homework #4cs-2301 b-term 20081 homework #4 strings, arrays, and malloc() cs-2301, system...

Homework #4CS-2301 B-term 2008 1

Homework #4Strings, Arrays, and malloc()

CS-2301, System Programming for Non-majors

(Slides include materials from The C Programming Language, 2nd ed., by Kernighan and Ritchie and from C: How to Program, 5th ed., by Deitel and Deitel)


Homework #4

• Due Friday, December 5, 11:59 PM

• Assignment:–– Read text from one or more files– Justify the text so that right and left margins

line up– Print justified text

• Somewhat bigger than previous homeworks


Objectives

• Get and interpret arguments from the command line

• Develop a program from multiple C files

• Read input from file

• Write output to file and/or stderr• Learn to use malloc(), realloc(), and free()

• Work with strings and arrays of pointers


Background Reading inKernighan & Ritchie

• Chapter 7 – especially• §7.5, File input and output

• §7.8, Miscellaneous functions

• Chapter 4• §4.11.1, File inclusion

• §4.11.3, Conditional inclusion


Definition – Module

• A C program that does not stand on its own• Not complete• Unable to execute when compiled• Requires other C programs to be compiled, linked,

with it before it can run

• (Usually) one of many C programs that together partition a large problem into small pieces

• More tractable than one large program• (Often) can be tested independently of larger system


Program Structure for Homework #4

• At least three modules– hw4.c

• Contains the main() function and any utility functions

• Reads and interprets command line– hw4ReadAndPrint.c

• Reads text from a file, invokes justify() function, prints justified lines

– hw4Justify.c• Implements justify() function• To convert unjustified text to justified lines


Program Structure (continued)

• Common include file – hw4.h– Function headers for principle functions– Any common data structures

• Included in each module– #include "hw4.h"

• Note straight quotes, instead of < >

• Compiled withgcc –Wall –o hw4 hw4.c hw4ReadAndPrint.c hw4Justify.c


Program Structure (continued)

• Common include file – hw4.h– Function headers for principle functions– Any common data structures

• Included in each module– #include "hw4.h"

• Note straight quotes, instead of < >

• Compiled withgcc –Wall –o hw4 hw4.c hw4ReadAndPrint.c hw4Justify.c

Why isn’t hw4.h

included in

this list of file

s to compile?


Homework #4 Program Operation

./hw4 –w100 –t5 file1.txt file2.txt ...

• Optional arguments•-w100, -wn – specifies how wide the justified lines

should be (in characters)•-t5, -tn – specifies the tab spacing

• Defaults to 80 characters wide and 5 character tabs

• Mandatory arguments• File names

• Each file contains text to justify

• Sample files provided – see assignment


Homework #4 Program Operation

./hw4 –w100 –t5 file1.txt file2.txt ...

• Optional arguments•-w100, -wn – specifies how wide the justified lines

should be (in characters)•-t5, -tn – specifies the tab spacing

• Defaults to 80 characters wide and 5 character tabs

• Mandatory arguments• File names

• Each file contains text to justify

• Sample files provided – see assignment

The program name is simply

the name of its file. Why is

“./” needed?


Definition – Justify text

• Partition text into lines• No more characters than line width

• Insert additional spaces to align right edges• Except if text ends with '\0'

• If '\t' is encountered, replace with spaces • To next multiple of tab spacing '\t'• Left edges of lines following '\t' aligned under

character following '\t'


Function main()

• Process arguments in command line in a loop• For each argument i, if argv[i] is

– -t or -w, set tab spacing or width to following number (no space)

– Otherwise, treat it as a file name

• If a file name– Open file– Call ReadAndPrint()

– Close file


Digression – File I/O

FILE *fopen(char *name, char *mode);

• Opens file with pathname name• File mode specifies kind of access

• Read only

• Read write

• Create, etc.

• Returns pointer to FILE•NULL if an error

• For this application, call•fp = fopen(argv[i], "r"); //readonly


File I/O (continued)

• int fclose(FILE *fp);– Closes the file pointed by fp– Returns zero if successful, EOF if error

• getc(FILE *stream)fgetc(FILE *stream)– Returns one character from stream or EOF at end of file

or if error• fprintf(FILE *stream, char* format, ...)

– Same as printf, but to file denoted by stream• Built-in file pointers of type FILE *

– stdin, stdout, stderr


Function main()

• Process arguments in command line in a loop• For each argument i, if argv[i] is

– -t or -w, set tab spacing or width to following number (no space)

– Otherwise, treat it as a file name

• If a file name– Open file using fp = fopen(argv[i], "r");– Call ReadAndPrint(fp, stdout, width, tab)– Close file using fclose(fp);


ReadAndPrint()

• Parameters:–– FILE *input, FILE *output– const int width, const int tab

• Allocates char *buffer using malloc()– Suitable size (defined in hw4.h)

• Reads text from input– One character at a time using fgetc()– Keep track of size of array, # of chars already in array– If necessary, use realloc() to increase size of array

• When '\n' of EOF is encountered, – Terminate string with '\0' – Call justify()

• …


ReadAndPrint() (continued)

• …justify() returns array of pointers to char *

• i.e., char **• Each pointer points to a line of justified text ending in '\0'

• ReadAndPrint() prints text to output• fprintf(output, "%s\n", line[i]); where line[i] is ith line returned

• After printing ith line, free(line[i]);• After printing all lines, free array of pointers


Code Fragment for ReadAndPrint

char *buffer = malloc(defaultBufSize);int bufSize = defaultBufSize;int position = 0;

while ((chr=fgetc(input))!= '\n') {if (rc==EOF) break;if (position>=bufSize-1) {

bufSize += defaultBufSize;buffer = realloc(buffer,bufSize);

}buffer[position++]=chr;

}

buffer[position] = '\0';


justify function

char **justify(const char *text,const int width, const int tab);

• Allocate array of pointers• For each line

• Allocate a character array of size width+1store pointer in array of pointers

• Fill character array with some text up to white space• Insert blanks to justify text• If end, add null pointer• If array of pointers is full, increase size


Code Fragment for Justify

char *line[] = malloc(defaultArrSize);int arrSize = defaultArrSize;int lineNum = 0, nextChr = 0;

while (text remaining) {if (lineNum>=arrSize-1) {

arrSize += defaultArrSize;line = realloc(line,arrSize);

}line[lineNum]=malloc(width+1);nextChr = justifyOne(line[lineNum],

text[nextChr], width, tab);lineNum++;

}

line[lineNum] = NULL;


justifyOne()

• Exercise for the student!


Summary — Homework #4

• Multi-module assignment

• Read and justify text from input file(s)• Specified on command line

• Call malloc() to allocate storage for – Text strings– Array of pointers to text strings

• Call realloc() to change size of array• Print errors on stderr


Questions?

Homework #4 dueFriday, December 5, 11:59 PM

homework #4cs-2301 b-term 20081 homework #4 strings, arrays, and malloc() cs-2301, system...

Documents

c slide

file hw4

c hw4readandprint

c hw4justify

c programs

c implements

multiple c files

file input