Comp 11 Lectures. Mike Shah. June 20, Tufts University. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

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1 Comp 11 Lectures Mike Shah Tufts University June 20, 2017 Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

2 Please do not distribute or host these slides without prior permission. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

3 Structs Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

4 Comp 11 - Pre-Class warm up How many different data types do you see on the student ID below. (e.g. Computer Science University is a string) Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

5 Lecture Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

6 Edsgar Dijkstra Figure 1: Edsgar Dijkstra was one of the pioneering computer scientists forming many foundational algorithms and ideas in computer science. His work on concurrency and distributed computing is what makes things like the cloud possible. Variations of Dijkstra s shortest path algorithm are what make your GPS pick a path for you to get from location A to B. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

7 Data Types The student name, a string An ID, perhaps an int or long A leading digit on the barcode, perhaps store as a char or int. A date (3 ints, month, day, year) And several more pieces Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

8 Program a Student ID So lets actually write the program for storing a student ID. 1 #i n c l u d e <i o s t r e a m > 2 #i n c l u d e <s t r i n g > 3 4 i n t main ( ) { 5 6 // Student Data 7 s t d : : s t r i n g name = Smith, Jane ; 8 l o n g i d = ; 9 c h a r d i g i t = 2 ; 0 s t d : : s t r i n g date = 5/31/17 ; 1 2 r e t u r n 0 ; 3 } Listing 1: A program that takes in a student ID Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

9 How does our program scale? By now, you know the next series of questions: What if I want to add another student? What abstraction might I use to help? Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

10 Building a student database How about an array? Or even a std::vector 1 #i n c l u d e <i o s t r e a m > 2 #i n c l u d e <s t r i n g > 3 #i n c l u d e <v e c t o r > 4 5 i n t main ( ) { 6 s t d : : v e c t o r <s t d : : s t r i n g > namevector ; 7 s t d : : v e c t o r <long > i d V e c t o r ; 8 s t d : : v e c t o r <char> d i g i t V e c t o r ; 9 s t d : : v e c t o r <s t d : : s t r i n g > d a t e V e c t o r ; 0 1 namevector. push back ( Smith, Jane ) ; 2 i d V e c t o r. push back ( ) ; 3 d i g i t V e c t o r. push back ( 2 ) ; 4 d a t e V e c t o r. push back ( 5/31/17 ) ; 5 6 r e t u r n 0 ; 7 } Listing 2: Now we can build a database of students Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

11 Populating our Student Database 1 // Add our f i r s t s t u d e n t 2 namevector. push back ( Smith, Jane ) ; 3 i d V e c t o r. push back ( ) ; 4 d i g i t V e c t o r. push back ( 2 ) ; 5 d a t e V e c t o r. push back ( 5/31/17 ) ; 6 // Add a second s t u d e n t 7 namevector. push back ( E i n s t e i n, A l b e r t ) ; 8 i d V e c t o r. push back ( ) ; 9 d i g i t V e c t o r. push back ( 2 ) ; 0 d a t e V e c t o r. push back ( 5/31/17 ) ; 1 // Add a t h i r d s t u d e n t 2 namevector. push back ( Granger, Hermione ) ; 3 i d V e c t o r. push back ( ) ; 4 d i g i t V e c t o r. push back ( 3 ) ; 5 d a t e V e c t o r. push back ( 5/31/17 ) ; Listing 3: Now we can build a database of students Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

12 Strategy to Update database Now what if a students information changes? How would we update our database. Well, we know we can access the student by an index. And that index will be the same for each field That is, accessing namevector[0], idvector[0], etc. holds our first students information in each vector. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

13 Accessing a student 1 // L e t s update our t h i r d s t u d e n t, Hermionie 2 namevector [ 2 ] = Granger, Hermione Jean ; 3 i d V e c t o r [ 2 ] = 0 ; 4 d i g i t V e c t o r [ 2 ] = 7 ; 5 d a t e V e c t o r [ 2 ] = 1/31/18 ; Listing 4: If we need to update our student Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

14 Does everyone like this solution? First, does this work and update Hermione s student information in each of the 4 vectors? What do you think about this approach in general? Figure 2: Hermione Granger is a fictional character from the Harry Potter book series. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

15 Potential problems Sometimes we forget things! 1 // Oops I f o r g o t 1 f i e l d 2 namevector. push back ( Smith, Jane ) ; 3 d i g i t V e c t o r. push back ( 2 ) ; 4 d a t e V e c t o r. push back ( 5/31/17 ) ; 5 // Oops I f o r g o t 2 f i e l d s! 6 namevector. push back ( E i n s t e i n, A l b e r t ) ; 7 d a t e V e c t o r. push back ( 5/31/17 ) ; 8 // Add a t h i r d s t u d e n t 9 namevector. push back ( Granger, Hermione ) ; 0 i d V e c t o r. push back ( ) ; 1 d i g i t V e c t o r. push back ( 3 ) ; 2 d a t e V e c t o r. push back ( 5/31/17 ) ; Listing 5: I forgot to push back all the fields in each of our vectors. Now Ms. Granger is not always index number 2 Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

16 Code Length We have to remember to work with 4 vectors all of the time. What if we want to pass this information to a function? We have to have four parameters that pass the correct data type What if I want to update my student record with a fifth field? We need to find every place in the code and update it. This leaves us prone to errors. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

17 Our next abstraction The point is, we want an abstraction to help make our lives easier A way to structure related pieces of data together. Luckily, C++ (And most languages) have a tool to do this. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

18 The struct Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

19 struct A struct is a C++ keyword It allows us to create a new data type that we define. This data type can consist of multiple primitives. All of the data stays together. Lets take a look Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

20 Our first struct 1 // We now have a t y p e c a l l e d Student ( with a c a p i t a l S ) 2 s t r u c t Student { 3 // Our 4 f i e l d s o f data 4 s t d : : s t r i n g name ; 5 l o n g i d ; 6 c h a r d i g i t ; 7 s t d : : s t r i n g date ; 8 } ; 9 // Known ad a POD p i e c e o f data Listing 6: We have created a new data type. It is our own custom data type that we have defined for our programs. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

21 Our struct in C++ 1 #i n c l u d e <i o s t r e a m > 2 3 // We d e f i n e our s t r u c t o u t s i d e o f main 4 s t r u c t Student { 5 // Our 4 f i e l d s o f data 6 s t d : : s t r i n g name ; 7 l o n g i d ; 8 c h a r d i g i t ; 9 s t d : : s t r i n g date ; 0 } ; 1 2 i n t main ( ) { 3 // We s i m p l y use the name o f our s t r u c t as 4 // the new d a t a t y p e 5 Student j a n e S m i t h ; 6 7 r e t u r n 0 ; 8 } Listing 7: Defining our struct Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

22 Setting Data in our struct 1 #i n c l u d e <i o s t r e a m > 2 s t r u c t Student { 3 s t d : : s t r i n g name ; 4 l o n g i d ; 5 c h a r d i g i t ; 6 s t d : : s t r i n g date ; 7 } ; 8 9 i n t main ( ) { 0 Student j a n e S m i t h ; 1 // Quite s i m i l a r to our v e r y f i r s t example! 2 j a n e S m i t h. name = Smith, Jane ; 3 j a n e S m i t h. i d = ; 4 j a n e S m i t h. d i g i t = 2 ; 5 j a n e S m i t h. date = 5/31/17 ; 6 7 r e t u r n 0 ; 8 } Listing 8: Note that the. operator is how we access our fields Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

23 Summarizing what is new and important terminology We create a struct before our main, which allows us to use a new data type. Our struct can have several data types, that when aggregated make another type.. The. (DOT) operator allows us to access and update each of a struct s individual fields. We can think of a struct as a large piece of data (POD) that we define. We like to think of these as objects rather than variables (which are our primitive types like int, float, char, bool, etc). We call Student, an object type. So janesmith is an instance of an object called Student Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

24 Adding more power to struct A struct has fields (member fields we call them) such as int, char, string, as previously seen. But we can also have member functions, that can be called on our objects. Lets create a member function, that outputs all of our fields values. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

25 Our output member function in use 1 s t r u c t Student { 2 s t d : : s t r i n g name ; 3 l o n g i d ; 4 c h a r d i g i t ; 5 s t d : : s t r i n g date ; 6 // We c r e a t e a r e g u l a r f u n c t i o n 7 // The d i f f e r e n c e i s, i t i s w i t h i n our s t r u c t. 8 // So t h i s f u n c t i o n can o n l y be used on Student s 9 v o i d output ( ) { 0 s t d : : cout << name << \n ; 1 s t d : : cout << i d << \n ; 2 s t d : : cout << d i g i t << \n ; 3 s t d : : cout << date << \n ; 4 } 5 } ; Listing 9: Defining our struct Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

26 output usage (warning text is tiny) caption 1 #i n c l u d e <i o s t r e a m> 2 3 s t r u c t Student{ 4 s t d : : s t r i n g name ; 5 long id ; 6 c h a r d i g i t ; 7 s t d : : s t r i n g d a t e ; 8 9 v o i d output ( ) { 0 s t d : : cout << name << \n ; 1 s t d : : cout << i d << \n ; 2 s t d : : cout << d i g i t << \n ; 3 s t d : : cout << d a t e << \n ; 4 } 5 }; 6 7 i n t main ( ) { 8 Student janesmith ; 9 janesmith. name = Smith, Jane ; 0 janesmith. i d = ; 1 j a n e S m i t h. d i g i t = 2 ; 2 janesmith. date = 5/31/17 ; 3 // C a l l our output method on janesmith. 4 // Remember janesmith s datatype i s of Student, so we can do t h i s 5 j a n e S m i t h. o u t p u t ( ) ; 6 7 r e t u r n 0 ; 8 } Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

27 Is this better than what we started with? 1 j a n e S m i t h. name = Smith, Jane ; 2 j a n e S m i t h. i d = ; 3 j a n e S m i t h. d i g i t = 2 ; 4 j a n e S m i t h. date = 5/31/17 ; Listing 10: Here we define individual fields one at a time This bit of the code is a bit ugly. How could we get rid of it? Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

28 add a set member function 1 s t r u c t Student { 2 s t d : : s t r i n g name ; 3 l o n g i d ; 4 c h a r d i g i t ; 5 s t d : : s t r i n g date ; 6 // Our new s e t method 7 v o i d s e t ( s t d : : s t r i n g name, l o n g i d, c h a r d i g i t, s t d : : s t r i n g d a t e ) { 8 name = name ; 9 i d = i d ; 0 d i g i t = d i g i t ; 1 date = d a t e ; 2 } 3 4 v o i d output ( ) { 5 s t d : : cout << name << \n ; 6 s t d : : cout << i d << \n ; 7 s t d : : cout << d i g i t << \n ; 8 s t d : : cout << date << \n ; 9 } 0 } ; Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

29 What is our set member function doing? We created a new member function called set that takes four parameters We then assign whatever values are passed into that function to our member variables. This sets the member variables by a simple function call. Remember, our member variables are defined within our struct, but outside of our function thus they are within scope. That is why they persist. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

30 Using set 1 i n t main ( ) { 2 Student j a n e S m i t h ; 3 // Ah, much b e t t e r. Now o n l y one l i n e o f code 4 j a n e S m i t h. s e t ( Smith, Jane, j a n e S m i t h. id, , 2, 5/31/17 ) ; 5 // We output a l l o f our f r e s h l y s e t member v a r i a b l e s 6 j a n e S m i t h. output ( ) ; 7 8 r e t u r n 0 ; 9 } Listing 12: We call the member function set on janesmith Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

31 Creating multiple students 1 i n t main ( ) { 2 Student j a n e S m i t h ; 3 j a n e S m i t h. s e t ( Smith, Jane, , 2, 5/31/17 ) ; 4 j a n e S m i t h. output ( ) ; 5 6 Student a l b e r t E i n s t e i n ; 7 a l b e r t E i n s t e i n. s e t ( E i n s t e i n, A l b e r t, , 2, 5/31/17 ) ; 8 a l b e r t E i n s t e i n. output ( ) ; 9 0 Student hermionegranger ; 1 hermionegranger. s e t ( Granger, Hermione, , 3, 5/31/17 ) ; 2 hermionegranger. output ( ) ; 3 4 r e t u r n 0 ; 5 } Listing 13: We create more students and they each have their own member variables unique to themselves Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

32 Full Example (Try on your own after class) 1 #i n c l u d e <i o s t r e a m> 2 3 s t r u c t Student{ 4 s t d : : s t r i n g name ; 5 long id ; 6 c h a r d i g i t ; 7 s t d : : s t r i n g d a t e ; 8 9 v o i d s e t ( s t d : : s t r i n g name, l o n g i d, c h a r d i g i t, s t d : : s t r i n g d a t e ){ 0 name = name ; 1 id = i d ; 2 d i g i t = d i g i t ; 3 date = date ; 4 } 5 6 v o i d o u tput ( ){ 7 s t d : : cout << name << \n ; 8 s t d : : cout << i d << \n ; 9 s t d : : cout << d i g i t << \n ; 0 s t d : : cout << d a t e << \n ; 1 } 2 }; 3 4 i n t main ( ){ 5 Student janesmith ; 6 janesmith. set ( Smith, Jane, , 2, 5/31/17 ) ; 7 j a n e S m i t h. o u t p u t ( ) ; 8 9 Student a l b e r t E i n s t e i n ; 0 alberteinstein. set ( Einstein, Albert, , 2, 5/31/17 ) ; 1 a l b e r t E i n s t e i n. output ( ) ; 2 3 Student hermionegranger ; 4 hermionegranger. set ( Granger, Hermione, , 3, 5/31/17 ) ; 5 hermionegranger. output ( ) ; 6 7 r e t u r n 0 ; 8 } Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

33 C++ everywhere! Or try it here! (Press run and see output at the bottom of the page) https: //wandbox.org/nojs/clang-4.0.0/permlink/couyabq0lmnir9wg Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

34 Final Takeaways We have discovered a new abstraction that allows us to move beyond primitive data structions. We can create and use member functions (In the same way we have seen member functions with string). We can also combine this abstraction with others we have learned. Passing a Student struct in a function Creating a vector of our Student struct is now possible. Hmm, maybe we should try the above in lab. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

35 Review of what we learned (At least) Two students Tell me each 1 thing you learned or found interesting in lecture. Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

36 No in-class activity today Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

37 5-10 minute break and then... Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38

38 To the lab! Lab: You should have gotten an and hopefully setup an account at prior to today. If not no worries, we ll take care of it during lab! Mike Shah (Tufts University) Comp 11 Lectures June 20, / 38