Statements execute in sequence, one after the other, such as the following solution for a quadratic equation:

Transcription

1 Control Structures Sequence Statements execute in sequence, one after the other, such as the following solution for a quadratic equation: double desc, x1, x2; desc = b * b 4 * a * c; desc = sqrt(desc); x1 = (-b - desc) / 2 * a; x2 = (-b + desc) / 2 * a; Statements may appear in a block or compound : { double desc, x1, x2; desc = b * b 4 * a * c; desc = sqrt(desc); x1 = (-b - desc) / 2 * a; x2 = (-b + desc) / 2 * a; In C all definitions must be at the beginning of a block. In C++ you may define variables at any point in the code: { double desc = b * b 4 * a * c; desc = sqrt(desc); double x1 = (-b - desc) / 2 * a; double x2 = (-b + desc) / 2 * a; Variables defined in a block are local to that block and override global variables and variables in outer blocks with the same name. { int a = 1, b = 2; cout << a << b << endl; { int a = 3; cout << a << b << endl; cout << a << b << endl; The above code outputs the following:

2 Selection If Statement The general form of the if and if- s follow: if (condition) if (condition) A may be a single terminated by a semicolon, or a compound. For example: int g; // grade, cout << "passed" << endl; cout << "passed" << endl; cout << "failed" << endl; { cout << "passed" << endl; cout << "good job" << endl; { cout << "failed" << endl; cout << "bad job" << endl; What does the following code output if the grade is 10? cout << "passed" << endl; cout << "good job" << endl; What does the following code output if grade is zero? if (grade = 0) cout << "failed" << endl;

3 Nested If Recall that control structures may be nested. Lets nest an If within another If : if (g > 90) cout << "super" << endl; cout << "good" << endl; if (g > 25) cout << "fair" << endl; cout << "poor" << endl; Well, that's a bit hard to read. Let's rewrite it with proper indenting: if (g > 90) cout << "super" << endl; // g = cout << "good" << endl; // g = if (g > 25) cout << "fair" << endl; // g = cout << "poor" << endl; // g = What is the output for the following code if the grade is 20? if (g > 90) cout << "pt1"; cout << "pt2";

4 Ternary Conditional Operator The following code evaluates i and j, placing the smallest number in min: if (i < j) min = i; min = j; Let's use the ternary operator to accomplish the same logic: min = i < j? i : j; The general form is as follows: condition? trueexpression : falseexpression If the condition evaluates to true (non-zero) then trueexpression is returned. Otherwise, falseexpression is returned. Not meant as a substitute for If-Else, ternary operators sometimes simplify logic. This is illustrated below, in a min function: int min(int x, int y) { int min(int x, int y) { int t; return (x < y)? x : y; if (x < y) t = x; t = y; return t; Switch Statement If you are repeatedly testing a variable for constant values, consider using the switch instead of an if- : if (a == 1) switch(a) { x = 5; case 1: if (a == 3) x = 5; x = 10; break; if (a == 5) case 3: x = 15; x = 10; if (a == 6) break; x = 15; case 5: case 6: x = 100; x = 15; break; default: x = 100; Control transfers directly to the appropriate case, then falls through until there is a break. On a break, control transfers to the next sequential after the switch.

5 There are several implementations of switch. For example, if there are only a few cases the compiler may use if- logic to implement the test. For many cases a jump table may be used and more efficient execution results. Is the switch a structured programming construct? Repetition You can do repetition with while, do-while, and for s. The general format for each follows: while (condition) do while (condition); for (init-stmt; condition; end-stmt) Here is code that counts down from 3 to 1 for all three constructs: // while int x = 3; while (x > 0) { x = x 1; // do-while int x = 3; do { x = x 1; while (x > 0); // for for (int x = 3; x > 0; x = x 1) In the last case, variable x is local to the for and cannot be referenced where. It should be noted that there is a compiler bug in Microsoft Visual C++, version 6, related to this construct. In version 6, variables defined as above have scope outside the for. As an example, the following code would cause a "duplicate symbols" compiler error in version 6: for (int x = 3; x > 0; x = x 1) for (int x = 3; x > 0; x = x 1)

6 For code that's compatible in version 6, and the rest of the world, declare the variable outside the for : int x; for (x = 3; x > 0; x = x 1) for (x = 3; x > 0; x = x 1) Alternatively, simply use different variable names: for (int x = 3; x > 0; x = x 1) for (int y = 3; y > 0; y = y 1) cout << y << endl; Break Statement You can prematurely terminate the execution of a loop with the break. int x = 3; while(1) { if (x == 0) break; x = x 1; When x is zero control transfers out of the while to the next sequential. In this case it would be better to test this value as part of the while condition. Consider the following code: bool done = false; while (1) { do { // 20 lines of code // 20 lines of code if (x == 0) break; if (x == 0) // 20 lines of code done = true; { // 20 lines of code while (!done);

7 In nested loops you can reference a label to break out by more than one level. bool done1 = false; loop: bool done2 = false; while(1) { do { // 20 lines of code1 // 20 lines of code1 while(1) { do { // 20 lines of code2 // 20 lines of code2 if (x == 0) break; if (x == 0) { // 20 lines of code3 done1 = true; if (x == 1) break loop; { // 20 lines of code4 // 20 lines of code3 if (x == 1) { done1 = true; done2 = true; { // 20 lines of code4 while (!done1); while (!done2); Is the break a structured programming construct? Continue Statement Use the continue to transfer control to the top of a loop. For example: for (int x = 1; x <= 15; x = x + 1) { if (x % 5 == 0) continue; This will output the sequence 1, 2, 3, 4, 6, 7, 8, 9, 11, 12, 13, 14. What output does the following code generate? int x = 1; while (x <= 15) { if (x % 5 == 0) continue; x = x + 1; Is the continue a structured programming construct? Goto Statement Use the goto to transfer control. For example: if (x >= 0) goto next; x = 0; next: For this simple example we could have just tested to see if x is less than zero. Use of the goto often leads to spaghetti code and banned in some installations. Here is one instance where use of the goto simplifies logic.

8 void process() { void process() if (error) { if (error) goto cleanup; // cleanup code... return; if (error) goto cleanup; cleanup: if (error) { // cleanup code // cleanup code return;... // cleanup code Alternatively you could place the cleanup code in a function and call it as needed. However, if the cleanup code needs access to several variables local to the process function, simply using a goto is often expedient. Is the goto a structured programming construct?