will be selected and executed, namely, the one corresponding to the first Cond i that is true. STYLE TIP Align if, else if s, and else and indent & align the statements in each part.

The intent is to implement a multi-alternative selection structure of the following form, where exactly one of the alternatives is selected and executed: 13 Stmt 1 Stmt 2 Stmt N Stmt N+1 Stmt 3

Actually, however, it implements a "waterfall" selection structure of the following form: Cond 1 Stmt 1 T F Stmt 2 Cond 2 TF Stmt N Cond N TF Stmt N

15 if (Cond 1 ) Stmt 1 else if (Cond 2 ) Stmt 2 else if (Cond 3 ) Stmt 3... else if (Cond N ) Stmt N else Stmt N+1 This form is surely more difficult to type with all its staggered indents. It also does not display as clearly the different alternatives and that exactly one of them will be selected. And it is treated by the compiler as a sequence of ______________ in which each else clause (except the last) is another if-else statement:

16 if (Cond 1 ) Stmt 1 else if (Cond 2 ) Stmt 2... else if (Cond N ) Stmt N else Stmt N+1 If Cond 1 is true, Stmt 1 is executed and the remaining statements are skipped; otherwise, control moves to Cond 2 ; if Cond 2 is true, Stmt 2 is executed and the remaining statements are skipped; otherwise, control goes to the next condition... if Cond N is true, Stmt N is executed and Stmt N+1 is skipped; otherwise, Stmt N+1 is executed.

17 if (score > 100 || score < 0) cerr << "Invalid score!\n"; else if (score >= 90) grade = 'A'; else if (score >= 80) grade = 'B'; else if (score >= 70) grade = 'C'; else if (score >= 60) grade = 'D'; else grade = 'F'; Example: Assigning letter grades: Using the nested-if form:

18... or the preferred if-else-if form: if (score > 100 || score < 0) cerr << "Invalid score!\n"; else if (score >= 90) grade = 'A'; else if (score >= 80) grade = 'B'; else if (score >= 70) grade = 'C'; else if (score >= 60) grade = 'D'; else grade = 'F'; Here's an example of program efficiency — not doing unnecessary computations. Note the simple conditions;we don't need (score = 90) (score = 80) (score = 70) (score = 60) Do you understand why?

Finally, there are situations where solving a problem requires that a statement be repeated, with the repetition being controlled b a ___________. Finally, there are situations where solving a problem requires that a statement be repeated, with the repetition being controlled by a ___________. 19 There are three parts to the repetition mechanism:There are three parts to the repetition mechanism: –Initialization –Repeated execution –Termination Now we look at one repetition statement in C++, the for statement:Now we look at one repetition statement in C++, the for statement: (§ 5.5) Repetitive Execution (§ 5.5)

20 for (InitializerExpr; LoopCondition; ModifierExpr) Statement where Statement can be either a single statement, or a compound statement. Does the initialization Causes termination — think "while this is true, do the following" Usually modifies something each time through the loop Need not all be on one line And again, note: A for statement is a single statement, even if it extends over many lines. Compared to for statements in other programming languages, C++'s has an unusual syntax.

Statement will be executed so long as ____________ ____________. InitializerExpr LoopCondition Statement ModifierExpr F T Statement is often called the _ of the loop. The for Loop for (InitializerExpr; LoopCondition; ModifierExpr) Statement 21 STYLE TIP Indent and align the statements in the body of the loop.

22 for (InitializerExpr; LoopCondition; ModifierExpr) Statement Each execution of LoopCondition Statement ModifierExpr LoopCondition Statement ModifierExpr is called one repetition or iteration of the loop. InitializerExpr LoopCondition Statement Modifierxpr F T _____

When LoopCondition becomes false,control proceeds to the statement ____________ ______________. InitializerExpr LoopCondition Statement ModifierExpr F T Note: if the LoopCondition is initially false, then the body of the loop will _____________executed. Note: if the LoopCondition is initially false, then the body of the loop will _____________executed. For this reason this is called a ___________ loop. for (InitializerExpr; LoopCondition; ModifierExpr) Statement 23

24Counting Output (suppose limit = 5): limit is 0? What if limit is 1? How get 1, 3, 5,... ? _____ __________ The "normal" use of the for loop is to count: int limit; cin >> limit; for (______________________________________) { cout << count << endl; cout << count << endl;} Declare and initialize the loop- control variable Check if loop-control variable has gone through all its values Inc-/dec-rement loop-control variable Could omit { }; loop body contains a single statement Scope Rule

Nested Loops Loops can also be nested: 1*1 = 1 1*2 = 2 1*3 = 3 2*1 = 2 2*2 = 4 2*3 = 6 Output (suppose limit1 = 2, limit2 = 3): 25 for (int val1 = 1; val1 <= limit1; val1++) { for (int val2 = 1; val2 <= limit2; val2++) { cout << val1 << '*' << val2 << " = " << val1 * val2 << endl; } Could omit { } since each loop body contains a single statement

Input Loops 26 There two different types of input loops: 1. ____________approach: Ask for the number of inputs to be entered and use a for loop. –This method requires knowing in advance how many inputs there are. Example: The Nyhoffian method of assigning letter grades: Find the mean of the numeric scores. This is the B- cutoff. Halfway between the mean and 100% is the A- cutoff. Halfway between the mean and 50% is the C- cutoff. 50% is the D- cutoff.

Example 27 /* Program to display a grading scale (Nyhoffian) for a test. Input: A list of test scores Output: Letter grade cutoffs */ #include using namespace std; int main() { cout << "Program finds letter grade cutoffs for a test.\n\n"; // First find the average double score, sum = 0.0, average; int numScores = 0;

28 cout << "Enter number of scores: "; cin >> numScores; for (int count = 1; count <= numScores; count++) { cout << "Enter a score: "; cin >> score; sum += score; } if (numScores > 0) average = sum / numScores; else { cerr << "\n*** No scores! Stopping program ***\n"; exit(1); } // Compute and display letter grade cutoffs. cout << "\nA-: " << (100 + average) /2 << "%\n" << "B-: " << average << "%\n" << "C-: " << (average + 50) / 2 << "%\n" << "D-: " << 50 << "%\n"; }

Execution of our example program: Program finds letter grade cutoffs for a test. Enter number of scores: 5 Enter a score: 77 Enter a score: 88 Enter a score: 66 Enter a score: 77 Enter a score: 99 A-: 90.7% B-: 81.4% C-: 65.7% D-: 50% 29

The second kind of input loop: 2. ___________ approach: Values are entered until the end of input is signaled by entering a sentinel (or flag) ― a value that signals the end of input. - A _________ loop is commonly used in many programming languages. -This method requires the availability of an appropriate sentinel value.

The while Loop Pattern: while (Expression) Statement Condition T F Statement Statement can be either a single or a compound statement, but is almost always compound. Repetition continues so long as Condition is true. 31

Using while for an Input Loop Pattern: Prompt for first value Read value while (value != sentinel) { Process the value Prompt for another value Read value } 32 Need the first input value so it can be compared with sentinel Get the next value

Example /* Program to display a grading scale (Nyhoffian) for a test. Input: A list of test scores Output: Letter grade cutoffs */ #include using namespace std; int main() { cout << "Program finds letter grade cutoffs for a test.\n\n"; // First find the average double score, sum = 0.0, average; int numScores = 0; 33

cout << "Enter a test score (-1 to quit): "; cin >> score; while (score >= 0) { numScores++; sum += score; cout << "Enter another score (-1 to quit): "; cin >> score; } if (numScores > 0) average = sum / numScores; else { cerr << "\n*** No scores! Stopping program ***\n"; exit(1); } // Compute and display letter grade cutoffs. cout << "\nA-: " << (100 + average) /2 << "%\n" << "B-: " << average << "%\n" << "C-: " << (average + 50) / 2 << "%\n" << "D-: " << 50 << "%\n"; } 34 sentinel

Execution of our example program, but using the while-loop version : Program finds letter grade cutoffs for a test. Enter a test score (-1 to quit): 77 Enter another score (-1 to quit): 88 Enter another score (-1 to quit): 66 Enter another score (-1 to quit): 77 Enter another score (-1 to quit): 99 Enter another score (-1 to quit): -1 A-: 90.7% B-: 81.4% C-: 65.7% D-: 50% 35