Week13.pptx

7/27/2019 Week13.pptx

1/38

CS1010: Programming Methodologyhttp://www.comp.nus.edu.sg/~cs1010/
http://www.comp.nus.edu.sg/~cs1010/http://www.comp.nus.edu.sg/~cs1010/

7/27/2019 Week13.pptx

2/38

Week 13: File Processing

Objectives:

Understand concepts of file I/O

Learn how to use stdlib functions for formatted,

character and line I/O

References:

Chapter 3, Chapter 7

Lessons 3.6, 3.8, 7.4

Week13 - 2CS1010 (AY2011/2 Semester 1)

7/27/2019 Week13.pptx

3/38

Week 13: Outline

1. Introduction

2. Opening a File and File Modes

3. Closing a File

4. I/O Functions

5. Formatted I/O with Demo #1

6. Detecting End of File & Errors with Demo #2

7. Character I/O with Demo #38. Exercise #1: Trimming Blanks

9. Line I/O with Demo #4

10. Video on Data Display Debugger (DDD)


0. Week 12 Exercise #3: Health Screen

7/27/2019 Week13.pptx

4/38

Wk12. Ex #3 (take-home): Health Screen (1/2)

CS1010 (AY2011/2 Semester 1) Week13 - 4

Write a program Week12_Health_Screen.c to read in alist of health screen readings

Each input line represents a reading consisting of 2 numbers:

a float value indicating the health score, and an int value

indicating the number of people with that score

You may assume that there are at most 100 readings

The input should end with the reading 0 0, or when 100

readings have been read.

As the readings are gathered from various clinics,

there might be duplicate scores in the input. You are todetermine how many unique scores there are.

A skeleton program Week12_Health_Screen.c is given.

Well discuss this at our next lecture.

7/27/2019 Week13.pptx

5/38

Wk12. Ex #3 (take-home): Health Screen (2/2)


A sample run is shown belowEnter score and frequency (end with 0 0):5.2135 33.123 42.9 30.87 2

2.9 28.123 63.123 27.6 32.9 40.111 5

0 0Number of unique readings = 7

Possible extension: Which is the score that has the

highest combined frequency? (Do this on your own.)

7/27/2019 Week13.pptx

6/38

1. Introduction: Input/Output

In C, input/output is done based on the concept of astream

A stream can be a file or a consumer/producer of data

Examples: screen, keyboard, hard disk, printer, network port

A stream is accessed using file pointervariable oftype FILE *

The I/O functions/macros are defined in header file

stdio.h Two types of streams: text and binary


7/27/2019 Week13.pptx

7/38

1. Introduction: Text vs Binary Streams

Text stream Consists of a sequence of characters organized into lines

Each line contains 0 or more characters followed by a

newline character \n

Text streams stored in files can be viewed/edited easily Example: source code of C program

Binary stream

Beyond the scope of CS1010


7/27/2019 Week13.pptx

8/38

1. Introduction: Standard Streams

Three streams are predefined for each C program:stdin, stdout, stderr

stdin points to a default input stream (keyboard)

stdout points to a default output stream (screen)

stderrpoints to a default output stream for error messages(screen)

printf writes output to stdout

scanf reads input from stdin

The 3 standard streams do not need to be declared,

opened, and closed


7/27/2019 Week13.pptx

9/38

1. Introduction: Constants

There are two useful constants used in file processing NULL: null pointer constant

EOF: used to represent end of file or error condition


7/27/2019 Week13.pptx

10/38

2. Opening a File and File Modes (1/2)

Prototype:

FILE *fopen(const char *filename, const char *mode)

ReturnsNULL if error; otherwise, returns a pointer ofFILE type

Possible errors: non-existent file, or dont have

permission to open the file

File mode depends on intended file operations


7/27/2019 Week13.pptx

11/38

2. Opening a File and File Modes (2/2)

File modes for text files:


Mode Meaning

r Open for reading

w Open for writing (file needs not exist)

a Open for appending (file needs not exist)

r+ Open for reading and writing, starting at beginning

w+ Open for reading and writing (truncate if file exists)

a+ Open for reading and writing (append if file exists)

We will focus only on r and w

Note that In r mode, file must already exist

In w mode, new data overwrite existing data (if any)

7/27/2019 Week13.pptx

12/38

3. Closing a File

Prototype:

int fclose(FILE *fp)

Allows a file that is no longer used to be closed

Returns EOF if error is detected; otherwise, returns 0

It is good practice to close a file after use.


7/27/2019 Week13.pptx

13/38

4. I/O Functions

Formatted I/O: fprintf, fscanf Uses format strings to control conversion between character

and numeric data

Character I/O: fputc,putc ,putchar , fgetc ,

getc , getchar , ungetc Reads and writes single characters

Line I/O: fputs,puts , fgets , gets

Reads and writes lines

Used mostly for text streams

Block I/O: fread, fwrite

Used mostly for binary streamsWeek13 - 13CS1010 (AY2011/2 Semester 1)

7/27/2019 Week13.pptx

14/38

5. Formatted I/O (1/4)

Uses format strings to control conversion betweencharacter and numeric data fprintf: converts numeric data to character form and

writes to an output stream

fscanf: reads and converts character data from an input

stream to numeric form

Both fprintf and fscanf functions can have

variable numbers of arguments

Example:


float weight, height;FILE *fp1, *fp2;. . .fscanf(fp1, "%f %f", &weight, &height);fprintf(fp2, "Wt: %f, Ht: %f\n", weight, height);

7/27/2019 Week13.pptx

15/38

5. Formatted I/O (2/4)

fprintf returns a negative value if an error occurs;otherwise, returns the number of characters written

fscanf returns EOF if an input failure occurs before

any data items can be read; otherwise, returns the

number of data items that were read and stored


=printf(" "); fprintf(stdout, " ");

scanf(" "); fscanf(stdin, " ");=

7/27/2019 Week13.pptx

16/38

5. Formatted I/O (3/4): Demo #1


#include int main(void){

FILE *infile, *outfile;char x;int y;

float z;

infile = fopen("demo1.in", "r");outfile = fopen("demo1.out", "w");

fscanf(infile, "%c %d %f", &x, &y, &z);

fprintf(outfile, "Data read: %c %d %.2f\n", x, y, z);

fclose(infile);fclose(outfile);return 0;

}

Week13_Demo1.c

File demo1.in:10 20 30

Whats the output in demo1.out?

7/27/2019 Week13.pptx

17/38

5. Formatted I/O (4/4): Demo #1 ver2


#include #include int main(void){

. . .

if ((infile = fopen("demo1.in", "r")) == NULL){printf("Cannot open file demo1.in\n");exit(1);

}if ((outfile = fopen("demo1.out", "w")) == NULL)

{printf("Cannot open file demo1.out\n");exit(2);

}. . .

}

Week13_Demo1_v2.c

Check if file can

be opened.

To use exit()

7/27/2019 Week13.pptx

18/38

6. Detecting End of File & Errors (1/2)

Each stream is associated with two indicators: errorindicator& end-of-file (EOF) indicator

Both indicators are cleared when the stream is opened

Encountering end-of-file sets end-of-file indicator

Encountering read/write error sets error indicator An indicator once set remains set until it is explicitly cleared

by calling clearerr or some other library function

feof returns a non-zero value if the end-of-file

indicator is set; otherwise returns 0 ferror returns a non-zero value if the error indicator

is set; otherwise returns 0

Need to include Week13 - 18CS1010 (AY2011/2 Semester 1)

7/27/2019 Week13.pptx

19/38

6. Demo #2: Using feof( ) (2/2)

Caution on using feof()


#include #include

int main(void)

{. . .

while (!feof(infile)){

fscanf(infile, "%d", &num);printf("Value read: %d\n", num);

}. . .

}

Week13_Demo2.c

Value read: 10Value read: 20Value read: 30

Value read: 30

Output:

10 20 30

Input file demo2.in

Why does the last line appear twice?

To be discussed in Week 13 discussion.

7/27/2019 Week13.pptx

20/38

7. Character I/O: Output Functions (1/4)

Output functions: fputc,putchar


fputc andputchar return EOF if a write erroroccurs; otherwise, return character written

int ch = 'A';FILE *fp;

putchar(ch); // writes ch to stdout

fp = fopen( ... );fputc(ch, fp); // writes ch to fp

7/27/2019 Week13.pptx

21/38

7. Character I/O: Input Functions (2/4)

Input functions: fgetc, getchar , ungetc


fgetc and getchar return EOF if a read error occurs

or end of file is reached; otherwise, return character

read Need to call eitherfeof orferror to distinguish the 2 cases

int ch;FILE *fp;ch = getchar(); // reads a char from stdinfp = fopen( ... );

ch = fgetc(fp); // reads a char from fp

7/27/2019 Week13.pptx

22/38

7. Character I/O: Demo #3 (3/4)


int copyFile(char *sourcefile, char *destfile){FILE *sfp, *dfp;if ((sfp = fopen(sourcefile, "r")) == NULL)

exit(1); // error - can't open source fileif ((dfp = fopen(destfile, "w")) == NULL) {

fclose(sfp); // close source fileexit(2); // error - can't open destination file

}while ((ch = fgetc(sfp)) != EOF) {

if (fputc(ch, dfp) == EOF) {fclose(sfp); fclose(dfp);

exit(3); // error - can't write to file}

}fclose(sfp); fclose(dfp);return 0;

}

Week13_CopyFile.c

7/27/2019 Week13.pptx

23/38

7. Character I/O: ungetc (4/4)

ungetc pushes back a character read from a stream

ungetc returns the character it pushes back

Example: Read a sequence of digits and stop at the

first non-digit


int ch;FILE *fp = fopen( ... );

while (isdigit(ch = getc(fp))){

// process digit read. . .

}ungetc(ch, fp); // pushes back last char read

7/27/2019 Week13.pptx

24/38

8. Exercise #1: Trimming Blanks

Write a program Week13_TrimBlanks.c that containsa function

int trimBlanks(char *infile, char *outfile)

that takes an input text file and produces a new text

file that is a duplicate copy of the input file except thateach sequence of consecutive blank characters is

replaced by a single blank character.

The function returns -1 if there is an error; otherwise,

it returns the number of blank characters trimmed. An incomplete program Week13_TrimBlanks.c is

given. A test input file trimblanks.in is also given.


7/27/2019 Week13.pptx

25/38

9. Line I/O: Output Functions (1/6)

Output functions: fputs,puts


fputs andputs return EOF if a write error occurs;otherwise, return a non-negative number

FILE *fp;

// writes to stdout with newline character appendedputs("Hello world!");

fp = fopen( ... );// writes to fp without newline character appendedfputs("Hello world!", fp);

7/27/2019 Week13.pptx

26/38

9. Line I/O: Input Functions (2/6)

Input functions: fgets, gets


fgets and gets store a null character at the end of the string

fgets and gets return a null pointer if a read error occurs orend-of-file is encountered before storing any character;otherwise, return first argument

Avoid using gets due to security issue

char s[100];FILE *fp;

gets(s); // reads a line from stdin

fp = fopen( ... );fgets(s, 100, fp); // reads a line from fp

7/27/2019 Week13.pptx

27/38

9. Line I/O: fgets (3/6)

Prototype:char *fgets(char *s, int n, FILE *fp) s is a pointer to the beginning of a character array

n is a count

fp is an input stream

Characters are read from the input stream fp into s

until a newline character is seen,

end-of-file is reached, or

n 1 characters have been read without encounteringnewline character or end-of-file

If the input was terminated because of a newline

character, the newline character will be stored in thearray before the terminating null character (\0)


7/27/2019 Week13.pptx

28/38

9. Line I/O: fgets (cont.) (4/6)

If end-of-file is encountered before any charactershave been read from the stream, fgets returns a null pointer

The contents of the array s are unchanged

If a read error is encountered, fgets returns a null pointer The contents of the array s are indeterminate

WheneverNULL is returned, feoforferror shouldbe used to determine the status


7/27/2019 Week13.pptx

29/38

9. Demo #5: Counting Lines (5/6)

Write a function that takes as input the name of a textfile and returns the number of lines in the input file.

If an error occurs, the function should return a

negative number.

Assume that the length of each line in the file is at

most 80 characters.


7/27/2019 Week13.pptx

30/38

9. Demo #5: Counting Lines (6/6)


#define MAX_LINE_LENGTH 80int countLines(char *filename){

int count = 0;FILE *fp;char s[MAX_LINE_LENGTH+1];

if ((fp = fopen(filename, "r")) == NULL)return -1; // error

while (fgets(s, MAX_LINE_LENGTH+1, fp) != NULL)count++;

if (!feof(fp)) // read error encounteredcount = -1;

fclose(fp);return count;

}

Week13_CountLines.c

7/27/2019 Week13.pptx

31/38

10. Data Display Debugger (DDD)

See video

http://www.youtube.com/watch?v=VF7IBEAA8Ig

http://www.youtube.com/watch?v=VF7IBEAA8Ighttp://www.youtube.com/watch?v=VF7IBEAA8Ig

7/27/2019 Week13.pptx

32/38

Summary for Today


In todays lecture we learnt

I/O on text files

I/O functions:

File operations: fopen, fclose, feof, ferror Formatted I/O: fprintf, fscanf

Character I/O: fputc, putchar, fgets, getchar, ungetc

Line I/O: fputs, puts, fgets, gets

7/27/2019 Week13.pptx

33/38


Post-CS1010 (1/2)

For CS/CM/CB students: CS1010 CS1020 CS2010orCS1010 CS2020 (accelerated)

For IS and EC students: CS1010 CS1020

CS1020 Data Structures and Algorithms I Emphasis on algorithms and data structures

Using an object-oriented programming language Java

Textbook: Data Abstraction & Problem Solving with Java: Walls

and Mirrors by F. Carrano and J. Prichard

7/27/2019 Week13.pptx

34/38


Post-CS1010 (2/2)

CS1020 has sit-in labs (like mini-PEs) every otherweek

Strengthen your programming skills to prepare for

CS1020

7/27/2019 Week13.pptx

35/38


CS1010 Final Examination

CS1010 Exam 21 November 2011, Monday, 1 3pm

Venue: To be announced by Registrars Office

Format MCQ section: 6 questions

5 other questions (comprising tracing questions and

programming questions)

Grading of CS1010 is not based on bell curve

7/27/2019 Week13.pptx

36/38


How to Prepare for Exams?

Preparing for Exams: A Guide for NUS Students http://www.cdtl.nus.edu.sg/examprep/
http://www.cdtl.nus.edu.sg/examprep/http://www.cdtl.nus.edu.sg/examprep/

7/27/2019 Week13.pptx

37/38


7/27/2019 Week13.pptx

38/38

End of File

Week13.pptx

Documents