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POINTERS ON C

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1

A Quick Start

1.1 Introduction

It is always difficult to start describing a programming language because little

details do not make much sense until one knows enough to understand the "big

picture." In this chapter, I try to give you a glimpse of the big picture by looking at a

sample program and explaining its workings line by line. This sample program also

shows you how familiar procedures are accomplished in C. This information plus the

other topics discussed in the chapter introduce you to the basics of the C language so

that you can begin writing useful programs.

The program we dissect reads text from the standard input, modifies it, and

writes it to the standard output. Program l.l first reads a list of column numbers. These

numbers are pairs and indicate ranges of columns in the input line. The list is

terminated with a negative number. The remaining input lines are read and printed,

then the selected columns from the input lines are extracted and primed. Note that the

first column in a line is number zero. For example, if the input is

4 9 12 20 -1

abcdefghijklmnopqrstuvwxyz

Hello there, how are you?

I

am

fine,

thanks.

See you!

Bye

then the program would produce:

Original input

:

abcdefghijklmnopqxstuvwxyz

Rearranged

line:

efghijmnopqrstu

Chapter 1 A Quick Start

2

Original input

:

Hello

there,

how

are you?

Rearranged

line:

o ther

how

are

Original

input

: I

am

fine,

thanks.

Rearranged

line: fine,

thanks.

Original

input

:

See

you!

Rearranged

line:

you!

Original

input

:

Bye

Rearranged

line:

The important point about this program is that it illustrates most of the basic

techniques you need to know to begin writing C programs.

/*

** This program reads input lines from standard input and prints

**

each

input

line,

followed by

just

some

portions of the

line,

to

** the standard output.

**

The

first

input

is

a

lint of

column numbers, which ends with a

**

negative number. The column numbers are paired and

specify

**

ranges

of

columns from

the input line that are to

be

printed.

** For example, 0 3 10 12 -1 indicates

that

only columns 0 through 3

**

and columns 10

through

12

will

be

printed.

*/

#include <stdio.h>

#inc1ude <stdlib.h>

#include <string.h>

#define

MAX_COLS

20

/*

max

# of columns to process

*/

#define

MAX_INPUT

1000

/*

max

len of input

&

output lines

*/

int

read_column_numbers(

int

columns[],

int

max

);

void rearrange( char *output, char const

*input,

int

n_columns,

int

const columns[]

);

int

main( void

)

{

int

n_columns;

/*

#

of

columns

to

process

*/

int

columns[MAX_COLS];

/*

the columns

to

process

*/

char input[MAX_INPUT];

/*array for input line */

char output[MAX_INPUT];

/*array for

output

line */

Program 1.1 Rearrange characters continue.

1.1 Introduction

3

/*

**

Read the

list

of

column numbers

*/

n_columns = read_column_numbers( columns, MAX_COLS

);

/*

**

Read, process and

print

the remaining lines of input

*/

while( gets(input

)

!=

NULL

){

printf(

"Original input

:

%s\n", input

);

rearrange( output, input,

n_columns, columns

);

printf(

"Rearranged

line:

%s\n", output

);

}

returnEXIT_SUCCESS;

}

/*

**

Read

the

list

of

column numbers, ignoring any

beyond

the specified

** maximum.

*/

int

read_column_numbers(

int

columns[],

int

max

)

{

int

num =

0;

int ch;

/*

**

Get the numbers, stopping

at

eof

or

when a number

is

<

0.

*/

while(

num < max &&

scanf( "%d", &columns[num]

)

==

1

&&columns[num] >= 0

)

num +=1;

/*

**

Make

sure

we have an even number

of inputs,

as

they are

** supposed to be paired.

*/

if(

num

%

2

!=

0

){

puts( "Last

column number

is not paired."

);

exit(

EXIT_FAILURE

);

}

/*

** Discard the rest of the line that contained the

final

Program 1.1 Rearrange characters continue.

4

Chapter 1 A Quick Start

** number.

*/

while( (ch

=

getchar()) !=

EOF && ch

!=

'\n' )

;

return num;

}

/*

** Process a line of input by concatenating the characters from

**

the indicated columns.

The

output

line is

the

NUL

terminated,

*/

void

rearrange( char *output, char const

*input,

in n_columns,

int

const columns[]

)

{

int

col;

/*

subscript for

columns

array

*/

int

output_col;

/*

output column counter

*/

int

len;

/*

length of input line

*/

len

=

strlen( input

);

output_col

=

0;

/*

**

Process each

pair of

column numbers.

*/

for(

col = 0; col < n_columns; col += 2

){

int

nchars = columns[col + 1] – columns[col] + 1;

/*

**

If

the input line

isn't

this long or the output

**

array

is

full,

we're done

*/

if(

columns[col]

>=

len

||

output_col

== MAX_INPUT

– 1

)

break;

/*

**

If

there

isn't

room

in the output array, only

copy

**

what

will

fit.

*/

if(

output_col

+

nchars

> MAX_INPUT

– 1)

nchars

= MAX_INPUT

– output_col – 1;

/*

Program 1.1 Rearrange characters continue.

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