CSc 17

1
CSc 17

• Chapter 10 - Strings

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Constants in Classes

• Before we begin our discussion on strings, one
  note related to defining constants in classes.
• You cannot define a constant using the style
  we've seen so far
• const MAX_NUMS 5

3
Constants in Classes
class MyClass public const MAX_NUMS 5
// illegal!!! int numsMAX_NUMS
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Constants in Classes

• There are two ways around this

class MyClass public static const MAX_NUMS
5 // ok int numsMAX_NUMS
class MyClass public enum MAX_NUMS 5
// ok int numsMAX_NUMS
5
Strings

• We will use two types of strings in this course
• C-style strings
• C-style strings

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C-style Strings ("C Strings")

• just an array of characters
• char filename128
• a special character is used to mark the end of
  the string (a "sentinel") this char takes up one
  of the array entries this means you can only
  store n-1 characters in a C string of dimension n
• This special character is called the null
  character, has an ASCII code of 0 and is written
  as '\0' (two chars to represent one!)

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C String Example

• char filename10
• 'N','U','M','S','.','T','X','T'
• N U M S . T X T ? ?

0 1 2 3 4 5 6 7 8 9
C does not guarantee that it will "pad" the
remainder of the string with null characters
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A Better Way to Initialize

• char filename10 "NUMS.TXT"
• N U M S . T X T \0 \0

0 1 2 3 4 5 6 7 8 9
C will "pad" the remainder of the string with
null characters
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C Strings

• You can leave off the dimension when using an
  initialization
• char filename "NUMS.TXT"
• C will allocate one more than the number of
  chars (to accommodate the null character). So the
  above example is equivalent to
• char filename9 "NUMS.TXT"

10
Copying C Strings

• C strings aren't simple variables like integers
  and doubles. You can't assign one string to
  another with
• char st110 "hello"
• char st210
• st2 st1 // illegal!

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Copying C Strings

• You can write a function to copy one string to
  another
• void strCopy(char destStr, char srcStr)
• int i0
• do
• destStri srcStri
• i
• while (srcStri ! '\0')
• char st110 "hello"
• char st210
• strCopy(st2, st1) // copy st1 into st2

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Copying C Strings

• Actually, there's a C string library that
  contains such a function. It's called strcpy
• To use this function, include the string library
• include ltstring.hgt
• // newer compilers can do
• // include ltcstringgt

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Testing Equality with C Strings

• As with copying one string to another, you can't
  use for comparing one string to another. You
  can do it syntactically, but you get unexpected
  results
• char st110 "hello"
• char st210 "hello"
• if (st1 st2)
• cout ltlt "EQUAL" ltlt endl
• else
• cout ltlt "NOT EQUAL" ltlt endl

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Testing Equality with C Strings

• To test equality with C strings, use the strcmp
  function from the string library
• strcmp(str1, str2)
• Returns -1 if str1 lt str2
• Returns 0 if str1 str2
• Returns 1 if str1 gt str2

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Other String Library Functions

• strcat(destStr, srcStr)
• strcat will concatenate srcStr at the end of
  destStr
• char st120 "cat"
• char st210 "in the"
• char st310 "hat"
• strcat(st1,st2)
• strcat(st1,st3)
• cout ltlt st1 ltlt endl

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Other String Library Functions

• strlen(str)
• strlen returns the length of the string str
• (loops until it finds the null character)

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Reading an Entire Line

• input streams have a member function called
  getline that will read in characters until the
  newline is found or a specified number of
  characters is read (whichever comes first)
• Example cin.getline(str, 10)

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Converting Numeric Strings

• int n atoi(str)
• converts an integer in C string form to numeric
  integer form (include stdlib.h)
• int n atoi("205") // returns 205
• int n atoi("20xy5") // returns 20
• int n atoi("xy205") // returns 0

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Converting Numeric Strings

• long n atol(str)
• converts a long in C string form to numeric long
  form

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Converting Numeric Strings

• double x atof(str)
• converts a double in C string form to numeric
  double form
• double x atof("25") // returns 25.0
• double x atof("20.1.3") // 20.1
• double x atof("20.1eee3") // 20.1
• double x atof("20.1e3") // 20100.0

21
Programming

• Write a function palindrome which takes one
  argument (a C string) and returns true if the
  string is a palindrome, false otherwise
• Write a program to input integers from the
  keyboard stopping when a blank line is entered
  (use getline and atoi). Output the number of
  lines entered as well as the sum of the numbers
  entered.