1 / 28

CS 108 Computing Fundamentals April 10, 2014

CS 108 Computing Fundamentals April 10, 2014. Factorial Via Recursion. First: answer = 1 if n = = 0 … factorial(0) = 1 Second: otherwise: factorial(n) = n * factorial ( n - 1 ) Notice that the identifier "factorial" appears on both sides of the assignment operator

angelo
Télécharger la présentation

CS 108 Computing Fundamentals April 10, 2014

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CS 108 Computing Fundamentals April 10, 2014

  2. Factorial Via Recursion • First: answer = 1 if n = = 0 … factorial(0) = 1 • Second: otherwise: factorial(n) = n * factorial( n - 1 ) • Notice that the identifier "factorial" appears on both sides of the assignment operator • How would we write a factorial function to implement these two rules into a C algorithm ?

  3. Factorial Via Recursion • First: 1 if n = 0 • Second: factorial( n ) = n* factorial( n-1 ) if n > 0 int factorial ( int n ) { if n = = 0 return 1 ; else return ( n * factorial ( n – 1 ) ); }

  4. Understanding Recursion • You can think of a recursive function call as if it were calling a completely separate function. • Understand this about recursively called functions: the operations that can be performed by recursively called functions are the same, but the data that is input to each recursively called function is different • The next slide might help to visualize the sameness of operations and the differences in data through the use of different (but very, very similar) functions... we’ll pass factorial_a the value 3.

  5. int factorial_b ( int n ) { if ( n = = 0 ) return 1; else return n * factorial_c ( n - 1) ; } Understanding Recursion int factorial_a ( int m ) { if ( m = = 0 ) return 1; else return m * factorial_b ( m - 1 ) ; } int factorial_c ( int q ) { if( q = = 0 ) return 1; else return q * factorial_d ( q - 1) ; } int factorial_d ( int r ) { if( r = = 0 ) return 1; else return r * factorial_e ( r - 1) ; }

  6. Let's develop a symbol table for the previous slide which is incorporated in this program (this program can handle 0 – 3 as inputs, which is enough to demonstrate the concepts) : • http://web.cs.sunyit.edu/~urbanc/cs_108_apr_08a.txt • Let's look at the addresses and values of the variables/parameters • http://web.cs.sunyit.edu/~urbanc/cs_108_apr_08b.txt • Notice that each version of the factorial functions does the same thing • Therefore, we can condense this into something more general

  7. Recursion Example: Factorial #include <stdio.h> // 2.c int factorial ( int ) ; int main (void) { int input = 0 , answer = 0; printf("\n\n The program computes the factorial of an entered integer. ") ; printf("\n Enter an postive integer: ") ; scanf("%d", &input ) ; answer = factorial ( input ) ; printf("\n\n %d factorial equals: %d \n\n" , input , answer ) ; return 0 ; } int factorial ( int passed_input ) { if ( passed_input = = 0)// if cutting, remove the extra space between = and = return 1 ; else return ( passed_input * factorial ( passed_input - 1 ) ) ; }

  8. Let's develop a symbol table for the previous slide which is incorporated in this program: • http://web.cs.sunyit.edu/~urbanc/cs_108_apr_08c.txt

  9. Questions to Guide The Use of Recursion • Can you define the problem in terms of smaller problem(s) of the same type? • Think about the factorial problem: factorial(n) = n * factorial ( n-1 ) if n > 0 • Does each recursive call reduce the size of the problem? • As the problem "shrinks", will you eventually reach a point where an obvious solution presents itself? • Again, think about the factorial problem: factorial (0) = 1

  10. Files and File Access • Thus far we have used volatile memory (primary storage) in our programs • Volatile memory has certain advantages and disadvantages • Long-term storage requires the use of secondary storage • Data files are used by programmers to store and retrieve data in secondary storage

  11. Files and File Access • Data file hierarchy: • bits (binary digits… 0’s and 1’s) • bytes (bits grouped together (in 8’s) to form a single character) • words or fields(logical groupings of bytes (characters) that, taken together, can represent data or information (data in context)) • records(logical groupings of fields that describe an object or entity) • files (a collection of records)

  12. Files and File Access • In C a file can refer to a data file, but in C a file can also refer to any concrete device with which we want to exchange data (monitor, printer, tape drive, hard disk, etc…) • In C a file stream is the flow of data between a program and a file • File streams are a series of bytes • File streams are not device dependent… all streams have the same behavior

  13. Files and File Access • File streams have two formats: • text stream(think readable data or text) • binary stream(non-readable… think .exe files) • We use pointers to manage file streams that read and write data… pointers that are used to manage file streams are called file pointers • C provides an internal structure named FILE that is used to declare file pointers • We use FILE as a data type when we declare a file pointer FILE *file_pointer_name ;

  14. #include <stdio.h> // Example of declaring a file pointer using FILE. int main (void) { FILE *read_ptr; // The names and the number of file pointers FILE *write_ptr; // used in program: determined by FILE *append_ptr; // the programmer. // We will need to assign a file to a file pointer. return (0) ; }

  15. Files and File Access • To work with files in C we must do four things: • create a file pointer (using FILE) • assign a file to the file pointer (using fopen) • opens the file (should test the file when opening) • process the file (read and/or write, transform) • close the file (using fclose) • Opening usually involves using the stdio.h function fopen • fopen is used in an assignment statement to pass a file pointer to a previously declared (by the programmer… us) file pointer

  16. Opening a File • fopen syntax: FILE *file_pointer_name ; file_pointer_name = fopen ("file_name", "file_mode"); • fopen example: FILE *read_ptr ; read_ptr = fopen ("student_1.dat", "r");

  17. Opening a File • File mode: specifies the way to open the file:"r" opens and existing file for reading"w" creates a text file for writing "a" opens an existing file for appending

  18. Opening a File • File mode: specifies the way to open the file:"r+" opens an existing file for reading and/or writing "w+" creates a text file for reading and/or writing "a+" opens or creates a file for appending "rb" creates a binary file for writing "wb" creates a binary file for writing "ab" opens an existing binary file for appending "r+b" opens an existing binary file for reading and/or writing "w+b" creates a binary file for reading and/or writing "a+b" opens or creates a binary file for appending

  19. Opening a File #include <stdio.h> //example of declaring and assigning a file pointer int main (void) { FILE *read_ptr ; read_ptr = fopen("student_1.dat", "r"); return (0) ; } • It’s considered "good practice" to check each file pointer assignment so that you know each file is successfully opened

  20. Opening a File and Checking It #include <stdio.h> //example of declaring and assigning a file pointer //then checking it int main (void) { FILE *read_ptr ; read_ptr = fopen("student_1.dat", "r"); if (read_ptr == NULL) printf("\n\nstudent_1.dat was not properly opened.\n"); else printf("\n\nstudent_1.dat is opened in the read mode.\n"); return (0) ; }

  21. Reading a File: Single Field/Single Record • fscanf is used like scanf for files… in stdio.h… then fclose #include <stdio.h> //3.c int main (void) { FILE *read_ptr ; char last_name [15]; read_ptr = fopen("student_1.dat", "r"); if (read_ptr == NULL) printf("\n\n student_1.dat was not properly opened.\n\n"); else { printf("\n\n student_1.dat is opened in the read mode.\n"); fscanf(read_ptr, "%s", last_name); printf("%s \n\n", last_name); fclose(read_ptr); } return (0) ; }

  22. Reading a File: Single Field/Multiple Records • feof is used to check for a file’s EOF marker #include <stdio.h> //4.c int main (void) { FILE *read_ptr ; char last_name [15]; read_ptr = fopen("student_2.dat", "r"); if (read_ptr == NULL) printf("\n\nstudent_2.dat was not properly opened.\n"); else { printf("\n\nstudent_2.dat is opened in the read mode.\n"); fscanf (read_ptr, "%s", last_name ); while ( !feof (read_ptr) ) { printf ("%s \n", last_name); fscanf (read_ptr , "%s", last_name ); } fclose(read_ptr); } return (0) ; }

  23. Reading a File: Multiple Fields and Records #include <stdio.h> //5.c int main (void) { FILE *read_ptr ; char last_name [15]; char major [15]; read_ptr = fopen("student_3.dat", "r"); if (read_ptr == NULL) printf("\n\nstudent_3.dat was not properly opened.\n"); else { printf("\n\nstudent_3.dat is opened in the read mode.\n"); fscanf (read_ptr, "%s %s", last_name , major); while ( !feof (read_ptr) ) { printf ("%s %s \n", last_name, major); fscanf (read_ptr , "%s %s", last_name , major); } fclose(read_ptr); } return (0) ; }

  24. Writing a File: Use fprintf and fclose #include <stdio.h> //6.c int main (void) { FILE *write_ptr ; char last_name [15]; char major [15]; write_ptr = fopen("student_3.dat", "w"); // need "w" for mode if (write_ptr == NULL) printf("\n\nstudent_3.dat was not properly opened.\n"); else { printf("\n\nEnter the last name and major: "); scanf ("%s %s", last_name , major); fprintf (write_ptr, "%s %s \n", last_name, major); } fclose (write_ptr); return (0); }

  25. Appending a File: Use fprintf and fclose #include <stdio.h> //7.c int main (void) { FILE *append_ptr ; char last_name [15]; char major [15]; append_ptr = fopen("student_3.dat", "a"); //need "a" for mode if (append_ptr == NULL) printf("\n\nstudent_3.dat was not properly opened.\n"); else { printf("\n\nEnter the last name and major: "); scanf ("%s %s", last_name , major); fprintf (append_ptr, "%s %s \n", last_name, major); } fclose (append_ptr); return (0) ; }

  26. Writing an Array to a File #include <stdio.h> //8.c int main(void) { FILE *scores; int score_array[10] = { 97, 65, 72, 84, 79, 81, 91, 93, 67, 73 }; int x; scores=fopen("test_scores.dat","w"); if(scores==NULL) printf("\n\ntest_scores.dat was not properly opened.\n"); else { for(x = 0 ; x < 10 ; x++ ) { fprintf(scores,"%d\n", score_array[x]); } fclose(scores); printf("All ten exam scores saved to disk.\n\n\n"); } return(0); }

  27. Reading an Array From a File #include <stdio.h> //9.c int main(void) { FILE *scores; int score_array[10] ; int x; scores=fopen("test_scores.dat","r"); if(scores==NULL) printf("\n\ntest_scores.dat was not properly opened.\n"); else { for(x = 0 ; x < 10 ; x++ ) { fscanf(scores,"%d", &score_array[x]); } fclose(scores); for(x = 0 ; x < 10 ; x++ ) printf("Exam #%d score is %d \n", x + 1, score_array[x]); } return(0); }

  28. #include <stdio.h> //10.c int main(void) { FILE *scores; int score_array[10] [3]; int x; scores=fopen("test_scores_2.dat","r"); if(scores==NULL) printf("\n\ntest_scores_2.dat was not properly opened.\n"); else { for(x = 0 ; x < 10 ; x++ ) { fscanf(scores,"%d %d", &score_array[x] [0], &score_array[x] [1]) ; score_array[x] [2] = score_array[x] [0] + score_array[x] [1] ; } fclose(scores); for(x = 0 ; x < 10 ; x++ ) printf("Student #%d scores and total: \t %d \t %d \t %d\n\n\n", x + 1, score_array[x] [0] , score_array[x] [1] , score_array[x] [2] ); } return(0); }

More Related