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C Programming – Part 6 File Input and Output. TEC 284. Streams. In C we use pointers to manage streams that read and write data Streams are just file or hardware devices such as a monitor or pointer which can be controlled by a C program
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C Programming – Part 6 File Input and Output TEC 284
Streams • In C we use pointers to manage streams that read and write data • Streams are just file or hardware devices such as a monitor or pointer which can be controlled by a C program • We use pointers to the stream in order to control them • To point to and manage a file stream in C, an internal data structure called FILE is used
FILE #include <stdio.h> main () { //create 3 file pointers FILE *pRead; FILE *pWrite; FILE *pAppend; } • Each FILE pointer can essentially open and manage a separate file
Opening and Closing Files • When working with files, the basic steps involve opening the file, processing the file and then closing the file • After opening the file, there should always be some error checking/handling to ensure the file was successfully opened before working on the file • The fopen() function is used • fopen() returns a FILE pointer when the file is successfully opened
Opening a file in C main () { FILE *pRead; pRead = fopen(“file1.txt”,“r”); } • fopen() takes two arguments • The name of the file to be opened • A flag to indicate how the file should be opened • In this example, the file file1.txt is opened in read-only mode
Text file open modes • r - read text mode • w - write text mode (truncates file to zero length or creates new file) • a - append text mode for writing (opens or creates file and sets file pointer to the end-of-file) • r+ - read and write text mode • w+ - read and write text mode (truncates file to zero length or creates new file) • a+ - read and write text mode (opens or creates file and sets file pointer to the end-of-file
fopen() results • If the file does not exist and it is opened with read mode (r), then the open fails. • If the file is opened with append mode (a), then all write operations occur at the end of the file regardless of the current file position. • On success a pointer to the file stream is returned. On failure a null pointer is returned
Opening a file in C with error handling main () { FILE *pRead; pRead = fopen(“file1.txt”,“r”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nFile was opened for reading”); } }
Check for NULL if(pRead == NULL) Can be shortened to if(pRead) • If pRead returns a non-NULL, the if condition is true. • If pRead returns NULL, the condition is false
Closing a file • After opening and processing a file, the file should be closed using fclose() • fclose() uses the FILE pointer to flush the stream and close the file • This frees up memory resources for the CPU and is good programming practice • fclose() takes a pointer to a file as its argument • fclose(pRead);
Reading a file • For simplicity, the test file the example is referring to is just a text file containing names Names.txt John Michael Frank Stacy Robert
Reading a file main () { FILE *pRead; char name[20]; pRead = fopen(“Names.txt”,“r”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nContents of file:\n”); fscanf(pRead, “%s”, name); while(!feof(pRead)) { //feof() checks for end-of-file //feof() returns a non-zero value when the end-of-file //marker is reached printf(“%s\n”,name); fscanf(pRead, “%s”, name); } fclose(pRead); } }
Results of the program Contents of file : John Michael Frank Stacy Robert
Reading multiple fields • fscanf() can read multiple fields by supplying a series of type specifiers for the second argument • E.g. fscanf(pRead, “%s%s”, name, hobby); • The %s specifier will read a series of characters until a white space is found, including a blank, new line or tab
Reading multiple fields test file Hobbies.txt Michael Programming Sheila Shopping Spencer Football Olivia Dancing
Code to read multiple fields main () { FILE *pRead; char name[20], char hobby[15]; pRead = fopen(“Hobbies.txt”,“r”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nName\tHobby\n\n”); fscanf(pRead, “%s%s”, name, hobby); while(!feof(pRead)) { //feof() checks for end-of-file //feof() returns a non-zero value when the end-of-file //marker is reached printf(“%s\t%s\n”,name, hobby); fscanf(pRead, “%s%s”, name, hobby); } fclose(pRead); } }
Results Name Hobby Michael Programming Sheila Shopping Spencer Football Olivia Dancing
Writing Data • Similarly to how printf() is used to send information to the screen fprintf() can be used to send information to a file • fprintf() uses a FILE pointer to write data to a file • The fprintf() function takes a FILE pointer, a list of data types and a list of values (or variables) to write the information to a file
Writing to a file main () { FILE *pWrite; char fName[20], char lName[20], char id[15], float gpa; pWrite = fopen(“Students.txt”,“w”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nEnter first name, last name, id and GPA\n”); printf(“Enter data separated by spaces: ”); //store data entered by the user into variables scanf( “%s%s%s%f”, fName, lName,id, &gpa ); //write variable contents separated by tabs fprintf(pWrite, “%s\t%s\t%s\t%.2f\n”,fName, lName, id, gpa); fclose(pWrite); } //end if }
Reading the same record back main () { FILE *pRead; char fName[20], char lName[20], char id[15], float gpa; pRead = fopen(“Students.txt”,“r”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { //print heading printf(“\nName\t\tID\t GPA\n\n”); //read field info from file and store in variables fscanf( “pRead”, “%s%s%s%f”, fName, lName,id, &gpa ); //print variable data to standard output printf(“%s %s\t%s\t%.2f\n”,fName, lName, id, gpa); fclose(pRead); } //end if }
Appending to a file • When opening a file using fopen() with a w argument value will erase any previous data stored in the file • Use the a attribute to append data at the end of the file • If the file does not exist, a new one will be created
Appending code example //This program appends to our previous Hobbies.txt file example main () { FILE *pWrite; char name[20], char hobby[20]; pWrite = fopen(“Hobbies.txt”,“a”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nEnter a new name and hobby: ”); //store data entered by the user into variables scanf( “%s%s”, name, hobby); //write variable contents separated by tabs fprintf(pWrite, “%s\t%s\n”,name, hobby); fclose(pWrite); } //end if }