Problem Solving �using Files

Rupesh Nasre.

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Mentor TAs: Ahmed, Vivek, Vimala, Akash,

Kankan, Rahul, Swati, Akshay, Ashok, Keshav

IIT Madras

May 2022

Files allow persistent data.

• Our programs so far were transient.
• Once a.out is terminated, the data is lost.
• In many applications, data entered one day should be available on some other day.
• Files make data persistent.
• Across multiple invocations of a.out
• Across multiple executables also (a.out, b.out)
• We can read data stored in files, or write to it.

File Operations

• fopen: opens a file.
• fclose: closes it.
• fscanf, fgets, fread: Read data from a file.
• fprintf, fputs, fwrite: Write data to a file.

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• These operations work with a file pointer.

Problem: Implement cat.

• cat allows printing contents of a file.

int main(int a, char *b[]) {

FILE *fp = fopen(b[1], "r");

char str[100];

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fgets(str, 100, fp);

while (!feof(fp)) {

puts(str);

fgets(str, 100, fp);

}

fclose(fp);

}

Does this miss the last line in the file?

$ ./a.out mycat.c

int main(int a, char *b[]) {

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FILE *fp = fopen(b[1], "r");

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char str[100];

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fgets(str, 100, fp);

...

• fgets retains \n in str. Either remove it prior to puts, or print it using printf without extra \n.
• Recall that gets does not include \n.
• Both fgets and gets end the string with \0.

If the file does not exist, or is not readable, fopen returns NULL.

File pointer

automatically

moves ahead.

Same as scanf.

Problem: Implement cp.

• Copy a file to another.

int main(int a, char *b[]) {

FILE *fpr = fopen(b[1], "r");

FILE *fpw = fopen(b[2], "w");

char str[100];

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fgets(str, 100, fpr);

while (!feof(fpr)) {

fputs(str, fpw);

fgets(str, 100, fpr);

}

fclose(fpr);

fclose(fpw);

}

If the file does not exist, w mode creates a new file.

Use "a" to append to a file.

Problem: Implement cp.

• Copy a file to another: from input to output.

int main(int a, char *b[]) {

FILE *fpr = fopen(b[1], "r");

FILE *fpw = fopen(b[2], "w");

char str[100];

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fgets(str, 100, fpr);

while (!feof(fpr)) {

fputs(str, fpw);

fgets(str, 100, fpr);

}

fclose(fpr);

fclose(fpw);

}

C provides two standard I/O devices: stdin and stdout.

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These are always open (and should not be closed).

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A third one is called stderr, which is primarily used to print error messages.

stdin);

stdin);

stdin)) {

stdout);

Same as gets

Ctrl-d is eof

Same as puts

Problem: Split text into words.

$ mycat file.in

Lexical analyzer groups a sequence of characters into tokens.

Syntax analyzer checks if these tokens adhere to a grammar syntax.

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$ mysplit file.in file.out

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$ mycat file.out

1: Lexical

2: analyzer

3: groups

4: a

5: sequence

6: of

7: characters

8: into

9: tokens.

10: Syntax

11: analyzer

12: checks

13: if

14: these

15: tokens

16: adhere

17: to

18: a

19: grammar

20: syntax.

int main(int a, char *b[]) {

FILE *fpr = fopen(b[1], "r");

FILE *fpw = fopen(b[2], "w");

char str[100];

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fscanf(fpr, "%s", str);

int lineno = 0;

while (!feof(fpr)) {

fprintf(fpw, "%d: %s\n", ++lineno, str);

fscanf(fpr, "%s", str);

}

fclose(fpr);

fclose(fpw);

}

Problem: Join words into text.

$ mycat file.out

1: Lexical

2: analyzer

3: groups

4: a

5: sequence

6: of

7: characters

8: into

9: tokens.

10: Syntax

11: analyzer

12: checks

13: if

14: these

15: tokens

16: adhere

17: to

18: a

19: grammar

20: syntax.

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$ myjoin file.out file.in

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$ mycat file.in

Lexical analyzer groups a sequence of characters into tokens.

Syntax analyzer checks if these tokens adhere to a grammar syntax.

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int main(int a, char *b[]) {

FILE *fpr = fopen(b[1], "r");

FILE *fpw = fopen(b[2], "w");

char str[100];

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int lineno;

fscanf(fpr, "%d: %s", &lineno, str);

while (!feof(fpr)) {

fprintf(fpw, "%s ", str);

if (strstr(str, ".")) fprintf(fpw, "\n");

fscanf(fpr, "%d: %s", &lineno, str);

}

fclose(fpr);

fclose(fpw);

}

Problem: Text correction

• Replace mispelt with misspelt in a file.
• Unlike previous problems, this demands reading as well as writing to the same file.
• fopen supports additional modes.
• r+: Allows reading and writing. Returns NULL if the file cannot be opened.
• w+: Allows reading and writing, initially erases the file content.
• After reading, file pointer FP moves ahead.
• Thus, after reading mispelt, FP needs to go back.

Problem: Text correction

int main(int a, char *b[]) {

FILE *fp = fopen(b[1], "r+");

char str[100];

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fscanf(fp, "%s", str);

while (!feof(fp)) {

if (strcmp(str, "mispelt") == 0) {

fseek(fp, -strlen("mispelt"), SEEK_CUR);

fprintf(fp, "misspelt");

}

fscanf(fp, "%s", str);

}

fclose(fp);

}

Problem: Text correction

From the current position (relative)

Also supported: SEEK_SET, SEEK_END

$ mycat file.in

If a word is mispelt it will be corrected.

$ ./a.out file.in

$mycat file.in

If a word is misspeltit will be corrected.

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In general, we can use another file for writing.

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r+ allows filesize to grow.

e.g., if mispelt is the last word in the file.

Problem: Student Records

• Read student records from user.
• Store in a file (RAM --> DISK).
• Retrieve records from the file (DISK --> RAM).

struct student {

char rollno[9];

char name[20];

char hostel[20];

int room;

};

filestud.h

#include "filestud.h"

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void writetofile(struct student *stud) {

FILE *fp = fopen("stud.dat", "w");

fprintf(fp, "%s\n%s\n%s\n%d\n",

stud->rollno, stud->name,

stud->hostel, stud->room);

fclose(fp);

}

void readfromfile() {

FILE *fp = fopen("stud.dat", "r");

…

fclose(fp);

}

filestud.c

#include "filestud.h"

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int main() {

struct student stud;

populate(&stud);

writetofile(&stud);

populate(&stud);

writetofile(&stud);

readfromfile();

}

filestudmain.c

After executing a.out, how many student‘s records will stud.dat contain?

To append to an existing

file, we can open it in

the append mode "a".

Application: Ticket Booking System

• N seats: 1 to N, initially unoccupied
• Booking needs name, assigns an unoccupied seat
• Cancellation needs seat number
• Update needs seat number, allows change of name.

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