Computer Science 1620 Strings. Programs are often called upon to store and manipulate text word...

Computer Science 1620

Strings

Programs are often called upon to store and manipulate textword processorsemailchatdatabaseswebpagesetc

In C++, we refer to sequence of characters as strings

we've been using strings since the beginning of the semestercout << "Hello world!" << endl;

String storage in C++ two ways

1) A character array (C-strings)2) The string type

are these the same?similar, except the string type is much more

flexible than the character array

C-Style StringsC Programmers did not have the string type to use a string, they had to use a character

array it is useful to be able to use character arrays

you may not always have the string typeyou will learn to appreciate the string type

How is a C-string stored? as an array of chars

index 0 holds first character index 1 holds second character …

'H' 'e' 'l' 'l' 'o' ' ' 'w' 'o' 'r' 'l' 'd' '!'

x[0] x[1] x[2] x[3] x[4] x[5] x[6] x[7] x[8] x[9] x[10] x[11]

char x[12];x[0] = 'H';x[1] = 'e';x[2] = 'l';x[3] = 'l';x[4] = 'o';x[5] = ' ';x[6] = 'W';x[7] = 'o';x[8] = 'r';x[9] = 'l';x[10] = 'd';x[11] = '!';

How does C/C++ know how big the string is?

Null-termination end-of-string symbolic constant shows program the end of the string corresponds to value 0

not character '0', this is value 48 numeric 0 has no corresponding character counterpart

can be specified in a number of ways '\0' 0

char x[13];x[0] = 'H';x[1] = 'e';x[2] = 'l';x[3] = 'l';x[4] = 'o';x[5] = ' ';x[6] = 'W';x[7] = 'o';x[8] = 'r';x[9] = 'l';x[10] = 'd';x[11] = '!';x[12] = 0;

'H' 'e' 'l' 'l' 'o' ' ' 'w' 'o' 'r' 'l' 'd' '!'

x[0] x[1] x[2] x[3] x[4] x[5] x[6] x[7] x[8] x[9] x[10] x[11]

Array must be big enough to store all characters plus the terminating null character.

Initializing character arrays character arrays can be initialized in the same ways as other

arrays

C++ allows you to initialize character arrays using a string literal

Advantages automatically puts null at the end easier to type

Must be big enough to store literal

char x[13] = {'H','e','l','l','o',' ','W','o','r','l','d','!', '\0'};char y[] = {'K', 'e', 'v', 'i', 'n', '\0'};

char x[13] = "Hello world!";char y[] = "Kevin";

char x[10] = "Hello world!"; // error

String Outputcout is equipped to handle null-terminated

character array

outputs each character until it finds the terminating null

char x[13] = "Hello world!";char y[] = "Kevin";

cout << x << endl;cout << y << endl;

String Inputcin is equipped to handle null-terminated

character arrayprogrammer must "guess" at a good size of

array, to accommodate inputpowers of 2 are common

char x[256];

cin >> x;

cout << x << endl;

Line Inputwhat is the output of the following program?

assume I type "Kevin John Grant" at the prompt

int main() {

char name[256];

cout << "Please enter your full name: ";

cin >> name;

cout << "Name: " << name << endl;

return 0;

cin data tokenized by white space.How do we read in the entire line?

getlineuse the cin.getline function takes (up to) three parameters

1) the character array to read into2) the maximum # of characters you wish to read

typically, the size of the array

3) an optional delimiting (stopping) character if omitted, default is '\n' (newline)

Line Inputwhat is the output of the following program?

assume I type "Kevin John Grant" at the prompt

int main() {

char name[256];

cin.getline(name, 256, '\n');

return 0;

Line Inputprevious program could also have been

written:

int main() {

char name[256];

cin.getline(name, 256);

return 0;

since no third parameter included, assumes '\n'

Advantages of Character Arrayseasy to use

setting/retrieving characters can be done using the array indexing syntax

char str[256]; str[5] = 'K'; cout << str[6];

C++ has some conventions for making strings easier to handle

placing string literals in char arrays handling char array input and output

Disadvantages of Character Arrays1) Static Size

the programmer must decide beforehand how much room to give a character array

this leads to two problems: a) too much space b) too little space

example: write a program that takes in a name of a person, and prints that name to the screen

Example Programhow big do we make our character array?

int main() {

char name[???];

cout << "Please enter your name: ";

cin.getline(name, ???);

cout << "Your name is: " << name << endl;

how about 10?

int main() {

char name[10];

to ensure correctness, better make it 50

int main() {

char name[50];

Here, we are wasting 44 characters.

Disadvantages of Character Arrays2) Difficult to modify

suppose I have a string that contains "Saskatoon"

I want to change that string to "Lethbridge"how do I do it?

Example Programhow do I make the new string hold

"Lethbridge"?

int main() {

char city[50] = "Saskatoon";

Example Programhow do I make the new string hold

"Lethbridge"?

int main() {

char city[50] = "Saskatoon";

city = "Lethbridge"; // will this work?

Example Programwe can only assign a string literal to a

character array upon declaration

int main() {

char city[50] = "Saskatoon"; // this works

city = "Lethbridge"; // this doesn't

The String typea programmer-defined typenot part of the actual C++ syntax

rather, defined in a library (like cmath)must include <string> to use this type

Advantages of String type over char array do not need to declare a size

the string will "resize" itself as necessary you will learn how this is accomplished in future courses

can reassign a string after it’s been declared just like an atomic variable

Example:

#include<iostream>

#include<string>

using namespace std;

int main() {

string city = "Saskatoon";

cout << city << endl;

city = "Lethbridge";

cout << city << endl;

return 0;

String size not specified.

Strings can be assigned new strings.

String Storage the string type stores its characters in an

arrayhowever, it does not store the null-

terminating character rather, it stores the size (# of characters) of

the stringhowever, these details are transparent to

the user

String Operations1) Output

strings can be output using cout the value sent to output will be whatever value is

stored in the string

#include<iostream>

#include<string>

int main() {

string city = "Saskatoon";

cout << city << endl; // outputs Saskatoon

city = "Lethbridge";

cout << city << endl; // outputs Lethbridge

return 0;

String Operations2) Input

strings can be input using cin the value received by the string will be whatever

the user types at the prompt

#include<iostream>

#include<string>

int main() {

string city;

cout << "Where are you from: ";

cin >> city;

cout << "You are from " << city << endl;

return 0;

String Operations 2) Input

the string type also allows line input use the getline function

note: not cin.getline takes three parameters

cin the string variable you are sending the value to an optional delimiting character ('\n' by default)

#include<iostream>

#include<string>

int main() {

string city;

getline(cin, city, '\n');

return 0;

String Operations 2) Input

remember that if you want to read until a carriage return, the '\n' is optional

#include<iostream>

#include<string>

int main() {

string city;

getline(cin, city);

return 0;

String Operations3) Assignment

a value can be assigned to a string using the assignment (=) operator

very flexible in what you can assign to a string variable

a string literal a null-terminated character array a single character another string

#include<iostream>

#include<string>

int main() {

string str;

str = "Hi";

cout << str << endl;

char ca[] = "How are ya?";

str = ca;

str = 'K';

string str2 = "Goodbye";

str = str2;

return 0;

String Operations4) Comparison

we can use a string with one of the relational operators (==, !=, <, <=, >, >=)

compared letter by letter (as discussed previously)

very flexible in what you can compare to a string variable

a string literal a null-terminated character array another string variable

#include<iostream>

#include<string>

int main() {

cout << boolalpha;

string str = "Hi";

cout << (str == "Hi") << endl;

char ca[] = "How are ya?";

cout << (str > ca) << endl;

string str2 = "Hello";

cout << (str != str2) << endl;

return 0;

String Operations5) Character Access

with character arrays (C-style strings), we could get any character we wished using the subscript operatorchar x[] = "Kevin";

cout << x[2] << endl; // outputs v

the string type gives us exactly the same featurestring x = "Kevin";

cout << x[2] << endl; // outputs v

x[0] = 'S';

x[3] = 'e';

cout << x << endl; // outputs Seven

String Operations6) Size

to obtain the size of a string, affix a .length() or .size() to the end of the variable name

these types of function calls (class function calls) will be explained later on in this course

string x = "Kevin";

cout << x.size() << endl; // outputs 5

x = "Lethbridge";

cout << x.length() << endl; // outputs 10

String Operations7) Concatenation

we can concatenate (fuse into one string) two strings using the + operator

the + operator allows several types to be concatenated with a string variable

a string literal a character array a single character another string

#include<iostream>

#include<string>

int main() {

string str;

str = "Hi";

cout << str + " there" << endl;

char ca[] = ", how are ya?";

cout << (str + ca) << endl;

cout << str + '!' << endl;

string str2 = "dden";

cout << (str + str2) << endl;

return 0;

These 6 operations allow some fun stuff with strings1: Write a program that reads a string from

a user, and counts the number of 'e' characters that occur in that input

1. Read in the grades 2. Calculate the average A

3. Add (70 – A) to each grade 4. Re-output the grades#include <iostream>#include <string> using namespace std;

int main() {

Step 1: Read a string from the user

Step 2: Count the number of e's in that string

Step 3: Output the number of e's in that string

return 0;}

int main() {

Step 1: Read a string from the user

return 0;}

int main() {

string input; cout << "Please enter a string: "; getline(cin, input);

return 0;}

int main() {

return 0;}

int main() {

Step 2.1: Begin a count at 0 Step 2.2 For each 'e' in string, increment count

return 0;}

int main() {

Step 2.1: Begin a count at 0 Step 2.2 For each 'e' in string, increment count

return 0;}

int main() {

int count = 0;

Step 2.2 For each 'e' in string, increment count

return 0;}

int main() {

int count = 0;

Step 2.2 For each 'e' in string, increment count

return 0;}

int main() {

int count = 0;

Step 2.2 For each character in stringStep 2.2.1 if the character is an 'e', inc. count

return 0;}

int main() {

int count = 0;

Step 2.2 For each character in stringStep 2.2.1 if the character is an 'e', inc. count

return 0;}

Since strings allow array syntax, we can use our rules from arrays:1) Begin an index at 02) Loop up to but not including size of string3) Increment one at a time4) Process character inside loop

int main() {

int count = 0; for (int i = 0;

Step 2.2.1 if the character is an 'e', inc. count

return 0;}

int main() {

int count = 0; for (int i = 0; i < input.size();

return 0;}

int main() {

int count = 0; for (int i = 0; i < input.size(); i++)

return 0;}

int main() {

int count = 0; for (int i = 0; i < input.size(); i++) {

Step 2.2.1 if the character is an 'e', inc. count }

return 0;}

int main() {

Step 2.2.1 if the character is an 'e' Step 2.2.1.1. increment count }

return 0;}

int main() {

Step 2.2.1 if the character is an 'e' Step 2.2.1.1. increment count }

return 0;}

int main() {

if (input[i] == 'e')

Step 2.2.1.1. increment count }

return 0;}

int main() {

Step 2.2.1.1. increment count }

return 0;}

int main() {

count++; }

return 0;}

int main() {

count++; }

return 0;}

int main() {

count++; }

cout << "Number of e's: " << count << endl; return 0;}

Example 2: Write a function that takes a string as its parameter, and returns the reverse of that string

string reverse(string input) {

Step 1: Create a new empty string

Step 2: Place each character of input into the new string, in reverse order

Step 3: Return the new string

Step 1: Create a new empty string

string result = "";

Step 2: For each character of input (in reverse order) Step 2.1: Place each character into the new string

string result = "";

Step 2: For each character of input (in reverse order) Step 2.1: Place each character into the new string

string result = "";

for (int i = input.size() – 1; i >= 0; i--) Step 2.1: Place each character into the new string

string result = "";

for (int i = input.size() – 1; i >= 0; i--) Step 2.1: Place each character into the new string

string result = "";

for (int i = input.size() – 1; i >= 0; i--) result = result + input[i];

string result = "";

for (int i = input.size() – 1; i >= 0; i--) result += input[i];

string result = "";

return result;

string result = "";

return result;}

int main() { string input; cout << "Please enter a string: "; getline(cin, input); cout << "The reverse of that string is: " << reverse(input) << endl; return 0;

Strings and const parameterssince the string we are sending to reverse is

not changed, we could send it as a reference parameter (so only one copy of the string exists in memory)

string reverse(string &input) {

string result = "";

return result;}

string reverse(const string &input) {

string result = "";

return result;}

By declaring parameter const, we guarantee that function will not change parameter.

Example 3: Write a function called change. Change takes a string s, and two characters, x and y. Change returns s with each occurrence of x changed to a y.Example:

string str = "Hello World"; cout << change(str, 'l', 'r') << endl;

// outputs Herro Worrd

string change(string s, char x, char y) { Step 1: Create a new, empty string

Step 2: Place each character of s into the new string.Step 2.1 If the character is an x, put a y instead

string change(string s, char x, char y) { Step 1: Create a new, empty string

string change(string s, char x, char y) { string result = "";

Step 2: For each character in s Step 2.1 Place character of s into new string, unless character is an x, in which case, put y into the new string

for (int i = 0; i < s.size(); i++) Step 2.1 Place character of s into new string, unless character is an x, in which case, put y into the new string

for (int i = 0; i < s.size(); i++) { Step 2.1 If character is an x Step 2.1.1 Place y in new string Step 2.2 Otherwise Step 2.2 Place character in string }

for (int i = 0; i < s.size(); i++) { if (s[i] == x) Step 2.1.1 Place y in new string Step 2.2 Otherwise Step 2.2 Place character in string }

for (int i = 0; i < s.size(); i++) { if (s[i] == x) result += y; Step 2.2 Otherwise Step 2.2 Place character in string }

for (int i = 0; i < s.size(); i++) { if (s[i] == x) result += y; else Step 2.2 Place character in string }

for (int i = 0; i < s.size(); i++) { if (s[i] == x) result += y; else result += s[i]; }

return result;

return result;}

int main() { string str = "Hello World!"; cout << change(str, 'l', 'r') << endl; return 0;}

String type there are other functions associated with

stringsyour textbook contains examples (find,

substr,swap)http://www.cppreference.com/cppstring/index.ht

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