CS31 Discussion 1E Spring 17 : week 05

Transcription

1 CS31 Discussion 1E Spring 17 : week 05 TA: Bo-Jhang Ho bojhang@cs.ucla.edu Credit to former TA Chelsea Ju

2 Road map of CS31 String Array Function Class Variable If / else Loops Pointer

3 Today s Agenda } Use project 3 to get more insights about functions } Peer discussion

4 When we talk about function } How function are executed in a program } Consumer-Provider relation

5 An example of a function int square(int a) { return a * a; } int main() { int a = 3; int b = 6; cout << square(a + b) << endl; } return 0; } Are two variable a s the same?

6 An example of a function int square(int a) { return a * a; } int main() { int a = 3; int b = 6; cout << square(a + b) << endl; } return 0; } Different functions have their own variable sets } Try it on VC++ / Xcode

7 What is your role? User Provider } How to use the function? } What s the goal of the function? } How do you expect users to use this function? } How do you implement it?

8 Math example User 10 cout <<

9 Math example User #include <iostream> using namespace std; int main() { cout <<???????????????????? << endl; } return 0;

10 Math example User } Step 1: Use your own words to ask Google

11 Math example User } Step 1: Use your own words to ask Google } Step 2: Read the parameter description carefully

12 Math example User #include <iostream> #include <cmath> using namespace std; int main() { cout << (1 + pow(2.7, 10.0)) << endl; } return 0;

13 Dice throw example User cout <<

14 Dice throw example User } Step 1: Use your own words to ask Google

15 Dice throw example User } Step 1: Use your own words to ask Google } Step 2: Read the parameter description carefully

16 Math example User #include <iostream> #include <cstdlib> using namespace std; int main() { cout << (rand() % 6 + 1) << endl; } return 0;

17 Math example User #include <iostream> #include <cstdlib> using namespace std; int main() { cout << (rand() % 6 + 1) << endl; } return 0; } Question: why it always give the same value?

18 Project 3 warm-up User } What s the difference between #include and #include <...>?

19 Project 3 warm-up User } *.h shows the interface of the functions } And this is what you need only } *.cpp should be included to make the program complete } You re welcome to check the details, but it s not necessary

20 Questions you have to ask yourself when developing a function Provider } What s the goal of the function? } How do you expect users to use this function? } What information have to pass to your function? } What message are you going to pass back? } How do you implement it?

21 1. What s the goal of the function Provider

22 Provider 2. How people are going to use this function A: Hey, I will give you a base and an exponent B: Sure, I will give you the result

23 Provider 2. How people are going to use this function

24 Provider 2. How people are going to use this function Please pass a coordinate (r, c) to me, remember to mention whether it s a horizontal line or a vertical line, also give me the length as well as the character. Let me know the mode: I can support both foreground and background. If I can successfully draw a line based on the parameters you gave me, I tell you true, otherwise I reply false.

25 Provider 2. How people are going to use this function true distance dir plotchar false Start at (r, c)

26 3. How to implement Provider } Modular design!

27 3. How to implement Provider } Modular design!

28 3. How to implement Provider } Modular design!

29 Determine function call graph void printspacestarline(int numspaces, int numstars); void printlayer(int startnumspaces, int startnumstars) { printspacestarline(startnumspaces, startnumstars); printspacestarline(startnumspaces - 1, startnumstars + 2); printspacestarline(startnumspaces - 2, startnumstars + 4); } int main() { printlayer(5, 1); // * // *** // ***** printlayer(4, 3); // *** // ***** // ******* printlayer(3, 5); // ***** // ******* // ********* printlayer(2, 7); // ******* // ********* //*********** printspacestarline(4, 3); // *** printspacestarline(4, 3); // *** return 0; }

30 Determine function call graph void printspacestarline(int numspaces, int numstars); void printlayer(int startnumspaces, int startnumstars) { printspacestarline(startnumspaces, startnumstars); printspacestarline(startnumspaces - 1, startnumstars + 2); printspacestarline(startnumspaces - 2, startnumstars + 4); } int main() { printlayer(5, 1); printlayer(4, 3); printlayer(3, 5); printlayer(2, 7); printspacestarline(4, 3); printspacestarline(4, 3); return 0; } main() paintlayer() paintspacestarline() Call direction

31 Determine function call graph int factorial(int n) { int prod = 1; for (int i = 2; i <= n; i++) prod *= i; return prod; } int main() { for (int k = 1; k <= 6; k++) { cout << "The factorial of " << k << " is " << factorial(k) << endl; } } void printfactorial(int n) { int prod = 1; for (int i = 2; i <= n; i++) prod *= i; for (int k = 1; k <= 6; k++) { cout << "The factorial of " << k << " is " << factorial(k) << endl; } } int main() { for (int k = 1; k <= 6; k++) printfactorial(k); } Function fun question 4 Function fun question 5 main() factorial() main() printfactorial() Call direction

32 Determine function call graph int factorial(int n) { int prod = 1; for (int i = 2; i <= n; i++) prod *= i; return prod; } void printfactorial(int n) { for (int k = 1; k <= n; k++) { cout << "The factorial of " << k << " is " << factorial(k) << endl; } } int main() { for (int k = 1; k <= 6; k++) printfactorial(k); } main() printfactorial() factorial()

33 3. How to implement Provider Always pay attention to what are available to you!

34 Project 3 warm-up Provider cleargrid() setchar() getchar() draw()

35 Project 3 warm-up Provider main() cleargrid() setchar() getchar() draw()

36 Project 3 Phase 2 Provider plotline() What s the call relation here? cleargrid() setchar() getchar() draw()

37 Project 3 Phase 2 Provider plotline() cleargrid() setchar() getchar() draw()

38 Project 3 Phase 3 Provider main() executecommands() plotline() cleargrid() setchar() getchar() draw()

39 Project 3 Phase 3 Provider main() executecommands() C F@ B@ H00 V00 plotline() cleargrid() setchar() getchar() draw()

40 Project 3 Phase 3 Provider main() executecommands() C F@ B@ H00 V00 plotline() cleargrid() setchar() getchar() draw()

41 Project 3 - executecommands() Provider } A parser question " * 69" "report.docx" "report.png" "int main() { return 0; }" Give me the calculation result Tell me the file type Generate a program

42 Project 3 - executecommands() Provider } A parser question } A typical approach: } Tokenize } Interpret / convert tokens "v2b h12fhh1fih0"

43 Project 3 - executecommands() Provider } A parser question } A typical approach: } Tokenize } Interpret / convert tokens "v2b h12fhh1fih0"

44 Project 3 - executecommands() Provider } A parser question } A typical approach: } Tokenize } Interpret / convert tokens } Why parsing is hard? } The length of tokens vary } There can be many exceptions and hard to come out a unified parsing strategy } In most cases, there are constraints across tokens (but not in this assignment) } Have to report precise error position

45 Project 3 - executecommands() Provider } Write down a lot of both good examples and bad examples

46 Project 3 - executecommands() Provider } Tokenize: Look forward strategy } Separate into several cases: } C } F@ } B@ } H-- } V } H2 } H22 } H-2 } H-22 "v2b h12fhh1fih0"

47 Discussion time } Find a partner and tell him/her what you have done, what program you encounter now } Brain storming the solution } Read code, use debugger help you, give suggestions