1. Files

2. Information Start with Your big homework!
Read through the task description and Your task variant
Create the input files with some basic data
Create your initial program, that just reads through the file and prints it out
Next week, we’ll start to work with structures – then you will be able to store them as required
2018
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3. Opening a file
Working with files requires us to use file pointers To declare a file pointer: FILE *fp; To open a file: filePointer = fopen(fileName, mode); Both filename and mode are type char* (char pointers aka strings)
fopen() returns a NULL-pointer when opening the file fails
You should always verify that the file was opened successfully!
2018
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4. Arguments for fopen Modes: File name: "r " – read
"w" – write (when the file already exists, it will be overwritten. If there is no file, it will be created)
"a" – append (write to the end of an existing file)
File name:
Full name of the file (name + extension) as a string (char*)
The file will be searched for in the same directory as the executable (relative path)
You can also specify the exact file location from the root (full path)
2018
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5. In code
You should always check if the file was opened successfully! When opening a file fails, reading from it will crash the program.
FILE *fi; // file pointer declaration
char inputFile[] = "numbers.txt";
fi = fopen(inputFile,"r"); //opening it
if (fi == NULL) // check for errors {
exit(1); // abort. Exit comes from stdlib.h }
2018
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6. Closing a file Resources should be freed when not needed
All of the files should be closed before exiting
Never use fclose() on an invalid or NULL pointer
To close a file: fclose(filePointer);
2018
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7. Functions for files (see stdio.h for more)
fscanf(filePointer, format, …); read formatted text from a file
fgets(destination, length, filePointer); read line by line or until length is exceeded
fprintf(filePointer, format, …); write formatted text to a file
fputs(contents, filePointer); write line by line to a file
fflush(filePointer); Writes the output buffer contents to the file
EOF - a constant to indicate the end of file. Typically -1 in C
2018
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8. Buffers Most file read/write functions are buffered
Data is not written each time fprintf(), fputs(), fwrite(), … is called nor is it read each time fscanf(), fgets(), fread(), … is called
Data is typically read as large chunks and passed to the appropriate functions as requested.
Data is typically written to the file once the buffer becomes full or the file is closed.
The contents of the buffer may be lost if the program crashes! This is especially important for log files!
You can force output buffer write using fflush()
2018
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9. Some additional information
MS Office, OpenOffice, …. don’t produce basic ASCII text files we are working with
Windows and Linux typically use a different line ending
You can open files in binary form instead of ASCII
Each time you call any read function, the next portion of the file will be passed to you (from buffer) Think of reading a book while tracking your progress with your finger
This position can be manipulated with
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10. Sample 1 (reading using EOF)
#include <stdio.h> int main(void) { FILE *fi; int temp; char inputFile[] = „numbers.txt“; fi = fopen(inputFile,"r"); while (fscanf(fo, "%d", &temp) != EOF) printf("Got: %d\n", temp); fclose(fp); return 0; }
2018
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11. Sample 2 (check success count)
#include <stdio.h> #define INPUT_FILE "numbers.txt" int main(void) { FILE *fi = fopen(INPUT_FILE,"r"); int used, quota; while (fscanf(fi, „%d %d“, &used, &quota) == 2) printf(“User is using %d from %d\n“, used, quota); fclose(fi); return 0; }
2018
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12. Sample 3 (writing to a file)
#include <stdio.h>
#define LIMIT 5
int main(void) {
FILE *fo = fopen(“numbers2.txt“,"w");
int i;
for (i = 0; i < LIMIT; i++)
fprintf(fo, "Output: %d\n", i);
fclose(fo);
return 0; }
2018
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13. Task 1 Read integers from input file (number count is not known)
The numbers will be divided between 2 separate files based on the following principle:
Even digits go to the first file
Odd digits go to the second file
Don’t forget some basic error proofing
Don’t overdo it, less is more in this case
2018
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14. Task 2 (part 1/3)
The input file contains statistics on different subjects (at least 5)
The data format on each line is as follows: <subject> <grade count> <grades>
Sample input data Programming Mechatronics Databases ……….
Read the original data from the file, store it in arrays and print it out
2018
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15. Task 2 (parts 2 and 3)
Part 2: You must be able to repeatedly perform following searches from the results
Search for a subject by name
Search for subjects where the average grade is more / less than a given value
Search for subjects that have more / less than given amount of grades
Search keywords and values are specified by the user from keyboard input
Part 3: Add logging
Each query must be logged
Each start of the program must be logged
The log file must contain the logs for each previous run
2018
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