Introduction to C Recursion, sorting algorithms, files

Transcription

1 Introduction to C Recursion, sorting algorithms, files

2 Teil I. recursion TU Bergakademie Freiberg INMO M. Brändel

3 RECURSION Recursion in programming is a way of solving a problem, by solving the same problem in a smaller instance first. Examples for problems that can be solved recursively are: Hint faculty of a number greatest common divisor quicksort binary search In praxis recursion reveals itself in a function, that calls itself with different arguments, than it was called with. TU Bergakademie Freiberg INMO M. Brändel

4 RECURSION Task 23 For an autodidactic approach go to science)#recursive_programs and convert the pseudocode for the greatest common divisor to C code! Enable user interaction in your program! Task 24 Write a program that computes the faculty of a user-given number recursively as well as iteratively! TU Bergakademie Freiberg INMO M. Brändel

5 Teil II. sorting algorithms TU Bergakademie Freiberg INMO M. Brändel

6 SORTING Sorting in programming is a very important topic. We will mostly sort lists of names, dates or numbers, but we are not limited to these types of data. When using a sorting algorithm important aspects are the time it needs to be programmed and the time it needs to sort. We will consider two examples for sorting algorithms: bubblesort: slow, but easy to comprehend quicksort: fast, but harder to comprehend TU Bergakademie Freiberg INMO M. Brändel

7 BUBBLESORT The bubblesort algorithm is the simplest sorting algorithm: Passing through the whole list indefinitely, checking neighbouring elements if they are in the correct order, swapping neighbouring elements if they are not until it passed through the whole list once without swapping any elements. Task 25 Write a program where an array is filled with random numbers and sorted with the bubblesort algorithm! TU Bergakademie Freiberg INMO M. Brändel

8 QUICKSORT The quicksort algorithm is an advanced sorting algorithm that relies heavily on recursion: Take a pivot element, e.g. the last of the list. Sort the lists elements, so that all elements smaller than the pivot are in the left part and all elements bigger are in the right part of the list. Swap the pivot with the first element of the right list. Use quicksort on both the left and the right part of the list. Task 26 Using partition_scheme as a source, write a program that sorts a randomly filled array with the quicksort algorithm! TU Bergakademie Freiberg INMO M. Brändel

9 FINAL C PROGRAM Task 27 - comparesort Write a program that fills an array with however many doubles from the interval [0, 1], as the user wants to! Print the array nicely formatted back to the user! Compute the mean, the minimum and the maximum and return these values to the user as well! Copy the array to two seperate arrays! Sort these array with both bubblesort and quicksort algorithm! Plot the sorted array! Measure the time needed for the sorting! Compare the times in a readable output! Use: functions, pointers, dynamic memory allocation, makefile,... TU Bergakademie Freiberg INMO M. Brändel

10 Teil III. files TU Bergakademie Freiberg INMO M. Brändel

11 FILES Files play an important role in data managment. We divide between binary files, text files and other files. Every file consists at the deepest level of bytes. We focus on the reading and writing of text files. The function fopen() creates an object of type FILE and gives us a pointer from the same type in return, so we can access the files input or output streams. F I L E fopen ( char filename, char mode ) ; The argument filename is the global or local system path to the file we want to access. TU Bergakademie Freiberg INMO M. Brändel

12 FILES The argument mode sets the purpose of the file access. mode r w a r+ w+ a+ description Opens an existing file for reading. Opens a file for writing. Overwriting content. Opens a file for writing. Appending content. Opens an existing file for reading and writing. Opens a file for writing. Overwriting content. Opens a file. Read from start. Write appending. To close an opened file we use: i n t f c l o s e ( F I L E fp ) ; TU Bergakademie Freiberg INMO M. Brändel

13 FILES To write a file we can choose between various functions: fputc(), writes a single character into the file. fputs(), writes a string into the file. fprintf(), writes a string into the file. To read from a file: fgetc(), gets a single character from the file. fgets(), gets a defined amount of characters at once, stops at a newline. fscanf(), gets characters from the file, stops at a space character. TU Bergakademie Freiberg INMO M. Brändel