6. Pointers, Structs, and Arrays. 1. Juli 2011

Transcription

1 1. Juli 2011 Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 1 of 50

2 Outline Recapitulation Pointers Dynamic Memory Allocation Structs Arrays Bubble Sort Strings Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 2 of 50

3 Recapitulation What is an enum? How is it encoded? What is a for loop? What is a declaration and what is a definition? What is a namespace for? What is a header file and how do we use it? What is an enum? What happens at the end of a scope? What is the difference between call-by-value and call-by-reference? Why is there a range for all primitive variables in C? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 3 of 50

4 6.1. Pointers Memory 21: 22; 20 // int a 23; 24; 25; 26; 27; 28: We have talked a lot about the mapping from variables to addresses. However, we have never analysed the real addresses. The memory is enumerated using an integer. Pointers are variables that hold the memory number (address) instead of a value. / / holds a f l o a t i n g p o i n t / / number double a ; i n t a ; / / holds address of a f l o a t i n g / / p o i n t number i n t b ; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 4 of 50

5 We Have Already Used Pointers Before : C/C++ only supports call-by-value. Call-by-reference is not a language concept. void foo ( i n t & a ) { / / a c t u a l l y, C/C++ does not support t h i s... i n t a = 20; foo ( a ) ; void foo ( i n t a ) { / / address now i s copied ( c a l l by value ) / / work on address of a, not on a d i r e c t l y... i n t a = 20; foo ( / address of a / ) ; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 5 of 50

6 Pointers And Addresses The * operator declares a pointer. i n t a ; i n t p ; The & operator returns the address of a variable (address operator or reference operator). p = &a ; The * operator makes an operation act on the location an pointers points to instead of the pointer itself (dereferencing operator. a += 10; ( p ) += 10; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 6 of 50

7 Pointers and the Memory Memory 21: 22; 20 // int a 23; 24; 22 // pointer p 25; 26; 27; 28: int a; a = 20; int *p; p = &a; a++; p++; (*p)++; *p++; // try this at home Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 7 of 50

8 Exercise void foo ( i n t & a ) { a+=1; i n t main ( ) { i n t a = 20; foo ( a ) ; std : : cout << a ;... Rewrite exactly this code without a return statement and without the reference operator. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 8 of 50

9 Solution void foo ( i n t a ) { ( a )+=1; / / i n d i r e c t memory access i n t main ( ) { i n t a = 20; / / d i r e c t memory access foo (&a ) ; std : : cout << a ;... There s three modifications. Analyse the call-by-value for the pointer. Which solution is the better/nicer solution? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 9 of 50

10 When to Use Pointers void foo ( i n t a ) { ( a )++;... a++; / / That could not have happened with references Pointers are difficult to handle (lots of syntactic overhead). We know how to do it, but do collaborators know? Avoid it whenever possible. Use C++ references instead. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 10 of 50

11 Pointer Syntax The pointer operator binds to the right neighbour. i n t a ; i n t b1 ; i n t b2 ; i n t b3 ; i n t c1, c2 ; i n t c3, c4 ; i n t c5, c6 ; i n t c7, c8 ; i n t c9 ; c7 = c8 ; c7 = c8 ; c7 = c8 ; c7 = c8 ; By the way, often people write int* p=0; to make the pointer point to nothing. However, also int* p = 20; would be fine (and for almost 100 percent is a bug). Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 11 of 50

12 Pointers and Scopes i n t p = &a ; f o r ( i n t i =0; i <2; i ++) { i n t a = 2 i ; p = &a ; std : : cout << a << std : : endl ; std : : cout << p << std : : endl std : : cout << a << std : : endl ; / / does t h i s work? std : : cout << p << std : : endl ; / / does t h i s work? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 12 of 50

13 Pointers and Scopes i n t p = &a ; f o r ( i n t i =0; i <2; i ++) { i n t a = 2 i ; p = &a ; std : : cout << a << std : : endl ; std : : cout << p << std : : endl std : : cout << a << std : : endl ; / / does t h i s work? std : : cout << p << std : : endl ; / / does t h i s work? With pointers, we can violate the end-of-scope rules, i.e. we can access variables that do not exist anymore. This is the principle of all these buffer overrun malware. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 12 of 50

14 6.2. Dynamic Memory Allocation Switching Off the Lifecycle Management double p ; p = new double ; / / now, p p o i n t s to an e x i s t i n g new v a r i a b l e p = 20; delete p ; / / now v a r i a b l e i s freed, but p s t i l l p o i n t s / / to t h i s v a r i a b l e. p = 30; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 13 of 50

15 Two Memory Leaks double p ; f o r ( i n t i =0; i <20; i ++) { p = new double ; delete p ; f o r ( i n t i =0; i <20; i ++) { double p = new double ; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 14 of 50

16 Two Memory Leaks double p ; f o r ( i n t i =0; i <20; i ++) { p = new double ; delete p ; f o r ( i n t i =0; i <20; i ++) { double p = new double ; The upper operation creates 20 doubles on the heap, but it destroys only one double in the end. Consequently, the remaining 19 doubles are lost for the future program execution. Let s sketch the memory layout! The second one destroys the pointer but not the space where the pointer is pointing to. This is why many applications crash after several hours of execution. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 14 of 50

17 6.3. Structs n-tuples / This represents one student i n our o n l i n e system / s t r u c t Student { i n t number ; double averagegrade ; ; Structs allow us to create user-defined data structures (n-tuples). Once declared, we can use them like built-in types. C/C++ takes care of the memory layout. This is particularly important for arrays of structs. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 15 of 50

18 Declaration and Definition / / T e l l s compiler t h a t something / / l i k e a StudentID does e x i s t s t r u c t StudentID ; / This represents one student i n our o n l i n e system / s t r u c t Student { i n t number ; double averagegrade ; StudentID id ; ; For pointers (and references), it is sufficient to tell the compiler that the type does exist (declaration). The declaration is needed for recursive definitions of structs. The declaration is needed for more the composition of structs. We ll first analyse struct composition. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 16 of 50

19 Struct Composition (Attributes) / / T e l l s compiler t h a t something / / l i k e a StudentID does e x i s t s t r u c t StudentID { i n t photonumber ; bool iswinterterm ; ; / This represents one student i n our o n l i n e system / s t r u c t Student { i n t number ; double averagegrade ; StudentID id ; / / i t isn t a pointer anymore ; Take a sheet of paper and sketch the memory layout for an instance of Student if the id is a pointer and if it is not a pointer. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 17 of 50

20 Memory Memory 21: 22; 40 // photono 23; true // winter term 24; 25; 26; 1234 // number 27; 3.4 // average grade 28: 22 // pointer 24; 25; 26; 1234 // number 27; 3.4 // average grade 28; 40 // photono 29; true // winter term 30: 31: Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 18 of 50

21 Structs Referencing Structs s t r u c t I n t e g e r E n t r y { i n t value ; I n t e g e r E n t r y next ; ; Take a sheet of paper and sketch the memory layout for an instance of IntegerEntry referencing another instance of IntegerEntry. What could be the reasoning behind such a data structure? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 19 of 50

22 Memory 21: 22; 40 // value 23; 26 // next 24; 25; 26; 23 // value 27; 0 // next 28: Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 20 of 50

23 Accessing the Elements of a Struct / / T e l l s compiler t h a t something l i k e a StudentID does e x i s t s t r u c t I n t e g e r E n t r y { i n t value ; I n t e g e r E n t r y next ; ;... IntegerEntry myentry ; myentry. value = 20; myentry. next = 0; IntegerEntry pentry = new IntegerEntry ; pentry >value = 20; pentry >next = 0; delete pentry ; The dot gives access to sub-fields. The -> operator gives access to sub-fields of pointers. Structs can be used with new as well. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 21 of 50

24 The Single Linked List / / T e l l s compiler t h a t something l i k e a StudentID does e x i s t s t r u c t I n t e g e r E n t r y { i n t value ; I n t e g e r E n t r y next ; ; void p r i n t L i s t ( I n t e g e r E n t r y f i r s t E n t r y ) { while ( f i r s t E n t r y! = 0 ) { std : : cout << f i r s t E n t r y >value << ; f i r s t E n t r y = f i r s t E n t r y >next ; void append ( I n t e g e r E n t r y f i r s t E n t r y, i n t value ) { while ( f i r s t E n t r y >next! = 0 ) { f i r s t E n t r y = f i r s t E n t r y >next ; IntegerEntry newentry = new IntegerEntry ; f i r s t E n t r y >next = newentry ; newentry >next = 0; newentry >value = value ; Can you rewrite append() recursively? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 22 of 50

25 Remove Element Operation Play around with the list and create a list [2, 4, 6, 8]. Write a remove operation that accepts an integer k and removes the kth entry from the list. Invoke it with remove(2). Print the result to the terminal. It should return [2, 4, 8]. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 23 of 50

26 An Excursus on Lists and Complexity the big-o notation f O(n k ) Doesn t grow faster than n k. Faster or slower refers to the leading term of the runtime polynomial. Constants or polynomials of lower order are not considered. Often, one writes = or says is of instead of is contained in. What does this mean for our list? What is the n there? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 24 of 50

27 An Excursus on Lists and Complexity the big-o notation f O(n k ) Doesn t grow faster than n k. Faster or slower refers to the leading term of the runtime polynomial. Constants or polynomials of lower order are not considered. Often, one writes = or says is of instead of is contained in. What does this mean for our list? What is the n there? Study the list in terms of n list entries. If we remove the first element, this is a fixed number of operations (O(1)). If we append an element, our while loop has to run over all n element before we can append an element (O(n)). If we search for an element, we have to study each element exactly once (O(n)). What could we do to make the append operation run faster? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 24 of 50

28 6.4. Arrays Application example: At the university, we wanna keep track of four grades a student did throughout his Master s studies. double analysis1 ; double analysis2 ; double linearalgebra ; double stochastics ;... / Takes grades, computes the average, and returns t h i s value. / double computeaverage ( const double& g1, const double& g2,... ) { double r e s u l t = g1+g2+g3+g4 ; r e s u l t /= 4. 0 ; r e t u r n r e s u l t ; Is this impractical, if we wanna store more than four values. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 25 of 50

29 Array Declaration Memory 21: 22; // grade[0] 23; // grade[1] 24; // grade[2] 25; // grade[3] 26; 27; 28: 22 // grade (pointer) double grade [ 4 ] ;... Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 26 of 50

30 Array Access Memory double grade [ 4 ] ;... 21: 22; // grade[0] 23; // grade[1] 24; // grade[2] 25; // grade[3] 26; 27; 28: 22 // grade (pointer) grade [ 0 ] = 1. 3 ; grade [ 2 ] = 3. 3 ; grade [ 3 ] = 1. 0 ; grade [ 1 ] = 4. 0 ; Depending on the context, [] either defines the size of an array (definition) or gives access to individual entries of an array (access). Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 27 of 50

31 Arrays & Pointers double grade [ 4 ] ; grade [ 0 ] = 1. 3 ; grade [ 2 ] = 3. 3 ; grade [ 3 ] = 1. 0 ; grade [ 1 ] = 4. 0 ; double p = grade ; i f ( grade [0]== p )... / / always t r u e i f ( grade [1]== ( p +1))... / / always t r u e An array variable is basically a pointer to the first element of the array. An array element access internally implies pointer arithmetics and dereferencing. Again, we thus have to range checks (size of array) at hand at all. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 28 of 50

32 Grades Revisited We need a range variable / Takes grades, computes the average, and returns t h i s value. / double computeaverage ( double grade, i n t numberofgrades ) { double r e s u l t = 0. 0 ; f o r ( i n t i =0; i<numberofgrades ; i ++ ) {... double scale = numberofgrades ; r e s u l t /= scale ; r e t u r n r e s u l t ;... double grade [ 4 ] ; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 29 of 50

33 Array Arguments double computeaverage ( double grade [ ], i n t numberofgrades ) {... double computeaverage ( const double grade [ ], i n t numberofgrades ) {... This time, the user is not allowed to modify any entry of grade. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 30 of 50

34 Pitfalls i n t gradesbsc, gradesmsc [ 5 ] ; gradesmsc [ 5 ] = 2. 3 ; gradesmsc [ 3 ] + + ; i n t p0 = gradesmsc ; i n t p1 = &gradesmsc [ 0 ] ; i n t p2 = &gradesmsc [ 1 ] ; gradesmsc++; / / t h a t does not work p2++; / / a r r r g h Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 31 of 50

35 Dynamic Arrays double grades = new double [ 4 5 ] ; delete [ ] grades ; We can create arrays on the heap. Size might be a variable, too. Corresponding delete has to be a delete[]. delete without brackets just deletes the first value. If we omit delete, we will get a memory leak. If we use the array after delete or before new, it points to garbage (remember: grades is only a pointer). Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 32 of 50

36 Array As Return Functions double createthreerandomgrades ( ) { double r e s u l t [ 3 ] ; r e s u l t [ 0 ] = 1. 3 ; r e s u l t [ 1 ] = 2. 7 ; r e s u l t [ 2 ] = 1. 0 ; r e t u r n r e s u l t ; double createthreerandomgrades ( ) { double r e s u l t = new double [ 3 ] ; r e s u l t [ 0 ] = 1. 3 ; r e s u l t [ 1 ] = 2. 7 ; r e s u l t [ 2 ] = 1. 0 ; r e t u r n r e s u l t ; At the end of the scope, the pointer is destroyed always. Arrays on the heap are not destroyed. This is called a factory mechanism. Someone else invoking the function has to delete the array. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 33 of 50

37 Multidimensional Arrays double matrix [ 4 ] [ 4 ] ; f o r ( i n t i =0; i <4; i ++) { f o r ( i n t j =0; j <4; j ++) { matrix [ i ] [ j ] = i == j? 1.0 : 0. 0 ; matrix [ 2 ] [ 1 ] = 1. 0 ; matrix [ 1 2 ] = 1. 0 ; What is the semantics of the for loop? Multidimensional arrays basically are flat arrays. C/C++ uses row-major format (different to FORTRAN). Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 34 of 50

38 Arrays of Structs s t r u c t Person { i n t age ; double weight ; ; Person couple [ 2 ] ; Person p = couple ; p++; C/C++ takes care of the memory padding. It stores the entries in the memory in the following order: couple[0].age, couple[0].weight, couple[1].age, couple[1].weight. The increment sets the pointer to the subsequent age address. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 35 of 50

39 Outlook C/C++ Arrays vs. FORTRAN FORTRAN is claimed to be the language for linear algebra as it is faster. FORTRAN does not provide pointers and dynamic data structures. Consequently, compiler can keep track of who has access where. Consequently, compiler can optimise aggressively (it tries to keep book of all possible values an array could have side-effect!). So, it is all a matter of exclusivity and the const operator. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 36 of 50

40 6.5. Sorting How can we sort an array with n entries? or: Yes We Can (watch a movie) ntqc8 Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 37 of 50

41 Bubble Sort void s o r t ( i n t e n t r i e s [ ], const i n t & numberofentries ) { bool sorted = true ; while ( sorted ) { sorted = f a l s e ; f o r ( i n t i =0; i<numberofentries 1; i ++ ) { i f ( e n t r i e s [ i ]> e n t r i e s [ i + 1 ] ) {... sorted = true ; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 38 of 50

42 Complexity of Bubble Sort Idea of bubble sort: Run over all elements in the list. Compare two subsequent elements whether they are in the correct order. Swap them if necessary. If a swap still had been necessary, run over list again. How expensive is the sorting? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 39 of 50

43 Complexity of Bubble Sort Idea of bubble sort: Run over all elements in the list. Compare two subsequent elements whether they are in the correct order. Swap them if necessary. If a swap still had been necessary, run over list again. How expensive is the sorting? The number of comparisons is a good metric. So, let n be the number of elements in our list. At most (worst case), we ll need n runs over the list. In the average case, we ll need n/2 runs over the list. In each run, we have to do n 1 comparisons. Overall, the complexity is O(n 2 ). More intelligent (but more complex) sorting algorithms need only O(n log n) comparisons. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 39 of 50

44 6.6. Strings C has no string concept. C++ has a nice string concept. C has chars. And C has arrays. Thus, C has arrays of chars. And C has a mapping of numbers to chars. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 40 of 50

45 Codes The ASCII Code Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 41 of 50

46 Termination Codes for Arrays Memory 21: 22; 24 23; 12 24; 71 25; 71 26; 14 27; 0 28: 29: 30: 22 // s Lets have the following mapping: 0 Reserved (termination) 12 a 14 o 24 H 71 l Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 42 of 50

47 Strings as Arrays char s [ 6 ] ; s [ 0 ] = H ; s [ 1 ] = a ; s [ 2 ] = l ; s [ 3 ] = l ; s [ 4 ] = o ; s [ 5 ] = 0 ; Lets have the following mapping: 0 Reserved (termination) 12 a 14 o 24 H 71 l Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 43 of 50

48 Strings are Arrays char s [ 6 ] ; s [ 0 ] = H ; s [ 1 ] = a ; s [ 2 ] = l ; s [ 3 ] = l ; s [ 4 ] = o ; s [ 5 ] = 0 ; char mystring = Hallo ; We don t want to pass an integer with each string. Thus, C appends a 0 as last character. Thus, the array has one element more than the string has chars. Pascal follows a different variant. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 44 of 50

49 Operations on Strings Memory 21: 22; 24 23; 12 24; 71 25; 71 26; 14 27; 0 28: 29: 30: 22 // s Write a function length that takes a string and counts the number of entries. This number then is returned. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 45 of 50

50 Length Operation i n t length ( const char [ ] s ) { i n t length = 0; while ( s [ length ]==0) { length ++; return length ; char mystring = Hallo ; i n t lengthofmystring = length ( mystring ) ; What is copied here (call-by-value)! What happens, if we write char a n o t h e r S t r i n g = mystring ; can we then call length for both strings? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 46 of 50

51 Copying Strings char mystring = This i s a t e s t ; char mycopy = mystring ; delete [ ] mystring ; length ( mystring ) What happens if we execute this code? Write an operation that copies a string. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 47 of 50

52 6.7. Trees Remember our single linked list (for double values). Imagine we wanna hold this list sorted all the time. What would an insert operation look like? What is the (average) runtime of insert in terms of elements? How could we reduce this runime to O(logn)? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 48 of 50

53 A list with three entries s t r u c t L i s t E n t r y { double value ; L i s t E n t r y s m a l l e r E n t r y ; L i s t E n t r y biggerentry ; ; Code the data structure from above, and create a list with three values {0.4, 1.2, 22 (in this order) as follows: Create list for 0.4 (with two null pointers). Create a list element for 1.2 and append it to 0.4. The append function checks whether it is bigger or equal to its current value. It is not, so it is appended to smallerentry. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 49 of 50

54 Trees The example we implemented is a binary tree, as each node has up to two children. The average depth of this tree is O(logn). If we search for an element (as we have to do if we insert), the search consequently also is in O(logn). There s hundreds of variants for trees with different properties. Tree algorithms and recursive code go hand in hand. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 50 of 50