CSc Introduc/on to Compu/ng. Lecture 8 Edgardo Molina Fall 2011 City College of New York

Transcription

1 CSc Introduc/on to Compu/ng Lecture 8 Edgardo Molina Fall 2011 City College of New York 18

2 The Null Statement Null statement Semicolon with nothing preceding it ; Do-nothing statement required for syntax purposes only C++ for Engineers and Scientists, Third Edition 19

3 for Loops for statement: A loop with a fixed count condition that handles alteration of the condition Syntax: for (initializing list; expression; altering list) statement; Initializing list: Sets the starting value of a counter Expression: Contains the maximum or minimum value the counter can have; determines when the loop is finished C++ for Engineers and Scientists, Third Edition 20

4 for Loops (continued) Altering list: Provides the increment value that is added or subtracted from the counter in each iteration of the loop If initializing list is missing, the counter initial value must be provided prior to entering the for loop If altering list is missing, the counter must be altered in the loop body Omitting the expression will result in an infinite loop C++ for Engineers and Scientists, Third Edition 21

5 for Loops (continued) C++ for Engineers and Scientists, Third Edition 22

6 for (int i=0; i<limit; i++) { // do something } Figure 5.7 for loop flowchart. for Loops (continued) C++ for Engineers and Scientists, Third Edition 23

7 A Closer Look: Loop Programming Techniques These techniques are suitable for pretest loops (for and while): Interactive input within a loop Includes a cin statement within a while or for loop Selection within a loop Using a for or while loop to cycle through a set of values to select those values that meet some criteria C++ for Engineers and Scientists, Third Edition 24

8 A Closer Look: Loop Programming Techniques (continued) C++ for Engineers and Scientists, Third Edition 25

9 A Closer Look: Loop Programming Techniques (continued) Evaluating functions of one variable Used for functions that must be evaluated over a range of values Noninteger increment values can be used C++ for Engineers and Scientists, Third Edition 26

10 A Closer Look: Loop Programming Techniques (continued) C++ for Engineers and Scientists, Third Edition 27

11 A Closer Look: Loop Programming Techniques (continued) Interactive loop control Variable is used to control the loop repetitions Provides more flexibility at run-time C++ for Engineers and Scientists, Third Edition 28

12 A Closer Look: Loop Programming Techniques (continued) C++ for Engineers and Scientists, Third Edition 29

13 Nested Loops Nested loop: A loop contained within another loop All statements of the inner loop must be completely contained within the outer loop; no overlap allowed Different variables must be used to control each loop For each single iteration of the outer loop, the inner loop runs through all of its iterations C++ for Engineers and Scientists, Third Edition 30

14 Nested Loops (continued) Figure 5.9 For each i, j loops. C++ for Engineers and Scientists, Third Edition 31

15 Nested Loops (continued) C++ for Engineers and Scientists, Third Edition 32

16 Nested loops (independent loops) #include <iostream> #include <cmath> using namespace std; int main() { double x, y; const double MAX_X = 5; const double MAX_Y = 4; } for (x = 1; x <= MAX_X; x++) { cout << "x = " << x << endl; } >./forloop.exe x = 1 y = 1 y = 2 y = 3 y = 4 x = 2 y = 1 y = 2 y = 3 y = 4 x = 3 y = 1 y = 2 for(y=1; y <= MAX_Y; y++){ y = 3 y = 4 cout << "\ty = " << y << endl; x = 4 } y = 1 y = 2 y = 3 y = 4 x = 5 y = 1 y = 2 y = 3 C++ for Engineers and Scientists, Third Edition y = 4 33 return 0; Sample Run

17 Nested loops (dependent loops) #include <iostream> #include <cmath> using namespace std; int main() { double x, y; const double MAX_X = 5; const double MAX_Y = 4; for (x = 1; x <= MAX_X; x++) { cout << "x = " << x << endl; } for(y=1; y <= x; y++){ cout << "\ty = " << y << endl; } return 0; >./forloop.exe x = 1 y = 1 x = 2 y = 1 y = 2 x = 3 y = 1 y = 2 y = 3 x = 4 y = 1 y = 2 y = 3 y = 4 x = 5 y = 1 y = 2 y = 3 y = 4 y = 5 Sample Run } C++ for Engineers and Scientists, Third Edition 34

18 Nested loops and using continue #include <iostream> #include <cmath> using namespace std; int main() { double x, y; const double MAX_X = 5; const double MAX_Y = 4; for (x = 1; x <= MAX_X; x++) { cout << "x = " << x << endl; } if (x < 4) { continue; } for(y=1; y <= x; y++){ cout << "\ty = " << y << endl; } return 0; >./forloop.exe x = 1 x = 2 x = 3 x = 4 y = 1 y = 2 y = 3 y = 4 x = 5 y = 1 y = 2 y = 3 y = 4 y = 5 Sample Run } C++ for Engineers and Scientists, Third Edition 35

19 do while Loops do while loop is a posttest loop Loop continues while the condition is true Condition is tested at the end of the loop Syntax: do statement; while (expression); All statements are executed at least once in a posttest loop C++ for Engineers and Scientists, Third Edition 36

20 do while Loops (continued) Figure 5.10 The do while loop structure. C++ for Engineers and Scientists, Third Edition 37

21 do while Loops (continued) Figure 5.11 The do statement s flow of control. C++ for Engineers and Scientists, Third Edition 38

22 Validity Checks Useful in filtering user-entered input and providing data validation checks Can enhance with if-else statement C++ for Engineers and Scientists, Third Edition 39

23 Common Programming Errors Making the off by one error: loop executes one too many or one too few times Using the assignment operator (=) instead of the equality comparison operator (==) in the condition expression Testing for equality with floating-point or doubleprecision operands; use an epsilon value instead C++ for Engineers and Scientists, Third Edition 40

24 Common Programming Errors (continued) Placing a semicolon at the end of the for clause, which produces a null loop body Using commas instead of semicolons to separate items in the for statement Changing the value of the control variable Omitting the final semicolon in a do statement C++ for Engineers and Scientists, Third Edition 41

25 Summary Loop: A section of repeating code, whose repetitions are controlled by testing a condition Three types of loops: while for do while Pretest loop: Condition is tested at beginning of loop; loop body may not ever execute; ex., while, for loops C++ for Engineers and Scientists, Third Edition 42

26 Summary (continued) Posttest loop: Condition is tested at end of loop; loop body executes at least once; ex., do while Fixed-count loop: Number of repetitions is set in the loop condition Variable-condition loop: Number of repetitions is controlled by the value of a variable C++ for Engineers and Scientists, Third Edition 43