1 Introduction to Computing Lecture 11 Character Strings Assist.Prof.Dr. Nükhet ÖZBEK Ege...

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Introduction to Computing Lecture 11

Character Strings

Assist.Prof.Dr. Nükhet ÖZBEKEge University

Department of Electrical&Electronics [email protected]

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Topics

• Strings– Representation – Declaration – Functions – Common mistakes– Index of a char in a string

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Introduction

• Grouping of characters is called a string

• As C implements the strings using arrays of type char, we could not explore string until we had dealt with arrays

• Strings are important, because we also have to manipulate textual data as well as numerical data

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On the Nature of Strings• Recall: Main memory

– contiguous array of cells– each cell has an address

0x1FFF 0x2000 0x2001 0x20020x1FFEetc.

5ch

On the Nature of Strings (cont.)• Recall: Variable declaration

– sets aside a “box” to contain a value

Example: char ch;

ch = ‘B’;

0x1FFF 0x2000 0x2001 0x20020x1FFEetc.

‘B’

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On the Nature of Strings (cont.)

Example: char name[5];

Specifies numberof cells in the array

• String declaration– sets aside an array of cells– each cell contains a char– address of first cell in the array

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On the Nature of Strings (cont.)• String declaration

– sets aside an array of cells– each cell contains a char– address of first cell in the array

Example: char name[5];

0x2000 0x2004

name

is 0x2000

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Character StringsDeclaration 1:

char name[5];

Declaration 2:

#define MAXLENGTH 5

char name[MAXLENGTH];

0x2000 0x2004

name

is 0x2000

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String Input/Output

#include <stdio.h>

#define MAXLENGTH 15

int main(){ char string1[MAXLENGTH]; char string2[MAXLENGTH];

scanf("%s %s", string1, string2); printf("%s %s\n", string1, string2);

return 0;}

No ampersand (&)!

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String Input/Output (Alternate functions)

#include <stdio.h>



gets(string1); gets(string2); puts(string1); puts(string2); return 0;}

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Character Strings

Initialization:

char name[5] = “Ali”;

A l i \0

Terminating Character:• Marks the end of string• Special char: ’\0’ • aka NUL (single L)

0x2000 0x2004

name

is 0x2000

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Character Strings

Initialization:

char name[5] = “Ali”;

Can storeat most 4 letters,because of `\0’

A l i \0

0x2000 0x2004

name

is 0x2000

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Character Strings

Declaration 3:

char name[] = “Ali”;

Takes up an extra cell for ‘\0’

A l i \0

0x2000 0x2003

name

is 0x2000

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Character Strings

Declaration 4:

char *name = “Ali”;

Result is “undefined”if you try to modify this

string.

A l i \0

0x3000 0x3003

0x3000

name

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Character Strings

Declaration 5:

char name[];

String with arbitrary length?

No! Will cause an error.

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Arrays of Strings

• One string is an array of characters, so an array of strings is a two-dimensional array of characters in which each row is a string

• Example– char days[7][10] = {“Monday”, “Tuesday”, “Wednesday”, “Thursday”, “Friday”, “Saturday”, “Sunday”};

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A Char in a String

• The size of a character string is fixed.

• Character at position index: –string[index]– first character has index 0

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char name[8] = “Aise”;int i = 2;

printf(“Char at index %d is %c.\n”, i, name[i]);

A Char in a String

output: Char at index 2 is s.

index 0 index 4

A i s e \0

0x3995 0x399C

name

is 0x3995

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A Char in a String

index 2

A i s e \0

0x3995 0x399C

name

is 0x3995

char name[8] = “Aise”;

name[2] = ‘X’;printf(“Name: %s\n”, name);

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A i X e \0

0x3995 0x399C

name

is 0x3995

output: Name: AiXe

index 2

char name[8] = “Aise”;

name[2] = ‘X’;printf(“Name: %s\n”, name);

A Char in a String

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String Operations

• #include <string.h> • Operations:

– Assignment: strcpy()– Concatenation: strcat()– Comparison: strcmp()– Length: strlen()

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#include <stdio.h>#include <string.h>



strcpy(string1, “Hello World!”); strcpy(string2, string1);

return 0;}

String Operation: Assignment

string1: <garbage>string2: <garbage>

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String Operation: Assignment#include <stdio.h>#include <string.h>




return 0;}string1: “Hello World!”string2: <garbage>

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String Operation: Assignment#include <stdio.h>#include <string.h>




return 0;}

string1: “Hello World!”string2: “Hello World!”

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char name1[5] = “Ali”;char name2[5] = “Sami”;

name2 = name1;

Common Mistake:

Wrong Assignment

Example 1:

Error: “LValue required....”

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char *name1 = “Ali”;char *name2 = “Sami”;

name2 = name1;

Common Mistake:

Bad Assignment

Example 2:

Better avoid initialising

strings this way.(Usually, no error message.)

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Common Mistake:

Bad Assignment

S a m i \0

0x3990 0x3994

A l i \0

0x2000 0x2003

0x2000

name1

0x3990

name2

char *name1 = “Ali”;char *name2 = “Sami”;

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Common Mistake:

Bad Assignment

S a m i \0

0x3990 0x3994

A l i \0

0x2000 0x2003

0x2000

name1

0x2000

name2

name2 = name1;

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Common Mistake:

Not enough space

A l i \0

0x2000 0x2003


strcpy(name, “Samir”);

name

is 0x2000

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Common Mistake:

Not enough space

S a m i r \0

0x2000 0x2003


strcpy(name, “Samir”);

Requires caution.

name

is 0x2000

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String Operation: Concatenation

char string1[MAXLENGTH];char string2[MAXLENGTH];

strcpy(string1, “Goodbye”);strcpy(string2, “, Cruel ”);

strcat(string1, string2);strcat(string1, string2);strcat(string1, “World!”);

string1: “Goodbye”string2: “, Cruel “

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string1: “Goodbye, Cruel ”string2: “, Cruel “


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string1: “Goodbye, Cruel , Cruel ”string2: “, Cruel “





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string1: “Goodbye, Cruel , Cruel World!”string2: “, Cruel “





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Common Mistake:

char name[5];

strcpy(name, “Ali”);strcat(name, “ Osman”);

Not enough space

A l i \0

0x2000 0x2004

name

is 0x2000

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Common Mistake:

char name[5];

strcpy(name, “Ali”);strcat(name, “ Osman”);

Not enough space

A l i O s m a

0x2000 0x2004

n \0name

is 0x2000

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strcpy(string1, “Apple”);strcpy(string2, “Wax”);

if (strcmp(string1, string2) < 0){ printf(“%s %s\n”, string1, string2);}else{ printf(“%s %s\n”, string2, string1);}

output: Apple Wax

String Operation: Comparison

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if (strcmp(string1, string2) < 0){ printf(“%s %s\n”, string1, string2);}else{ printf(“%s %s\n”, string2, string1);}

String Operation: Comparison

Returns:

negative if string1 < string2

zero if string1 == string2

positive if string1 > string2

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if (string1 < string2){ printf(“%s %s\n”, string1, string2);}else{ printf(“%s %s\n”, string2, string1);}

Common Mistake:

Wrong Comparison

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output: 5

Number of char’sbefore the `\0’.

char string1[100];

strcpy(string1, “Apple”);

printf(“%d\n”, strlen(string1));

String Operation: Length

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Common Mistake:

char name[5];

strcpy(name, “Osman”);

Not enough space

Don’t forget the ‘\0’.

O s m a n \0

0x3990 0x3994

name

is 0x3990

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Character Strings as Parameters

• Strings as formal parameters are declared as char* or char[]– Examples:

void Greet ( char* name )

void Greet ( char name[] )

• As pointer to the first element of the string (array of chars).

• Changes to the string inside the function affect the actual string.

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Example: hello3.c

#include <stdio.h>#include <string.h>#define NAMELEN 50

/* Print a simple greeting to the user. */

void Greet ( char * name )

{

strcat(name,"! How are you?");

}

int main(){ char user[NAMELEN];

printf("Who are you? ");

scanf("%s", user);

Greet(user);

printf("%s\n", user);

return 0;}

user

Ali

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scanf("%s", user);

Greet(user);


return 0;}

Example: hello3.c




{


}

name user

Ali\0

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scanf("%s", user);

Greet(user);


return 0;}

Example: hello3.c




{


}

user

Ali! How are you?\0

name

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scanf("%s", user);

Greet(user);


return 0;}

Example: hello3.c




{


}

user

Ali! How are you?\0

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scanf("%s", user);

Greet(user);


return 0;}

More of scanf demystified

No ampersand (&) in scanf with strings!

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sscanf function

• sscanf function works exactly like scanf except that instead of taking the data values for its output parameters from the standard input device, it takes data from the string that is its first argument

• Examplesscanf(“85 96.2 hello”,”%d%lf%s”,&n,&val, word);

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Summary

• A string is a contiguous array of chars• The string identifier is the address of the first char in

the string• Individual chars are accessed using the str[index]

notation• There are C library functions for copying,

concatenating and comparing and finding lengths of strings

1 Introduction to Computing Lecture 11 Character Strings Assist.Prof.Dr. Nükhet ÖZBEK Ege...

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