PHY 107 – Programming For Science

Today’s Goal  Get familiar with opening & closing files  Declaring variables for use with files  Using variables to open files for reading & writing  Closing files and understand why you should do so ALL  Understand what it means: ALL I/O is file I/O  Common bugs to avoid when using files in C/C++  Get a better understanding of scanf & printf  Files as variables: know what their types are

Image To Sharpen  I have a (fuzzy) 1024 x 768 picture to sharpen 786,432  Only 786,432 numbers to type in to yesterday's lab  Will also need to click each pixel to update with result

More Data Entry Positions  Testing improved jet designs for oeingB-ay  Using program to simulate designs' lift & drag  5 possible designs (each 150MB) to test this iteration  Once results available, will tweak & retest designs  Need room of touch typists for all this data entry

This Is (Semi-Real) Problem  Large hadron collider eventually work as designed  No black hole when smashing particles at high speeds 28.5 GB/min  Creates 28.5 GB/min for nerds seeking truth & beauty

This Is (Semi-Real) Problem  Large hadron collider eventually work as designed  No black hole when smashing particles at high speeds 28.5 GB/min  Creates 28.5 GB/min for nerds seeking truth & beauty  Hired trained monkeys to type all 244,813,135,872 bits

This Is (Semi-Real) Problem  Large hadron collider eventually work as designed  No black hole when smashing particles at high speeds 28.5 GB/min  Creates 28.5 GB/min for nerds seeking truth & beauty  Hired trained monkeys to type all 244,813,135,872 bits college students

Yeah, Right  Real world demands we use files for most I/O  Data files used to start and/or end most projects  May contain: game levels, analysis results, CCD pics  Way to read & write files needed to be useful

Opening a File  To use file, we need variable to use in program  Numbers, cStrings, and booleans mixed in the file  Previous type (& arrays) do not make sense to use  C provides new types to refer to file itself

Types Used For Files  Within program, may use file in 2 possible ways  To read file, pointer to FILE will be needed  Also need variable of type FILE* to write to a file

File Variable Declaration  Types are new, but still just declaring variables  Requires type & name of variable being declared  Names for human use; normal scoping rules apply  Cannot assign variables  Cannot assign variables but can use as parameters files are NOT numbers  Cannot use in equations: files are NOT numbers FILE* fout; FILE* fin; FILE* bob, *bobBob, *bobBobBob, *bobJr; FILE* thisIsAFileVariable; FILE* monkey = "foo.txt"; FILE *fourFile = 4;

File Variable Declaration  Types are new, but still just declaring variables  Requires type & name of variable being declared  Names for human use; normal scoping rules apply  Cannot assign variables  Cannot assign variables but can use as parameters files are NOT numbers  Cannot use in equations: files are NOT numbers FILE* fout; FILE* fin; FILE* bob, *bobBob, *bobBobBob, *bobJr; FILE* thisIsAFileVariable; FILE* monkey = "foo.txt"; FILE *fourFile = 4;

File Variable Declaration  Types are new, but still just declaring variables  Requires type & name of variable being declared  Names for human use; normal scoping rules apply  Cannot assign variables  Cannot assign variables but can use as parameters files are NOT numbers  Cannot use in equations: files are NOT numbers FILE* fout; FILE* fin; FILE* bob, *bobBob, *bobBobBob, *bobJr; FILE* thisIsAFileVariable; FILE* monkey = "foo.txt"; FILE *fourFile = 4;

Name That File  Two ways to open a file once we have the name  No matter which used, must know name of file  Defaults to current directory, if only a name specified  By including in name, can use other directories  No standard way to do this – depends on the OS

Name That File  Two ways to open a file once we have the name  No matter which used, must know name of file  Defaults to current directory, if only a name specified  By including in name, can use other directories  No standard way to do this – depends on the OS

Opening the File  Can open file when variable declared char nameLoc[] = "bobbobbob"; char sndName[] = "csi.txt"; FILE* fin = fopen("image.dat”,”r”); FILE* fout = fopen("image.dat”,”w”); FILE* bobism = fopen(nameLoc,”a”); FILE* cookies = fopen(sndName,”r+”);  Even after declaration, files can be opened FILE* because; FILE* isaidso; because = fopen("mightMakes.right”,”r”); scanf(“%s”, sndName); isaidso = fopen(sndName,”w”);

Did I Do Good?  May not always be successful opening a file  Cannot open and read file that does not exist  May not have permission to access a certain file  Cannot do impossible & write to nonexistent drive  NULL pointer returned if fopen did not work  If call succeeded, then will have variable != NULL  variable == NULL when attempt to open file fails  May not work before calling fopen or after fclose

Did I Do Good?  May not always be successful opening a file  Cannot open and read file that does not exist  May not have permission to access a certain file  Cannot do impossible & write to nonexistent drive  NULL pointer returned if fopen did not work  If call succeeded, then will have variable != NULL  variable == NULL when attempt to open file fails  May not work before fopen or after fclose

Examples Of How NULL Works char sndName[]; FILE *because, *foo=fopen(”f.txt”,”r”); FILE *isaidso, *bar=fopen(”snafu”,”w”); isaidso = NULL; printf(“%p\n”,because); printf(“%p\n”,foo); printf(“%p\n”,bar); printf(“%p\n”, isaidso); because=fopen(”doesNotExist.txt”,”r”); isaidso=fopen(”doesNotExist.txt”,”w”); printf(“%p\n”, because); printf(“%p\n”, isaidso);

Examples Of How NULL Works char sndName[]; FILE *because, *foo=fopen(”f.txt”,”r”); FILE *isaidso, *bar=fopen(”snafu”,”w”); isaidso = NULL; printf(“%p\n”,because); printf(“%p\n”,foo); printf(“%p\n”,bar); printf(“%p\n”, isaidso); because=fopen(”doesNotExist.txt”,”r”); isaidso=fopen(”doesNotExist.txt”,”w”); printf(“%p\n”, because); printf(“%p\n”, isaidso);

Upon Opening The Location Is…  Once open, read & write from start of file  As logical a choice as any other location to start at  If reading, can start getting all data from file  When writing to existing file what will happen?

Oops!

Opening a File  Within program, may use file in 2 possible ways  Reading data from a file that already exists  Create or erase (as appropriate) & write file from scratch

Opening a File 3

Closing File  Important to close files once you are done  Program will delay saving data to make it faster  Crashes cause data loss if saves had been waiting  Until file is closed may be locked from other uses  Immediately saved on close, so insures data is safe  Can open more files if limit of open files reached FILE* mets = fopen(”nlChamp.txt”,”r”); FILE* yankees = fopen(”evil.txt”,”a”); fclose(mets); fclose(yankees);

Today's Key Point  Because of its history, all C/C++ I/O is file based  Obvious when file is source of data or target of write  But also true when reading from keyboard  Writing to screen also considered file I/O

Today's Key Point

What Are printf & scanf ?  To use printf & scanf we also need: #include

What Are printf & scanf ?  To use printf & scanf we also need: #include  Include statement needed for file I/O #include

What Are printf & scanf ?  To use printf & scanf we also need: #include  Include statement needed for file I/O #include  There is a reason: this is not an accident  printf & scanf are special versions of file I/O functions

Deep in Bowels of stdio.h  In stdio.h find 2 functions included in code: int printf( ) { fprintf(stdout, ); } int scanf( ) { fscanf(stdin, ); }  Already written code reading/writing from a file  Now must specify FILE* variable & not be as lazy

Input Using scanf

Input Using fscanf and FILE*

Output using printf  Already seen how to print text using printf printf(“Hello World\n”);  Prints out whatever is placed between quotes  Print variables’ values with specifier and adding after  Use escape sequences for fancier text output \n  newline (move to start of next line) \t  tab (go to next column that is multiple of 8) \\  \ (backslash character) \”  “ (quotation mark)

Output using fprintf & FILE*  File output easy to write text using fprintf FILE * fOut = fopen(fileName, “w”); fprintf(fOut,“Hello World\n”);  Prints out whatever is placed between quotes  Print variables’ values with specifier and adding after  Use escape sequences for fancier text output \n  newline (move to start of next line) \t  tab (go to next column that is multiple of 8) \\  \ (backslash character) \”  “ (quotation mark)

See How Easy It Is? #include #include int main(void) { int sum = 0; int val; printf("-1 to quit or sum nums typed\n”); scanf(“%d”,&val); while (val != -1) { sum += val; printf(”%d\n”, val); scanf(“%d”,&val); } return 0; }

See How Easy It Is? #include #include int main(void) { FILE* fin; FILE* fout; int sum = 0; int val; fprintf(fout,"-1 to quit or sum nums typed\n”); fscanf(fin,“%d”,&val); while (val != -1) { sum += val; fprintf(fout,”%d\n”, val); fscanf(fin, “%d”,&val); } return 0; }

And When We Run It? #include #include int main(void) { FILE* fin; FILE* fout; int sum = 0; int val; fprintf(fout,"-1 to quit or sum nums typed\n”); fscanf(fin,“%d”,&val); while (val != -1) { sum += val; fprintf(fout,”%d\n”, val); fscanf(fin, “%d”,&val); } return 0; }

And When We Run It?

Must Open File Before Using  How file opened unimportant, just that is open  Open when declared, by assigning in declaration FILE* bobIn=fopen("file.txt","r"); FILE* whyNot=fopen(strFromUser,"w");  Open later in program using same fopen() call bobIn=fopen("file.txt","r"); whyNot=fopen(strFromUser,"w");  Variable must refer to open file or else it crashes  Often add need to use NULL to protect from crashs

See How Easy It Is? #include #include int main(void) { int sum = 0; int val; printf("-1 to quit or sum nums typed\n”); scanf(“%d”,&val); while (val != -1) { sum += val; printf(”%d\n”, val); scanf(“%d”,&val); } return 0; }

See How Easy It Is? #include #include int main(void) { FILE* fin = fopen(“input.dat”,”r”); FILE* fout = fopen(“results.csv”,”w”); if (fin != NULL && fout != NULL) { int sum = 0; int val; fprintf(fout,"-1 to quit or sum nums typed\n”); fscanf(fin,“%d”,&val); while (val != -1) { sum += val; fprintf(fout,”%d\n”, val); fscanf(fin, “%d”,&val); } } return 0; }

See How Easy It Is? #include #include int main(void) { FILE* fin = fopen(“input.dat”,”r”); FILE* fout = fopen(“results.csv”,”w”); if (fin != NULL && fout != NULL) { int sum = 0; int val; printf("-1 to quit or sum nums typed\n”); fscanf(fin,“%d”,&val); while (val != -1) { sum += val; fprintf(fout,”%d\n”, val); fscanf(fin, “%d”,&val); } } return 0; }

Variables are Variables  Like all variables, can use files as parameters  Argument must also be file variable for it to compile  Reading & writing continues through the function  Can only go forward in file no matter what  Within the function, "file position marker" continues  Cannot unring bell, does not overwrite file on return  As with scanf, will not reread after function

But…  Like all variables, can use files as parameters  Argument must also be file variable for it to compile  Reading & writing continues through the function  Can only go forward in file no matter what  Within the function, "file position marker" continues  Cannot unring bell, does not overwrite file on return  As with scanf, will not reread after function

But…  Like all variables, can use files as parameters  Argument must also be file variable for it to compile  Reading & writing continues through the function  Can only go forward in file no matter what  Within the function, "file position marker" continues  Cannot unring bell, does not overwrite file on return  As with cin to read, will not reread after function

Butt…  Like all variables, can use files as parameters  Argument must also be file variable for it to compile  Reading & writing continues through the function  Can only go forward in file no matter what  Within the function, "file position marker" continues  Cannot unring bell, does not overwrite file on return  As with cin to read, will not reread after function

Variables are Variables  Like all variables, can use files as parameters  Argument must also be file variable for it to compile  Reading & writing continues through the function  Can only go forward in file no matter what  Within the function, "file position marker" continues  Cannot unring bell, does not overwrite file on return  As with scanf, will not reread after function PARAMETER MUST BE PASS-BY-REFERENCE

Your Turn  Get into your groups and try this assignment

For Next Lecture  Another use of data files on pages  Must we use text or are there faster approaches?  What was the point of learning binary numbers?  Could we predict size of file we read/write in program?  Weekly Assignment available tomorrow  Due next Tuesday; break from assignments over  Programming Assignment #3 available Friday