Lecture 4. Default Arguments. Set defaults in function prototype. int myfunction( int x = 1, int y = 2, int z = 3 );

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1 Lecture 4 More on functions Default arguments Recursive functions Sorting and Searching (e.g. bubble sort, linear search, binary search) Declaration file (specification), definition file (implementation) Compiling and linking Pointers Pointer arithmetic Relationship between pointers and arrays Lec 4 Programming in C++ 1 Default Arguments Set defaults in function prototype int myfunction( int x = 1, int y = 2, int z = 3 ); myfunction(3) x = 3, y and z get defaults (rightmost) If not enough parameters, rightmost get their default values Default values Can be constants, global variables, or function calls int myfunction( int x = 1, int y = f(5), int z = 3 ); See Fig03_23.cpp Lec 4 Programming in C++ 2 Lec 4 Programming in C++ 1

2 Recursive Functions Recursive functions Functions that call themselves There must be a base case 0! = 1 N! = N * (N-1)! If not base case N > 0 Break problem into smaller problem(s) Launch new copy of function to work on the smaller problem (recursive call/recursive step) Slowly converges towards base case Function makes call to itself inside the return statement Eventually base case gets solved Answer works way back up, solves entire problem Lec 4 Programming in C++ 3 Fibonacci Series Fibonacci series: 0, 1, 1, 2, 3, 5, 8... Each number sum of two previous ones Example of a recursive formula: fib(n) = fib(n-1) + fib(n-2), if n > 1 C++ code for Fibonacci function long fibonacci( long n ) { if ( n == 0 n == 1 ) // base case return n; else return fibonacci( n - 1 ) + fibonacci( n 2 ); } Lec 4 Programming in C++ 4 Lec 4 Programming in C++ 2

3 Fibonacci Series f( 3 ) return f( 2 ) + f( 1 ) return f( 1 ) + f( 0 ) return 1 return 1 return 0 Lec 4 Programming in C++ 5 Fibonacci Series Order of operations return fibonacci( n - 1 ) + fibonacci( n - 2 ); Do not know which one executed first C++ does not specify Recursive function calls Each level of recursion doubles the number of function calls 30 th number = 2^30 ~ 4 billion function calls Exponential complexity Lec 4 Programming in C++ 6 Lec 4 Programming in C++ 3

4 Sorting Arrays Sorting data Searching is more efficient Bubble sort See Fig04_16.cpp Several passes through the array Successive pairs of elements are compared If increasing order (or identical), no change If decreasing order, elements exchanged Repeat these steps for every element Lec 4 Programming in C++ 7 Sorting Arrays (cont.) Swapping variables int x = 3, y = 4; y = x; x = y; What happened? Both x and y are 3! Need a temporary variable Solution int x = 3, y = 4, temp = 0; temp = x; // temp gets 3 x = y; // x gets 4 y = temp; // y gets 3 Lec 4 Programming in C++ 8 Lec 4 Programming in C++ 4

5 Sorting Arrays (cont.) Example: Go left to right, and exchange elements as necessary One pass for each element Original: Pass 1: (elements exchanged) Pass 2: Pass 3: (no changes needed) Pass 4: Pass 5: Largest elements "bubble" to the top (like 7 in this example) Lec 4 Programming in C++ 9 Search in Arrays Non-sorted array Use linear search See Fig04_19.cpp Sorted array Use binary search More efficient See Fig04_20.cpp Lec 4 Programming in C++ 10 Lec 4 Programming in C++ 5

6 Multiple Source-File Programs Most programs are built from a number of source code files Each file contains a collection of related function definitions, class implementation Advantages: easy to develop, debugg, software reusability There are header files (.h) and source files (.cpp) One source has the main() function Lec 4 Programming in C++ 11 file1.h const int K=9; int f(int); file1.cpp #include file1.h int f(int i) { // code goes here} file2.h int square(int,int); double g(char*); file2.cpp #include file2.h int square(int a) { return a*a; } double g(char*) { // code goes here} #include file1.h #include file2.h int main() { std::cout << f(5) << std::endl; Return(square(f(K)); } prog.cpp Lec 4 Programming in C++ 12 Lec 4 Programming in C++ 6

7 Multiple Source-File Programs See program in fig3_10.cpp: it calls function rolldice in file rolldice.cpp At compilation of fig3_10.cpp function rolldice() must be declared. Function rolldice() is declared in header file rolldice.h rolldice.h is included in fig3_10.cpp Lec 4 Programming in C++ 13 Compiling 1. Preprocessing Preprocessor processes all preprocessor directives (# ) E.g. #include f1.h 2. Compiling Translating the source code to executable code, with some holes Each.cpp can be compiled separetely Lec 4 Programming in C++ 14 Lec 4 Programming in C++ 7

8 Compiling (cont.) Syntax errors are detected Type checking is performed When all compiler errors are removed, the translation to machine code takes place The result of compilation is stored in an object file (.o) with object code An object file does not have information enough to run by itself Lec 4 Programming in C++ 15 Linking Put several object files together and create an executable (.exe) Calls to functions outside the file with the main() are linked to their definitions It checks that all called functions exist somewhere among the object files If they are not found you get linking error Functions (object code) from standard library are also linked Lec 4 Programming in C++ 16 Lec 4 Programming in C++ 8

9 Compiling and Linking 1. Separate compilation g++ -c file1.cpp g++ -c file2.cpp file1.cpp file2.cpp file1.o file2.o g++ -c prog.cpp prog.cpp 2. Linking g++ prog.o file1.o file2.o o prog.exe prog.o file1.o file2.o prog.exe prog.o Lec 4 Programming in C++ 17 A Problem An application may have dozens, hundreds of files Compilation can be time consuming You just want to recompile the files that are out date But, to administrate many separately compiled files can be a problem. There is a solution Lec 4 Programming in C++ 18 Lec 4 Programming in C++ 9

10 make and makefile Tools to support compilation and linking The program make takes care of what must be recompiled when you do changes in some file A makefile consists of many entries. Each entry has A target (usually a file) The dependencies (files on which the target depends on) And, commands to run prog.o: prog.cpp file1.h file2.h g++ -c prog.cpp prog.exe: prog.o file1.o file2.o file1.h file2.h g++ prog.o file1.o file2.o o prog.exe Lec 4 Programming in C++ 19 Project Most IDE (Code::Blocks, Borland, DEV- C++) use the concept project to support compilation and linking (instead of having the programmer to write make files) GNU Makefor Windows Lec 4 Programming in C++ 20 Lec 4 Programming in C++ 10

11 Pointer Variables Pointer variables Contain memory addresses as values Normally, variable contains specific value Pointers contain address of variable that has specific value (indirect reference) countptr Indirection Referencing value through pointer Pointer declarations * indicates variable is pointer int *myptr; declares pointer to int Multiple pointers require multiple asterisks int *myptr1, *myptr2; count count Lec 4 Programming in C Pointer Variables Can declare pointers to any data type Card *CardPtr = NULL; Pointer initialization Initialized to 0, NULL, or address of a variable 0 or NULL points to nothing Lec 4 Programming in C++ 22 Lec 4 Programming in C++ 11

12 Pointer Operators: & & (address operator) Returns memory address of its operand Example int y = 5; int *yptr; yptr = &y; //yptr gets address of y yptr y 5 yptr y address of y is value of yptr Lec 4 Programming in C++ 23 Pointer Operators: * * (indirection/dereferencing operator) Returns the variable the pointer is pointing to *yptr returns y (because yptr points to y) *yptr = 9; // assigns 9 to y * and & are inverses of each other See fig05_04.cpp Lec 4 Programming in C++ 24 Lec 4 Programming in C++ 12

13 Pointer Expressions and Pointer Arithmetic Pointer arithmetic Increment/decrement pointer (++, --) Add/subtract an integer to/from a pointer( +, +=, -, -=) Pointers may be subtracted from each other Pointer arithmetic meaningless unless performed on pointer to array 5 element int array on a machine using 4 byte ints vptr points to first element v[0], which is at location 3000 vptr = v; vptr += 2; //sets vptr to 3008 //vptr points to v[2] location v[0] v[1] v[2] v[3] v[4] v vptr Lec 4 Programming in C++ 25 Pointer Expressions and Pointer Arithmetic Subtracting pointers Returns number of elements between two addresses vptr2 = &(v[2]); vptr = &(v[0]); vptr2 - vptr == 2 Pointer can be assigned to another pointer if both of same type int *ptr1, *ptr2; int x; ptr1= &x; ptr2= ptr1; Lec 4 Programming in C++ 26 Lec 4 Programming in C++ 13

14 Pointer Comparison Use equality and relational operators Comparisons meaningless unless pointers point to members of same array int *ptr1, *ptr2; if (ptr1 == ptr2) ; if (ptr1 > ptr2) ; if (ptr1 == NULL) ; Example: could show that one pointer points to higher numbered element of array than other pointer Lec 4 Programming in C++ 27 Relationship Between Pointers and Arrays Arrays and pointers closely related Array name like constant pointer Pointers can do array subscripting operations Accessing array elements with pointers int b[10], *bptr; bptr = b; Element b[n] can be accessed by *(bptr + n) (Called pointer/offset notation) Addresses &b[3] same as (bptr + 3) Lec 4 Programming in C++ 28 Lec 4 Programming in C++ 14

15 Relationship Between Pointers and Arrays Array name can be treated as pointer b[3] same as *(b + 3) Pointers can be subscripted (pointer/subscript notation) bptr[3] same as b[3] The two following function prototypes are equivalent int* f(char[]); int* f(char *); See Fig05_20.cpp See Fig05_21.cpp Lec 4 Programming in C++ 29 Lec 4 Programming in C++ 15