CSC Programming for Science Lecture 18: More Data Files

Question of the Day How can you add 5 matches to these 6 and make 9?

Today’s Goal Today’s lecture discusses using & manipulating data stored on disk  C commands to open and close files, read, and write contents of opened files After today’s lecture, you should be ready to write programs that reads & writes files Also know a few extra debugging tips

Data File Review To use a file, must first create a file pointer  File pointer is variable of type FILE *  Created using fopen(“FILENAME”, “ACCESS”);  Once complete, call fclose(filePointer); Open file pointer used in I/O functions  Read data using fscanf(filePointer,...);  Check if at end of file feof(filePointer); To use multiple files, must use multiple file pointers

File Pointer Variables Need to be careful using & declaring FILE *fin; // Good FILE *bob, jim; // Bad! jim lacks * power FILE *ham, *eggs; // Yes, now we are cooking! Variables only describe files  Assignment to variable does not affect files  Variable’s values meaningful only to C bob = ham; // Both use same file now int i = bob; // Gets warning, i’s value = ?

Opening a File To use a file, must first open the file: FILE* fopen(“FILENAME”, “ACCESS”); ACCESS specifies how file will be used  “r”  can read from file  “w”  will overwrite contents of file  “a”  will append contents of file Cannot read files that do not exist  Can write & append them, however

Opening a File (2) fopen returns NULL when unsuccessful  NULL is symbolic constant defined in stdio.h  Using NULL file pointer will crash program Returns some other value on success  Value not meaningful to humans, anyway

Writing to File First assign file pointer to the opened file fprintf(filePointer,...) writes to file  First argument is file pointer to write to  Rest of arguments identical to printf

Writing To Screen Always been using fprintf fprintf used to print to screen  FILE* stdout “auto-magically” exists  stdout simplifies printing to screen double a, b;... printf(“%lf %lf”, a, b); is just shorthand for: double a, b;... fprintf(stdout, “%lf %lf”, a, b);

Writing To Screen (2) stdout does not print immediately  Will be delayed for performance purposes  Can be a problem when debugging FILE* stderr similar to stdout  Both created at startup  Each normally prints to screen  Printing to stderr appears immediately  stderr typically used to print error messages

Always Close Your Files Once finished using a file, close it  Done using fclose(filePointer) function  Must re-open closed file pointer before using Closing file can be important  Data may not be recorded until file closed  Often need to close & re-open file to switch from reading it to writing it

Stupid Programming Tricks Redirection/piping takes advantage of file flexibility  Redirect files to appear as if they were typed in from keyboard  Redirect output to be stored directly in files  Neat trick that works on (nearly) all OSs

Redirect Input Use “<“ on the command line: a.out < hawaii.txt  Program uses scanf, sees input on keyboard  Input actually is contents of hawaii.txt  Great way to automate testing

Redirect Standard Output Use “>“ on the command line: a.out > miami.txt  Program uses printf or fprintf(stdout,…);  Nothing appears on screen, but instead saved to miami.txt Contents of miami.txt overwritten, however  Saves program results, useful to handle lots of debugging output a.out >> miami.txt appends results

Redirect Standard Error Use “2>“ on the command line: a.out 2> phoenix.txt  Program uses fprintf(stderr,…);  Only redirects stderr, other output appears as normal  Can be combined with “>”, but not to same file a.out > tahiti.txt 2> sanDiego.txt

Your Turn Get into groups and complete daily activity

For Next Lecture Complete week #7 weekly assignment Start programming assignment #2