Strings - Frontier Homepage Powered by Yahoofrontiernet.net/~abartczak/Asmts/05/Chapter09_Updt.pdfChapter 9 Strings Original Slides: Kun-Mao Chao Updated: Andrew Bartczak Copyright

Chapter 9 Strings

Original Slides:

Kun-Mao Chao

Updated:

Andrew Bartczak

Copyright ©2004 Pearson Addison-Wesley. All rights reserved. 9-2

Strings

• C implements the string data structure using arrays of type char.

• You have already used the string extensively.

– printf(“This program is terminated!\n”);

– #define ERR_Message “Error!!”

• Since string is an array, the declaration of a string is the same as declaring a char array.

– char string_var[30];

– char string_var[20] = “Initial value”;


String Literals

• String literal values are represented by sequences of

characters between double quotes (“)

• Examples

– “” - empty string

– “hello”

• “a” versus ‘a’

– ‘a’ is a single character value (stored in 1 byte) as the ASCII

value for a

– “a” is an array with two characters, the first is a, the second is

the character value \0


Referring to String Literals

• String literal is an array, can refer to a single character

from the literal as a character

• Example:

printf(“%c”,”hello”[1]);

outputs the character ‘e’

• During compilation, C creates space for each string

literal (# of characters in the literal + 1)

– referring to the literal refers to that space (as if it is

an array)


Duplicate String Literals

• Each string literal in a C program is stored at a different

location

• So even if the string literals contain the same string,

they are not equal (in the == sense)

• Example:

– char string1[6] = “hello”;

– char string2[6] = “hello”;

– but string1 does not equal string2 (they are stored at different

locations)


Changing String Variables

• Allocate an array of a size large enough to hold the string (plus 1 extra value for the delimiter)

• Examples (with initialization):

char str1[6] = “Hello”;

char str2[] = “Hello”;

char *str3 = “Hello”;

char str4[6] = {‘H’,’e’,’l’,’l’,’o’,’\0’};

• Note, each variable is considered a constant in that the space it is connected to cannot be changed

str1 = str2; /* not allowable, but we can copy the contents

of str2 to str1 (more later) */


Changing String Variables cont.

• Can change parts of a string variable

char str1[6] = “hello”;

str1[0] = ‘y’;

/* str1 is now “yello” */

str1[4] = ‘\0’;

/* str1 is now “yell” */

• Important to retain delimiter (replacing str1[5] in the original string with something other than ‘\0’ makes a string that does not end)

• Have to stay within limits of array


Memory Storage for a String

• The string is always ended with a null character

‘\0’.

• The characters after the null character are

ignored.

• e.g., char str[20] = “Initial value”;

n i t i a l v a l u e ? ? … I \0

[0] [13]


Distinction Between Characters and Strings

• The representation of a char (e.g., ‘Q’) and a

string (e.g., “Q”) is essentially different.

– A string is an array of characters ended with the null

character.

Q

Character ‘Q’

Q \0

String “Q”


Arrays of Strings Introduction

• An array of strings is a two-dimensional array of

characters in which each row is one string.

– char names[People][Length];

– char month[5][10] = {“January”,

“February”, “March”, “April”,

“May”};


String Input

• Use %s field specification in scanf to read string

– ignores leading white space

– reads characters until next white space encountered

– C stores null (\0) char after last non-white space char

– Reads into array (no & before name, array is a pointer)

• Example:

char Name[11];

scanf(“%s”,Name);

• Problem: no limit on number of characters read (need one for delimiter), if too many characters for array, problems may occur


String Input (cont)

• Can use the width value in the field specification to

limit the number of characters read:

char Name[11];

scanf(“%10s”,Name);

• Remember, you need one space for the \0

– width should be one less than size of array

• Strings shorter than the field specification are read normally, but C always stops after reading 10 characters


String Input (cont)

• Edit set input %[ListofChars]

– ListofChars specifies set of characters (called scan set)

– Characters read as long as character falls in scan set

– Stops when first non scan set character encountered

– Note, does not ignored leading white space

– Any character may be specified except ]

– Putting ^ at the start to negate the set (any character BUT list is allowed)

• Examples:

scanf(“%[-+0123456789]”,Number);

scanf(“%[^\n]”,Line); /* read until newline char */


String Output

• Use %s field specification in printf:

characters in string printed until \0 encountered

char Name[10] = “Rich”;

printf(“|%s|”,Name); /* outputs |Rich| */

• Can use width value to print string in space:

printf(“|%10s|”,Name); /* outputs | Rich| */

• Use - flag to left justify:

printf(“|%-10s|”,Name); /* outputs |Rich | */


Input/Output of a String Example

#include <stdio.h>

void main() {

char LastName[11];

char FirstName[11];

printf("Enter your name (last , first): ");

scanf("%10s%*[^,],%10s",LastName,FirstName);

printf("Nice to meet you %s %s\n",

FirstName,LastName);

}


Input/Output of a String

• The placeholder %s is used to represent string

arguments in printf and scanf.

– printf(“Topic: %s\n”, string_var);

• The string can be right-justified by placing a

positive number in the placeholder.

– printf(“%8s”, str);

• The string can be left-justified by placing a

negative number in the placeholder.

– Printf(“%-8s”, str);


Right and Left Justification of

Strings

The “%8s” placeholder displays a string which is right-

justified and in 8-columns width.

If the actual string is longer than the width, the displayed field

is expanded with no padding.


An Example of Manipulating String

with scanf and printf

The dept is the initial memory address

of the string argument. Thus we don’t

apply the & operator on it.


Execution of scanf ("%s", dept);

• Whenever encountering a white space, the scanning stops

and scanf places the null character at the end of the string.

• e.g., if the user types “MATH 1234 TR 1800,” the string

“MATH” along with ‘0’ is stored into dept.


Array of Strings

• Sometimes useful to have an array of string values

• Each string could be of different length (producing a

ragged string array)

• Example:

char *MonthNames[13]; /* an array of 13 strings */

MonthNames[1] = “January”; /* String with 8 chars */

MonthNames[2] = “February”; /* String with 9 chars */

MonthNames[3] = “March”; /* String with 6 chars */

etc.


Array of Strings Example

#include <stdio.h>

void main() {

char *days[7];

char TheDay[10];

int day;

days[0] = "Sunday";

days[1] = "Monday";

days[2] = "Tuesday";

days[3] = "Wednesday";

days[4] = "Thursday";

days[5] = "Friday";

days[6] = "Saturday";



printf("Please enter a day: ");

scanf("%9s",TheDay);

day = 0;

while ((day < 7) && (!samestring(TheDay,days[day])))

day++;

if (day < 7)

printf("%s is day %d.\n",TheDay,day);

else

printf("No day %s!\n",TheDay);

}



int samestring(char *s1, char *s2) {

int i;

/* Not same if not of same length */

if (strlen(s1) != strlen(s2))

return 0;

/* look at each character in turn */

for (i = 0; i < strlen(s1); i++)

/* if a character differs, string not same */

if (s1[i] != s2[i]) return 0;

return 1;

}


String Library Functions

• The string can not be copied by the assignment

operator ‘=’.

– e..g, “str = “Test String”” is not valid.

• C provides string manipulating functions in the

“string.h” library.

– The complete list of these functions can be found in

Appendix B of the textbook.


Some String Functions from String.h

Function Purpose Example

strlen Returns the number of characters

in a string strlen(“Hi”); // ret. 2

strcpy Makes a copy of a string strcpy(s1, “Hi”);

strcat Appends a string to the end of

another string strcat(s1, “more”);

strcmp Compare two strings

alphabetically strcmp(s1, “Hu”);

strcspn Calculate length of initial segment

of str1 characters not in str2. strcspn(“There ”, “er”);

strtok Breaks a string into tokens by

delimiters.

strtok(“Hi, Chao”, “ ,”);

strchr Searches the first occurrence of a

character in a string. strchr(s1,’u’);

strstr Searches the first occurrence of a

substring in a string. strchr(s1, “Hi”);


Functions strcpy and strncpy

• Function strcpy copies the string in the second argument into the first argument.

– e.g., strcpy(dest, “test string”);

– The null character is appended at the end automatically.

– If source string is longer than the destination string, the overflow characters may occupy the memory space used by other variables.

• Function strncpy copies the string by specifying the number of characters to copy.

– You have to place the null character manually.

– e.g., strncpy(dest, “test string”, 6); dest[6] = ‘\0’;

– If source string is longer than the destination string, the overflow characters are discarded automatically.


Extracting Substring of a String (1/2)

• We can use strncpy to extract substring of one string.

– e.g., strncpy(result, s1, 9);


Extracting Substring of a String (2/2)

• e.g., strncpy(result, &s1[5], 2);


Functions strcat,strcat and strlen

• Functions strcat and strncat concatenate the fist string argument with the second string argument. – strcat(dest, “more..”);

– strncat(dest, “more..”, 3);

• Function strlen is often used to check the length of a string (i.e., the number of characters before the fist null character).

– e.g., dest[6] = “Hello”; strncat(dest, “more”, 5-strlen(dest)); dest[5] = ‘\0’;


Functions strchr and strstr

• Function strchr searches for the first occurrence of a character ch in a string str.

– e.g.,dest[6] = “Hello”

strchr(dest, ‘H’);

• Function strstr searches for the first occurrence of a substring substr in a string str.

– e.g., dest[6] = “Hello”; strstr(dest, “He”);


Function strcspn

• Function strcspn returns the number of characters in the initial segment of string str1 which are not in the string

– e.g.,dest[6] = “Hello”

ivalue = strcspn(dest, “aho”);

The return ivalue is 4 since string dest spans four characters before ‘o’ is found.


String Comparison (1/2)

• Suppose there are two strings, str1 and str2.

– The condition str1 < str2 compare the initial memory

address of str1 and of str2.

• The comparison between two strings is done by

comparing each corresponding character in them.

– The characters are compared against the ASCII table.

– “thrill” < “throw” since ‘i’ < ‘o’;

– “joy” < joyous“;

• The standard string comparison uses the strcmp and

strncmp functions.


String Comparison (2/2)

Relationship Returned Value Example

str1 < str2 Negative “Hello”< “Hi”

str1 = str2 0 “Hi” = “Hi”

str1 > str2 Positive “Hi” > “Hello”

• e.g., we can check if two strings are the same by

if(strcmp(str1, str2) != 0)

printf(“The two strings are different!”);


Input/Output of Characters and

Strings

• The stdio library provides getchar function which gets the next character from the standard input.

– ch = getchar(); is the same as scanf(“%c”, &ch);

– Similar function are putchar, gets, puts:

char str[50] ;

printf(“Enter a staring: “) ;

gets(str);


Input/Output of Characters Example

#include<stdio.h>

int main()

{

char c;

printf(“Enter character: “);

c = getc(stdin);

printf(“Character entered: “);

putc(c, stdout);

return(0);

}


Input/Output of Strings Example

#include<stdio.h>

int main()

{

char str[50];

printf(“Enter a string: “);

gets(str);

printf(“You entered %s”, str);

return(0);

}

#include <stdio.h> int main() { char str[50]; printf("Enter a string : "); gets(str); printf("You entered: %s", str); return(0); }


File Input/Output of Characters and

Strings

• For IO from/to the file, the stdio library also provides corresponding functions.

– getc: reads a character from a file.

– Similar functions are putc, fgets, fputs.


Character Analysis and Conversion

• The <ctype.h> library defines facilities for character

analysis and conversion.

Functions Description

isalpha Check if the argument is a letter

isdigit Check if the argument is one of the ten digits

isspace Check if argument is a space, newline or tab.

tolower Converts the lowercase letters in the

argument to upper case letters.


Conversions Between Strings

Numbers

• The <stdlib.h> defines some basic functions for

conversion from strings to numbers:

– atoi(“123”) converts a string to an integer.

– atol(“123”) converts a string to a long integer.

– atof(“12.3”) converts a string to a float.

• However, there is no functions such as itoa, itof,

…etc,

– because there is a function called sprintf which can

converts many formats to a string.


The sprintf and sscanf Functions

• The sprintf function substitutes values for

placeholders just as printf does except that it

stores the result into a character array

– sprintf(s, “%d%d%d”, mon, day,

year);

• The sscanf function works exactly like scanf

except that it takes data from the string as its

input argument.

– sscanf(“ 11 22.2 Hello”, “%d%lf%s”,

&num, &val, word);

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Strings - Frontier Homepage Powered by Yahoofrontiernet.net/~abartczak/Asmts/05/Chapter09_Updt.pdfChapter 9 Strings Original Slides: Kun-Mao Chao Updated: Andrew Bartczak Copyright