Computer Programming- Lecture 5

Lecture 5
C-Strings

Computer Programming I 1

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];
0 1 2 3 4

item
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:
s[0] s[1] s[2] s[3] s[4] s[5] s[6] s[7] s[8] s[9]

H i M o m ! 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 A C-style string in C++ is
The declaration: basically an array of
characters, terminated by a
char name[10]="Hello";
null character ('0')

is equivalent When we count the
length of the string,
char name[10]={'H','e','l','l','o','0'}; we do not count the
terminating null
character.

H e l l o 0

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


The declaration:

If the size of the array is not
char name[]="Hello"; specified, then the array
size would be equal to the
is equivalent length of the string plus 1

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

H e l l o 0

(the size of the array is 6, the length of the string is 5)


The declaration: The declaration:

char name[6]="H"; char name[]="H";

is equivalent is equivalent

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

H 0 H 0
(the size of the array is 6, (the size of the array is 2,
But the length of the string is 1) But the length of the string is 1)


Null string
The declaration: (empty string) The declaration:

char name[6]=""; char name[]="";

is equivalent is equivalent

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

0 0
(the size of the array is 1, (the size of the array is 1,
the length of the string is 0) the length of the string is 0)


The declaration:

char name[]="H";
H 0
(the size of the array is 2,
is NOT equivalent the length of the string is 1)

char name='H'; H
(this is not an array nor a string,
it is simply a character)


char name[11]="John Connor";

J o h n C o n n o r

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

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> strcpy()
using namespace std;
int main() {
char source[] = "Judgement Day"; Copy one string into
char dest[] = "Rise of the Machine"; another. Copy string
strncpy(dest,source);
source to dest,
cout << "source = " << source << endl;
cout << " dest = " << dest << endl; stopping after the
} terminating null
character has been
moved.
source = Judgement Day
dest = Judgement Day

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() { This is an initialization, it
char dest[20] = "Judgement Day";
}
is valid.

int main() { int main() {
char dest[20]; char dest[20];
dest = "Judgement Day"; strcpy(dest,"Judgement Day");
} }
This is an assignment, it is
NOT valid. Use strcpy to assign a c-string
to another c-string

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


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> strcmp()
using namespace std; Compare one string to another starting
int main() { with the first character in each string and
char string1[] = "Hello"; continuing with subsequent characters
char string2[] = "Hello World"; until the corresponding characters differ
char string3[] = "Hello World"; or until the end of the strings is reached.
char string4[] = "Aloha";
int n;

n = strcmp(string1, string2);
cout << string1 << "tt" << string2 << "t==> " << n << endl;
Hello Hello World ==> -1
cout << string2 << 't' << string3 << "t==> " << n << endl;
Hello World Hello World ==> 0
cout << string3 << 't' << string4 << "tt==> " << n << endl;
}
Hello World Aloha ==> 1

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

if ( string1==string2 )
cout << "Same"; output:
else Different
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.


#include <iostream>
#include <cstring>
int main() {

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

Use strcmp() function instead for string comparison.


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>
strcat()
int main() {
char dest[30] = "Rise of "; Appends one string to
char source[] = "The Machine"; another. strcat()
appends a copy of
strcat( dest, source ); source to the end of
cout << " dest = " << dest << endl; dest. The length of
cout << "source = " << source << endl; the resulting string is
}
strlen(dest) +
dest = Rise of The Machine strlen(source).
source = The Machine

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";

s1 s2
H e l l o 0 W o r l d 0

strcat(s1,s2);

s1 s2
H e l l o W o r l d 0 W o r l d 0


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
must #include this for using
Consider this program : strlen() function

#include <cstring> Declaring name to be an
#include <iostream> array of characters

int main()
{ To treat the array of characters as C-
char name[10]; string, we just use the name of the
cout << "Name => ";
array when dealing with I/O stream.
cin >> name;

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

cout << "Name length is " << strlen(name);
}
Standard function to return the
length of the string


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

Sample run 1: Sample run 2:
Name => Hello there
Name => hello
Name is [Hello]
Name is [hello]
Name length is 5
Name length is 5
[remark: the cin will stop
[remark: no problem] 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
line
including spaces into a single C-string
variable
 char a[80];
cout << "Enter input:n";
cin.getline(a, 80);
cout << a << End Of Outputn";
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


More C-String Input and Output
Sample run:
consider:
Name => HelloThereHowAreYou
#include <cstring> Name is [HelloThereHowAreYou]
#include <iostream> Name length is 19
int main() {
char name[10];
cout << "Name => ";
cin >> name;
cout << "Name is [“ << name << "]n";
cout << “The Length: “ << strlen(name);
}

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 !!!]

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

#include <iostream> sample run 1: only 9
using namespace std; character
alibabamuthu
int main () { [alibabamu] read, NOT 10.
char name[20]; user press
cin.get(name,10); sample run 2: enter, stop
ali reading
cout << "[" << name << "]"; because 'n' is
[ali]
} the default
delimiter.

Specifies the maximum number of
bytes to be filled into the character 9 characters read,
array, in the case here, 10 character including 2 tab 't'
characters.
will be filled, meaning 9 characters sample run 3:
will be read (NOT 10) from input
ali baba muthu
stream, the 10th character is null
[ali baba ]
character ("0').


#include <iostream> sample run 1:
using namespace std; same as
alibabamuthu before
int main () { [alibabamu]
char name[20];
cin.get(name,10,'z'); sample run 2: user press
enter, 'n'
cout << "[" << name << "]"; ali
is read as
baba
} valid input
muthu
since 'n' is
[ali
NOT the
baba
delimiter.
]
the character 'z' is used
as delimiter the delimiter
sample run 3: (which is 'z' in this
afizuddin case) is encounter,
[afi] thus stop reading


#include <iostream>
int main () { output:
char name[20]; ab cde
cin >> name; [ab]
cout << "[" << name << "]";
}

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

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


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

#include <iostream>
using namespace std; sample run 1:
int main () { mary
[mary] This
char name1[10],name2[10],name3[10]; peter solved the
cin.getline(name1,10); [peter] delimiter
john problem!!
cout << "[" << name1 << "]n"; [john]
cin.getline(name2,10);
cout << "[" << name2 << "]n";
cin.getline(name3,10);
cout << "[" << name3 << "]n";
}


#include <iostream>
using namespace std; sample run 1:
int main () { marypeter
[mary]
char name1[10],name2[10],name3[10]; pan
cin.getline(name1,10,'p'); john
[eter
cout << "[" << name1 << "]n"; pan
cin.getline(name2,10,'j'); ]
cout << "[" << name2 << "]n"; carl
[ohn
cin.getline(name3,10,'a'); c]
cout << "[" << name3 << "]n";
} Why?


Assigning Characters Into C-Strings
#include <cstring>
#include <iostream> Strings are treated just
like an array of
int main() { character values here.
char str[11]; The null character is
str[0] = 'H'; str[1] = 'I'; str[2] = '0'; used to indicate the
cout << str << endl;
end of the string when
for (int i=0; i<=9; i++) str[i] = 'A' + i; processed by the
str[i] = '0'; standard library such
as puts() function.
str[5] = '0';
for (int i=0; i<=10; i++)
cout << "[" << static_cast<int>( str[i] ) << "]";
cout << endl; output:
HI
ABCDEFGHIJ
str[5] = 'A';
cout << str << endl; [65][66][67][68][69][0][71][72][73][74][0]
system("pause"); ABCDE
} ABCDEAGHIJ


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

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

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


Array of C-Strings
The declaration:
declares one string of which the maximum
length is 29.
char actor[30];
A c-string is an one-dimensional array of
characters.

How do I declared an array of c-strings?

one solution:
This declare an array of 5 strings, of which
char actor[5][30];
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" };

0 1 2 3 4 5 6 7 8 9
actor[0] A r n o l d 0
actor[1] N i c k 0
actor[2] C l a i r e 0
actor[3] K r i s t a n n a 0


Example:
#include <iostream>
#include <cstring>
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 Stallon
0 Arnold Schwarzenegger
1 Nick Stahl
2 Claire Danes
Computer Programming I 3 Kristanna Loken 52

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
atof
are found in the library cstdlib
 To use the functions use the include
directive

#include <cstdlib>


C-String conversion to numbers
#include <iostream>
#include <cstring>
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 12
output: 1234 123 123 0 12.34
1234 123 123 0 12


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

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