computer programming- lecture 5

1Computer Programming I

Lecture 5C-Strings


Outline

C-strings Functions for C-strings C-strings input C-strings output C-strings to numbers


Problems

How do I store the names of students? How do I specify filenames? How to I manipulate text?


String Taxonomy


An Array Type for Strings C-strings can be used to represent strings of

characters C-strings are stored as arrays of characters C-strings use the null character '\0' to end a string

The Null character is a single character

To declare a C-string variable, declare an array of characters:

char s[11];


Arrays A series of elements (variables) of the same type Placed consecutively in memory Can be individually referenced by adding an index to

a unique name Example, an array to contain 5 integer values of type

int called item:int item[5];

item0 1 2 3 4

int


An Array Type for Strings C-strings can be used to represent strings of

characters C-strings are stored as arrays of characters C-strings use the null character '\0' to end a string

The Null character is a single character

To declare a C-string variable, declare an array of characters:

char s[11];


C-string Details Declaring a C-string as char s[10] creates

space for only nine characters The null character terminator requires one space

A C-string variable does not need a size variable The null character(‘\0’) immediately follows the last

character of the string

Example:

??\0!moMiHs[9]s[8]s[7]s[6]s[5]s[4]s[3]s[2]s[1]s[0]


C-string Declaration

To declare a C-string variable, use the syntax:

char Array_name[ Maximum_C_String_Size + 1];

+ 1 reserves the additional character needed by '\0'


Initializing a C-string To initialize a C-string during declaration:

char my_message[20] = "Hi there."; The null character '\0' is added for you

Another alternative: char short_string[ ] = "abc"; but not this: char short_string[ ] = {'a', 'b', 'c'};


C-string error

This attempt to initialize a C-string does not cause the ‘\0’ to be inserted in the array char short_string[ ] = {'a', 'b', 'c'};


C-style string

char name[10]={'H','e','l','l','o','\0'};

char name[10]="Hello";

is equivalent

The declaration:A C-style string in C++ is basically an array of characters, terminated by a null character ('\0')

H e l l o \0

(the size of the array is 10, the length of the string is 5)

When we count the length of the string, we do not count the terminating null character.


char name[]={'H','e','l','l','o','\0'};

char name[]="Hello";

is equivalent

The declaration:

If the size of the array is not specified, then the array size would be equal to the length of the string plus 1

H e l l o \0



char name[6]={'H','\0'};

char name[6]="H";

is equivalent

The declaration:

H \0

(the size of the array is 6, But the length of the string is 1)

char name[]={'H','\0'};

char name[]="H";

is equivalent

The declaration:

H \0

(the size of the array is 2, But the length of the string is 1)


char name[]={'\0'};

char name[]="";

is equivalent

The declaration:

\0


Null string(empty string)

char name[6]={'\0'};

char name[6]="";

is equivalent

The declaration:

\0



char name='H';

char name[]="H";

is NOT equivalent

The declaration:

H \0


H

(this is not an array nor a string,it is simply a character)


char name[11]="John Connor";

Note: If you just want to have an array of characters, then this MAY be ok.

J o h n C o n n o r

Warning: if you want to use this as string with the C’s standard string manipulation library, then you may run into problems because the string is not null terminated.


Assignment With C-strings

This statement is illegal:

a_string = "Hello"; This is an assignment statement, not an

initialization The assignment operator does not work

with C-strings


Assignment of C-strings A common method to assign a value to a

C-string variable is to use strcpy, defined in the cstring library Example: #include <cstring>

… char a_string[ 11];

strcpy (a_string, "Hello");

Places "Hello" followed by the null character in a_string


#include <iostream>#include <cstring>using namespace std;int main() { char source[] = "Judgement Day"; char dest[] = "Rise of the Machine"; strncpy(dest,source); cout << "source = " << source << endl; cout << " dest = " << dest << endl;}

source = Judgement Day dest = Judgement Day

strcpy()

Copy one string into another. Copy string source to dest, stopping after the terminating null character has been moved.

WARNING : Make sure the destination string has enough allocated array space to store the new string. The compiler will not warn you about this, you MAY probably corrupted some part of the computer memory if your destination array of characters is not big enough to store the new string. This is DANGEROUS, so be careful !!!


int main(){ char source[] = "Judgement Day"; char dest[] = "Rise of the Machine";

dest = source; . . .}

WARNING : You cannot assign one string to another just as you can't assign one array to another !!!

The compiler will give an error.


int main() { char dest[20]; dest = "Judgement Day";}

WARNING : You cannot assign one string to another just as you can't assign one array to another !!! Use strcpy() function instead.

int main() { char dest[20] = "Judgement Day";}

int main() { char dest[20]; strcpy(dest,"Judgement Day");}

This is an initialization, it is valid.

This is an assignment, it is NOT valid. Use strcpy to assign a c-string

to another c-string


A Problem With strcpy

strcpy can create problems if not used carefully strcpy does not check the declared length

of the first argument

It is possible for strcpy to write characters beyond the declared size of the array


A Solution for strcpy Many versions of C++ have a safer version of

strcpy named strncpy strncpy uses a third argument representing the

maximum number of characters to copy Example: char another_string[10];

strncpy(another_string, a_string_variable, 9);

This code copies up to 9 characters into another_string, leaving one space for '\0'


== Alternative for C-strings The = = operator does not work as expected with

C-strings The predefined function strcmp is used to compare C-

string variables Example: #include <cstring>

… if (strcmp(c_string1, c_string2)) cout << "Strings are not the same."; else cout << "String are the same.";


strcmp's logic strcmp compares the numeric codes of

elements in the C-strings a character at a time If the two C-strings are the same, strcmp returns 0

0 is interpreted as false

As soon as the characters do not match strcmp returns a negative value if the numeric code in

the first parameter is less strcmp returns a positive value if the numeric code in the

second parameter is less Non-zero values are interpreted as true


#include <iostream>#include <cstring>using namespace std;int main() { char string1[] = "Hello"; char string2[] = "Hello World"; char string3[] = "Hello World"; char string4[] = "Aloha"; int n;

strcmp()Compare one string to another starting with the first character in each string and continuing with subsequent characters until the corresponding characters differ or until the end of the strings is reached.

n = strcmp(string1, string2); cout << string1 << "\t\t" << string2 << "\t==> " << n << endl;

n = strcmp(string2, string3); cout << string2 << '\t' << string3 << "\t==> " << n << endl;

n = strcmp(string3, string4); cout << string3 << '\t' << string4 << "\t\t==> " << n << endl;}

Hello Hello World ==> -1

Hello World Hello World ==> 0

Hello World Aloha ==> 1


#include <iostream>#include <cstring>using namespace std;int main() { char string1[] = "Hello"; char string2[] = "Hello";

if ( string1==string2 ) cout << "Same"; else cout << "Different"; }

WARNING : You should not use the logical comparison to compare whether two C-strings are identical.

The compiler will NOT give you any compile-time errors nor warning, but you will not be getting the results you thought you would get.

Different

output:


Use strcmp() function instead for string comparison.

Same

output:

#include <iostream>#include <cstring>using namespace std;int main() { char string1[] = "Hello"; char string2[] = "Hello";

if (strcmp(string1,string2)==0 ) cout << "Same"; else cout << "Different"; }


More C-string Functions The cstring library includes other functions

strlen returns the number of characters in a string int x = strlen( a_string);

strcat concatenates two C-strings The second argument is added to the end of the first The result is placed in the first argument Example:

char string_var[20] = "The rain"; strcat(string_var, "in Spain");

Now string_var contains "The rainin Spain"


The strncat Function

strncat is a safer version of strcat A third parameter specifies a limit for the

number of characters to concatenate Example:

char string_var[20] = "The rain"; strncat(string_var, "in Spain", 11);


#include <iostream>#include <cstring>using namespace std;int main() { char dest[30] = "Rise of "; char source[] = "The Machine";

strcat( dest, source ); cout << " dest = " << dest << endl; cout << "source = " << source << endl;}

dest = Rise of The Machinesource = The Machine

strcat()

Appends one string to another. strcat() appends a copy of source to the end of dest. The length of the resulting string is strlen(dest) + strlen(source).

WARNING : Make sure the destination string has enough allocated array space to store the new string. The compiler will not warn you about this, you MAY probably corrupted some part of the computer memory if your destination array of characters is not big enough to store the new string. This is DANGEROUS, so be careful !!!


char s1[12]="Hello";char s2[]="World";

H e l l o \0

s1W o r l d \0

s2

H e l l o W o r l d \0

s1W o r l d \0

s2

strcat(s1,s2);


C-String functions in C's Standard Library

Must #include <cstring> Most common functions:

strlen() – get the length of the string without counting the terminating null character.

strncpy() - copy one string to another string strcmp() - compare two string strncat() - concatenates / join two string


C-string Output

C-strings can be output with the insertion operator Example: char news[ ] = "C-strings";

cout << news << " Wow." << endl;


C-string Input The extraction operator >> can fill a C-

string Whitespace ends reading of data Example: char a[80], b[80];

cout << "Enter input: " << endl; cin >> a >> b; cout << a << b << "End of Output";could produce: Enter input: Do be do to you! DobeEnd of Output


Simple C-String Input and Output

#include <cstring>#include <iostream>using namespace std;

int main(){ char name[10]; cout << "Name => ";

cin >> name; cout << "Name is [" << name << "]" << endl;

cout << "Name length is " << strlen(name);}

Consider this program :Declaring name to be an array of characters

Standard function to return the length of the string

must #include this for using strlen() function

To treat the array of characters as C-string, we just use the name of the array when dealing with I/O stream.


Name => helloName is [hello]Name length is 5

#include <cstring>#include <iostream>using namespace std;int main() { char name[10]; cout << "Name => "; cin >> name; cout << "Name is [“ << name << "]\n"; cout << "Name length is “ << strlen(name);}

Sample run 1:

[remark: no problem]

Name => Hello thereName is [Hello]Name length is 5

Sample run 2:

[remark: the cin will stop scanning after the space character is encountered.


Reading an Entire Line

Predefined member function getline can read an entire line, including spaces getline is a member of all input streams getline has two arguments

The first is a C-string variable to receive input The second is an integer, usually the size of the

first argument specifying the maximum number of elements in the first argument getline is allowed to fill


Using getline The following code is used to read an entire

lineincluding spaces into a single C-string variable char a[80];

cout << "Enter input:\n";cin.getline(a, 80);cout << a << End Of Output\n";

and could produce: Enter some input: Do be do to you! Do be do to you!End of Output


getline syntax Syntax for using getline is

cin.getline(String_Var, Max_Characters + 1);

cin can be replaced by any input stream Max_Characters + 1 reserves one element for the null

character


#include <cstring>#include <iostream>using namespace std;int main() { char name[10]; cout << "Name => "; cin >> name; cout << "Name is [“ << name << "]\n"; cout << “The Length: “ << strlen(name);}

Name => HelloThereHowAreYouName is [HelloThereHowAreYou]Name length is 19

Sample run:

Warning: we have allocated only 10 characters to store the string, where has all the extra characters gone to?

Answer : We have probably corrupted some part of the memory, this is DANGEROUS, so be careful !!!]

More C-String Input and Outputconsider:


#include <iostream>using namespace std;int main () { char name[20]; cin.get(name,10); cout << "[" << name << "]";}

alibabamuthu[alibabamu]

ali[ali]

sample run 2:

sample run 1:

ali baba muthu[ali baba ]

sample run 3:

user press enter, stop

reading because '\n' is

the default delimiter.

9 characters read, including 2 tab '\t' characters.

Input functions : character array input usingInput functions : character array input using cin.get(...)

only 9 character read, NOT 10.

Specifies the maximum number of bytes to be filled into the character array, in the case here, 10 character will be filled, meaning 9 characters will be read (NOT 10) from input stream, the 10th character is null character ("\0').


#include <iostream>using namespace std;int main () { char name[20]; cin.get(name,10,'z'); cout << "[" << name << "]";}

alibabamuthu[alibabamu]

alibabamuthu[alibaba]

sample run 2:

sample run 1:

afizuddin[afi]

sample run 3:

user press enter, '\n' is read as valid input since '\n' is NOT the delimiter.

the character 'z' is used as delimiter the delimiter

(which is 'z' in this case) is encounter, thus stop reading

same as before


#include <iostream>using namespace std;int main () { char name[20]; cin >> name; cout << "[" << name << "]";}

ab cde[ab]

output:

Remember : cin.get(...) read whitespaces as valid input character unless the whitespace is delimiter

#include <iostream>using namespace std;int main () { char name[20]; cin.get(name,10); cout << "[" << name << "]";}

output:

ab cde[ab cde]


#include <iostream>using namespace std;int main () { char name1[10],name2[10],name3[10]; cin.getline(name1,10); cout << "[" << name1 << "]\n"; cin.getline(name2,10); cout << "[" << name2 << "]\n"; cin.getline(name3,10); cout << "[" << name3 << "]\n";}

sample run 1:

Input functions : overcome delimiter problem using cin.getline(...)

mary[mary]peter[peter]john[john]

This solved the delimiter problem!!


#include <iostream>using namespace std;int main () { char name1[10],name2[10],name3[10]; cin.getline(name1,10,'p'); cout << "[" << name1 << "]\n"; cin.getline(name2,10,'j'); cout << "[" << name2 << "]\n"; cin.getline(name3,10,'a'); cout << "[" << name3 << "]\n"; }

sample run 1:marypeter[mary]panjohn[eterpan]carl[ohnc]

Why?


Assigning Characters Into C-Strings#include <cstring>#include <iostream>using namespace std;

int main() { char str[11]; str[0] = 'H'; str[1] = 'I'; str[2] = '\0'; cout << str << endl;

for (int i=0; i<=9; i++) str[i] = 'A' + i; str[i] = '\0'; cout << str << endl;

str[5] = '\0'; for (int i=0; i<=10; i++) cout << "[" << static_cast<int>( str[i] ) << "]"; cout << endl; cout << str << endl;

str[5] = 'A'; cout << str << endl; system("pause");}

HIABCDEFGHIJ[65][66][67][68][69][0][71][72][73][74][0]ABCDEABCDEAGHIJ

Strings are treated just like an array of character values here. The null character is used to indicate the end of the string when processed by the standard library such as puts() function.

output:


WARNING : You can not use arithmetic addition in order to concatenate two C-string. The compiler will give you compile-time errors.

#include <iostream>#include <cstring>using namespace std;int main() { char dest[30] = "Rise of "; char source[] = "The Machine";

dest = dest + source; cout << " dest = " << dest << endl; cout << "source = " << source << endl;}

Computer Programming I 50

Array of C-Strings

char actor[30];

The declaration:declares one string of which the maximum length is 29.A c-string is an one-dimensional array of characters.

How do I declared an array of c-strings?

char actor[5][30];

one solution:This declare an array of 5 strings, of which the maximum length of the string is 30.That means, an array of strings is actually a two-dimensional array of characters.


char actor[4][10] = { "Arnold", "Nick", "Claire", "Kristanna" };

A r n o l d \0

N i c k \0

C l a i r e \0

K r i s t a n n a \0

actor[0]

actor[1]

actor[2]

actor[3]

0 1 2 3 4 5 6 7 8 9


#include <iostream>#include <cstring>using namespace std;int main() { char actor[4][30] = { "Sylvester Stallon", "Nick Stahl", "Claire Danes" }; char anotherActor[30] = "Arnold Schwarzenegger";

cout << anotherActor << endl; cout << actor[0] << endl;

strcpy ( actor[0], anotherActor ); strcpy ( actor[3], "Kristanna Loken" );

for (int i=0; i<=3; i++) cout << i << " " << actor[i] << endl;} Arnold Schwarzenegger

Sylvester Stallon0 Arnold Schwarzenegger1 Nick Stahl2 Claire Danes3 Kristanna Loken

Example:


C-String to Numbers "1234" is a string of characters 1234 is a number When doing numeric input, it is useful to read

input as a string of characters, then convert the string to a number Reading money may involve a dollar sign Reading percentages may involve a percent sign


C-strings to Integers To read an integer as characters

Read input as characters into a C-string, removing unwanted characters

Use the predefined function atoi to convert the C-string to an int value

Example: atoi("1234") returns the integer 1234 atoi("#123") returns 0 because # is not a digit


C-string to long

Larger integers can be converted using the predefined function atol

atol returns a value of type long


C-string to double

C-strings can be converted to type double using the predefined function atof

atof returns a value of type double Example: atof("9.99") returns 9.99

atof("$9.99") returns 0.0 because the $ is not a digit


Library cstdlib The conversion functions

atoi atol

atofare found in the library cstdlib

To use the functions use the include directive

#include <cstdlib>


#include <iostream>#include <cstring>using namespace std;int main() { char str1[] = "1234"; char str2[] = "0123"; char str3[] = " 123"; char str4[] = "$123"; char str5[] = "12.34"; int n1,n2,n3,n4,n5; double d1,d2,d3,d4,d5; long l1,l2,l3,l4,l5; n1 = atoi(str1); n2 = atoi(str2); n3 = atoi(str3); n4 = atoi(str4); n5 = atoi(str5); d1 = atof(str1); d2 = atof(str2); d3 = atof(str3); d4 = atof(str4); d5 = atof(str5); l1 = atol(str1); l2 = atol(str2); l3 = atol(str3); l4 = atol(str4); l5 = atol(str5); cout << n1 << " " << n2 << " " << n3 << " " << n4 << " " << n5 << endl; cout << d1 << " " << d2 << " " << d3 << " " << d4 << " " << d5 << endl; cout << l1 << " " << l2 << " " << l3 << " " << l4 << " " << l5 << endl;}

1234 123 123 0 121234 123 123 0 12.341234 123 123 0 12

output:

C-String conversion to numbers


Predefined Functions

toupper(char_exp) – convert lowercase character to uppercase

tolower(char_exp) – convert uppercase character to lowercase

isupper(char_exp) – tests if character is an uppercase letter

islower(char_exp) – tests if character is a lowercase letter

isalpha(char_exp) – tests if character is alphanumeric

isdigit(char_exp) – tests if character is digit

isspace(char_exp) – tests if character is white-space character


Predefined Functions (cont.)

atoi(str_exp) ingr1 = atoi(str1); Parses string interpreting its content as a number and returns an int value

atof (str_exp) dbl1 = atof(str1); Parses string interpreting its content as a floating point number and returns a value of type double

atol (str_exp) lng1 = atol(str1); Parses string interpreting its content as a number and returns a long value.


The End

computer programming- lecture 5

Education

represent

computer programming

null character

null character

single character

char shortstring

style string

string variable