C++ : An Introduction Lecture 11: Namespaces; Strings again - - PowerPoint PPT Presentation

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C++ : An Introduction Lecture 11: Namespaces; Strings again - - PowerPoint PPT Presentation

Dec 28, 2022 25 likes •262 views

C++ : An Introduction Lecture 11: Namespaces; Strings again Introducing Namespaces Namespaces are a relatively new C++ feature What problem do namespaces solve? What if you buy two different general-purpose class libraries from two

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SLIDE 1

C++ : An Introduction

Lecture 11: Namespaces; Strings again

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SLIDE 2

Introducing Namespaces

  • Namespaces are a relatively new C++ feature
  • What problem do namespaces solve?

What if you buy two different general-purpose class libraries from two different vendors #include "vendor1.h" #include "vendor2.h" and then it turns out that the headers have this in them: // vendor1.h class String { ... }; // vendor2.h struct String { ... }; Error at Compile time!!!

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SLIDE 3

Namespaces

  • Namespace defines scope

– Place identifiers and variables within namespace – Access with namespace_name::member – Guaranteed to be unique namespace Name { contents } – Unnamed namespaces are global -- need no qualification – Namespaces can be nested

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SLIDE 4

Using namespaces

  • using statement

using namespace namespace_name; – Members of that namespace can be used without preceding namespace_name:: – Can also be used with individual member – Examples

using namespace std; Discouraged by some programmers, because includes entire contents of std using std::cout; Can write cout instead of std::cout

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SLIDE 5

Vendor problem solved

  • // vendor1.h

namespace Vendor1 { class String { ... }; }

  • // vendor2.h

namespace Vendor2 { struct String { ... }; }

  • There are no longer two declarations

for String

  • Instead declarations exist for

Vendor1::String and Vendor2::String.

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SLIDE 6

Vendor problem solved 2

  • What happens if you say:

using namespace Vendor1; using namespace Vendor2;

  • No Problem!

String s1

ERROR

Vendor1::String s1; Vendor2::String s2;

OK

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SLIDE 7

1 #include <iostream> 2 using namespace std; // use std namespace 3 4 int integer1 = 98; // global variable 5 6 // create namespace Example 7 namespace Example 8 { // declare two constants and one variable 9 const double PI = 3.14159; 10 const double E = 2.71828; 11 int integer1 = 8; 12 13 void printValues(); // prototype 14 15 namespace Inner // nested namespace 16 { enum Years { FISCAL1 = 1990, FISCAL2, FISCAL3 }; 17 } // end Inner 18 } // end Example

This integer1, different from the global integer1. This includes all of std, allowing us to use cout and endl.

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SLIDE 8

19 namespace // unnamed namespace -- vars global 20 { double doubleInUnnamed = 88.22; // declare variable 21 } // end namespace 22 23 int main() 24 { // output value doubleInUnnamed of unnamed namespace 25 cout << "doubleInUnnamed = " << doubleInUnnamed; 26 // output global variable 27 cout << "\n(global) integer1 = " << integer1; 28 // output values of Example namespace 29 cout << "\nPI = " << Example::PI << "\nE = " 30 << Example::E << "\ninteger1 = " 31 << Example::integer1 << "\nFISCAL3 = " 32 << Example::Inner::FISCAL3 << endl; 33 Example::printValues(); 34 } // end main

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SLIDE 9

More String Handling

  • Strings in C++ are arrays of characters
  • Are terminated by a NUL (\0)

character

  • The string “hello” is really

‘h’, ‘e’, ‘l’, ‘l’, ‘o’, ‘\0’ = 6 bytes

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SLIDE 10

Primitive Strings – String Literal

char *str = "Hello"; // static string literal for (int i = 0; str[i] != '\0'; i++) cout << str[i];

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SLIDE 11

Primitive Strings – String Buffers char *str = new char[255]; cin >> str; // reads till a whitespace cout << str;

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SLIDE 12

Automatic Type Conversion

Can do arithmetic operations on characters according to the ASCII table cout << (char) ('d' + 3) << “\n”; cout << (‘d’ + 3) << “\n”;

g 103

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SLIDE 13

ASCII

  • The ASCII character set is limited to

256 characters due to using only 1 byte

  • Other character sets exist that use

more than one byte – Unicode, Kanji

  • Due to ASCII being represented by an

integer this is legal

if (ch >= ‘a’ && ch <= ‘z’) …

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SLIDE 14

The cstring Library

  • Provides string processing functions (inherited

from C)

  • Some functions in the library

int strlen(const char *s); char *strcpy(char *dest, const char *src); char *strcat(char *dest, const char *src); int strcmp(const char *s1, const char *s2); char *strchr(const char *s1, int c); char *strstr(const char *s1, const char *s2);

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SLIDE 15

Example Implementations

int strlen (const char *s) { int n = 0; while (*s++) n++; return n; }

While the char pointed to by s is not the terminating NUL (and increment s anyway), then …

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SLIDE 16

Example Implementations

char *strchr(const char *s, int c) { while (1) { if (*s == c) return (char *) s; if (*s++ == ‘\0’) return NULL; } }

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SLIDE 17

Example Implementations

char *strcpy(char *dest, const char *src) { char *p = dest; while(*p++ = *src++) ; //empty loop return (dest); }

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SLIDE 18

Example Implementations

char *strcat(char *dest, const char *src) { char *p = strchr(dest, 0); strcpy(p, src); return (dest); }

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SLIDE 19

Is this valid?

int main() { char *str = "Hello World!"; char s[100]; char *p; p = strcat(str, " How are you today?"); cout << str << '\n'; cout << p << '\n'; }

NO! str is a static string

strcpy(s, str); p=strcat(s, “How are you today?”);

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SLIDE 20

Exercise

  • Explain the differences between the three

comparisons below:

char *p = "help"; char *q = "hello"; if (strcmp(p, q) >= 0) … if (*p >= *q) … if (p >= q) …

String p is greater or equal to string q Dereference p and q and compare first characters Compare the memory addresses

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SLIDE 21

Exercise 8.1

Children often code messages by substituting letters by others further in the

  • alphabet. For example, substituting letters

3 places further translates “hello yacht” into the secret message “khoor bdfkw”.

1. Write a function which takes a string, and puts its encrypted version into a second array parameter. 2. Can you write your decrypt method so that the same method can be used for encryption and decryption?

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Exercise 8.1 Solution

#include <iostream.h> #include <ctype.h> int str_offset = 3; void crypt(char *src, char *dst) { for (int i = 0; src[i] != '\0'; i++) { if (tolower(src[i]) >= 'a' && tolower(src[i]) <= 'z') { dst[i] = tolower(src[i]) + str_offset; if (dst[i] > 'z') dst[i] = 'a' + dst[i] - 'z' - 1; if (dst[i] < 'a') dst[i] = 'z' - 'a' + dst[i] + 1; } else { dst[i] = src[i]; } dst[i] = '\0'; }

void crypt(char *src, char*dst, const int offset) ;

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SLIDE 23

Exercise 8.1 Solution

void main() { char *str = "Hello, Zet"; char *enc = new char[sizeof(str)]; char *dec = new char[sizeof(str)]; crypt(str, enc); cout << "The original string contained: " << str << '\n'; cout << "The encrypted string contains: " << enc << '\n'; str_offset = -3; crypt(enc, dec); cout << "The decrypted string contains: " << dec << '\n'; delete []enc; delete []dec; }