Chp 9 Puppetting On Strings

Question 1

What are strings?

Answer 1

Character arrays.

Question 2

What is a string constant?

Answer 2

Its a one dimensional array terminated by a null (‘\0’).

char name[ ] = { ‘B’, ‘i’, ‘p’, ‘u’, ‘\0’}

Question 3

Null character :

Answer 3

\0
Has two chars but is only 1.
\ indicates special char ahead.
\0 and 0 are diff.
ASCII values 0 and 48 respectively.

Question 4

What if a string is not terminated by null char?

Answer 4

Then its merely a collection of chars, other than null, functions working with string have no choice to know bout ends.

Question 5

Initialise a String:

Answer 5

char name[] = “abcdef”;

C inserts a null character automatically.

Question 6

Memory map of string ‘abcdef’ :

Answer 6

a,b,c,d,e,f,\0

Stored contiguously. 1 char takes 1 byte.

Question 7

How to print out a string when len is unknown?

Answer 7

while ( i != ‘\0’ )

we can make use of this statement.
OR
while ( *p != ‘\0’ )

Question 8

Base address:

All notations for element:

Answer 8

We can get it simply by mentioning the name of array.

num[i] = *(num + i) = *(i + num) = i[num]

Question 9

Format specifier for string:
in printf
and in scanf

and working of scanf with string?

Answer 9

%s
now clue here
scanf( ‘%s’ , name );
is valid as name carries base address, or basically address where we want the string.
When enter key is hit, scanf places ‘\0’ in the array.

Question 10

Precautions

Scanf and String:

Answer 10

1. Length of string should not exceed dimension of character array, as no bound checking, so it might result in overriding imp data.
2. scanf() cannot receive multi-word string.

Question 11

How to deal with limitation of scanf that it cant accept multi word string:

2 ways:

(2 functions in here )

Answer 11

gets().
Receives only one string but it can be multi-word.

Also, puts() : It can display only one string at a time.
And it puts cursor on the next line unlike printf().
OR
scanf( “%[^\n]s”, name );

Question 12

Pointers and String:

2 options to store the string sorta..

Answer 12

chr str[] = “Hello”;
chr *p = “Hello”;

We cannot assign a string to another. But we can assign a char pointer to another.
str1 = str; /* Error /
p = q; / q is a char pointer, No error */

String once defined can’t be initialised again. Valid for char pointers.

str = "Bye";   /* Error*/
p = "Bye";      /* works */

Question 13

Standard lib functions:

Answer 13

Check table.
for now:
strlen(string)
strcmp(string,string)
strcpy(target,string)
strcat()

Question 14

strlen()

Answer 14

Counts no. of char present in string.

It takes in the base address of string then counts until ‘\0’ encountered and returns length.
It doesn’t count ‘\0’.

Question 15

strcpy()

Answer 15

strcpy( target, source );

Copies Contents of source in target.
It accepts base address of both strings, string gets copied char by char, no short cut.
Remeber after chars being copied, after loop, target should have ‘\0’ at the end.

Question 16

The const keyword:

Answer 16

const qualifier ensures that ur program does not inadvertently alter a variable you intend to keep constant.

const char q = ‘hello’; / String fixed, pointer not/
q = ‘M’; / Error /
q = “Bye”; /Works/

char const q = “Hello”; / Same as above */

char const t = “Hello”; / Pointer const, string not*/

const can be used for int, float,…etc

Question 17

strcat()

Answer 17

strcat(target, source);
Concatenate source at the end of target.
Base addresses passed.

Question 18

strcmp()

Answer 18

strcmp( string1, string2 );
returns 0 for equal string, otherwise difference between ASCII values of the char when they first don’t match.
Examples:
strcmp ( “Jerry”, “Jerry” ); /* 0 /
strcmp ( “Jerry”, “Ferry” ); / 4 /
strcmp ( “Jerry”, “Jerry Boy” ); / -32 */
‘\0’ - ‘ ‘ = 0 - 32 = -32

Question 19

2 D Array of characters:

Answer 19

char name[6][10] = { 6 strings of dimension 10 or less. };
[] first subscript give number of names.
second gives length of each item.

Question 20

How to get 2D string, from keyboard?

Answer 20

for (i = 0; i<6 ; i++ )

{ scanf( ‘%s’, &name[i][0] ); }

Question 21

Memory map of 2D array of chars:

Answer 21

char name[6][10] = { 6 strings of dimension 10 or less. };

each string will be at a diff of 10 bytes in location.
each ends with ‘/0’.
Not necessary every name occupies all the 10 bytes, so unoccupied bytes are wasted , unused.

Question 22

How to take care of wasted bytes in 2D array of chars?

Answer 22

By Array of pointers to strings.

Question 23

Array Of Pointers to strings:

Benefits of using it?

Answer 23

char *p[6] = {Name of 6 strings};

Since the name contains the base address. No need to mention 6 on LHS in [].

Now we have not saved all the bytes wasted by 2D array method as we need 6*2 bytes for this array.
[2 bytes per int]

Bytes used optimally and manipulation easier.

Question 24

Exchange names using 2 D array and array of pointers to strings:

Answer 24

for i in 0 to 9
t = names[2][i]; names[2][i] = names[3][i];
names[3][i] = t;
10characters interchanged here

char *temp;
temp = name[2]; name[2] = name[3];
name[3] = temp;
Here just addresses exchanged.

Question 25

Limitation of Array of Pointers to String:

Answer 25

1. You cannot take values from keyboard in
   char *name[6];
   cuz when declared it contains garbage value, we cant send these garbage values to scanf() as the addresses where it should keep the received strings.

Question 26

The solution to Limitation of Array of Pointers to String:

How is the solutions quality?

Answer 26

#include "alloc.h"
char *name[6];
char n[50];
int len, i;
char *p;
for i in 0 to 5
{ scanf( '%s', &n);
  len = strlen(n);
 /* Allocate space for making copy of name
/* void* to char*, typecasting.
  p = malloc(len +1 )
  strcpy(p,n);
  name[i] = p;
}
malloc() : Needs no. of bytes to be allocated and returns base address of the chunk of memory it allocates.
Address returned is always of type void*.
It allocates memory dynamically, unlike arrays where static memory allocation takes place.

This suffers in performance as we need to allocate memory and then do the copying of string for each name received.