Strings

Table of contents

1. Table of contents
2. The string class
   1. Definition
   2. Conversion
3. The array representation
4. String methods
   1. Concatenation && Comparison
5. Ctype Library
6. String Stream
   1. Exercice

The string class

Definition

In C++ a string is a colleciton of characters. In order to use this class, we must introduce its header <string>.

#include<string>
...
string s = "hello"

• A literal string is group of characters enclosed between ”“.

• We cannot use the single character delimiter ` ` as it’s reserved for a single character.
  • "This is a string"
  • 'This will lead to an error'

• In the API level, strings in C++ have the same functionality as in Python or Java.

• One major difference with those languages, is that in C++, string a mutables. Which means that we could change their values.
• Another difference, is that C++ has two types of strings.
  1. Those compatible with the C language.
  2. Those OOP oriented type which uses classes.

Conversion

The class method to_string is very useful to convert number to their string equivalent.

#include <iostream>
#include <string>
 
int main() 
{
    double f = 23.43;
    double f2 = 1e-9;
    double f3 = 1e40;
    double f4 = 1e-40;
    double f5 = 123456789;
    std::string f_str = std::to_string(f);
    std::string f_str2 = std::to_string(f2); // Note: returns "0.000000"
    std::string f_str3 = std::to_string(f3); // Note: Does not return "1e+40".
    std::string f_str4 = std::to_string(f4); // Note: returns "0.000000"
    std::string f_str5 = std::to_string(f5);
    std::cout << "std::cout: " << f << '\n'
              << "to_string: " << f_str  << "\n\n"
              << "std::cout: " << f2 << '\n'
              << "to_string: " << f_str2 << "\n\n"
              << "std::cout: " << f3 << '\n'
              << "to_string: " << f_str3 << "\n\n"
              << "std::cout: " << f4 << '\n'
              << "to_string: " << f_str4 << "\n\n"
              << "std::cout: " << f5 << '\n'
              << "to_string: " << f_str5 << '\n';
}

If we want more control on the number of floating digits, we use a more advanced method sprintf (which stand string print formatted).

It syntax is given as follows:

std::sprintf(buffer, "fmt", value);

• fmt: where is desired format (like you learn in C).
• value: the value that we want to convert.
• buffer: The string called also buffer to store the result.

Also for the inverse conversion (string to number), we use the following functions:

• stoi: to get an integer. en entier.
• stol: to get a long
• stof: to get a float
• stod: to get a double.

The array representation

A String is represented in the memory as an array of characters char. Each one of those characters could be obtained using the [] operator.

string S = "Fear the Tree"

In order to get the ih character we use

• S[i] : not safe
• S.at(i) : check memory (safer but slower).

char ch1 = S[3];  //'r'
char ch2 = S.at(2); //'a'

To illustrate the difference between those two methods, let’s consider the following calls.

char ch1 = S[13];  // Runtime error, nothing is signaled in complied time (with default flags)
char ch2 = S.at(13);  //Will throw an exception

• In order to loop between the elements of a string, you have the following loops:

//Index based
for(size_t i = 0; i<chaine.size(); i++)
    cout<<chaine[i];
cout<<endl;

//foreach
for(auto ch: chaine)
    cout<<ch;
cout<<endl;

• The characters are encoded using the ASCII codes as showed int the following figure:

The Cesar encoding is a method in cryptography which encodes a character by translating it by integer k. Hence for example if we consider the example of the letter `A` and the integer k=3. The code of 'A' will be 'D', the code of 'B' will be 'E' and so forth.

• This representation will allow us to use mathematical operations such as the Cesar encoding to manipulate the characters.

Exercise: Write a program to encode the string “ATTACKATDOWN” with a Cesar encoding with size k=5.

#include <iostream>
#include <string>

using namespace std;

int main(int argc, char *argv[])
{
    string message = "ATTACK AT DAWN";
    int key = 5;
    string encoded = "";  //Encoded message

    for(auto ch: message)
    {
        //Only encode letters 
       if( ch >= 'A' && ch<='Z')        
       {
        //Formula change
        char  dis= ( ch - 'A' + key) % 26;
        encoded += 'A' + dis;
       }
       else
        encoded += ch;
    }
    cout<<encoded<<endl;
    return 0;
}

Input: ATTACK AT DAWN
Output: FYYFHP FY IFBS

String methods

In this section, we will present the available methods for the string object.

Concatenation && Comparison

• Like Java and Python, we could concatenate two strings using the += operator.

string s1 = "Programming in "
string s2 = s1 + " CPP";  //s2 = Programming in CPP
s1 += "?";      //Add the string "?" 

• Unlike Java, we could directly compare strings using the == operator.

string s3 = "Zebra";
if ( s3 == "Tiger" )
    cout<<"Different"<<endl;

if (s3 < "Tiger")
    cout<<"Inferior"<<endl;

• Also those strings are mutables.

s3.append("Giraffe"); // s2 now is  "ZebraGiraffe"
s3.erase(4,3); // erase three characters from  4,
s3[5] = 'i'; // change a single character
s3[9] = 'e'; //Danger

• Here is a list of useful methods:

  • s.append(str): Add a string at the end.

  • s.compare(str): Like Java, this is a comparator which return \([-1,0,1]\) according the comparison result.

  • s.erase(index, lenght): delete [length] character starting from [index].
  • s.find(str): Search the string [str] if it exists ottherwise npos.
  • s.insert(index, str): insert the string [str] at the position [index].
  • s.size(): return the size of the string.
  • s.substr(start, length): return the string with lenght with [length] starting from start.

Ctype Library

The library ctype offers a set of useful function to verify the nature of character.

• #include<ctype>

  • isalnum: return true if the character is alphabetic or numeric.
  • isalpha: return true is the character is alphabetic.
  • islower: lower case letter.
  • isupper: upper case letter.
  • isdigit: verify if it is a digit..
  • isxdigit: is an hexadecimal digit.
  • iscntrl: is control character.
  • isspace: for the space character.
  • tolower: to convert a letter (not a string) into a lower case.
  • toupper: Same but to upper case.

• Example:

string mixed = "ab80c3d27"
cout<<"digits  "<<mixed<<": "<<endl;

for(auto ch: mixed)
    if( isdigit(ch) )
        cout<<ch<<endl;

//Convert all in upper case
string s = "my string";
for(auto & ch: s)
    ch = toupper(ch)
cout<<"Now "<<s<<endl;

String Stream

The String stream is a useful tool to manipulate streams using the power of streams. For example if we want to concatenate string, the += operator will use a quadratic time (more to know in coming chapters) to reallocate the memory. A better solution is to use stringstream with the stream syntax using the « and » operator to manipulate data.

• operator«: to get (read) values into the stream.
• operator»: to extract values from the stream.
• str: to get the current string.
• clear: to clear (delete) the content of the string.

To illustrate the use of those streams, let’s suppose we want to process the data of a student ("Adil", "Fes", 23) and we want to create a string in the form name [state: Fes] [age: 23] to represent this student. We could achieve this using stream:

Stduent S{"Adil", "Fes", 23};

//Construct the stream
stringstream SS;

//Creating the string using what you learn for << and >>
ss<<S.name<<" [state: "<<S.state<<"] [age: "<<S.age<<"]";

//Printing the stream
cout<<ss.str()<<endl;

We could also use them as input stream (like cin)

string str("Hello from the dark side");

Let suppose we want to get all the words of this string ( difficult exercise just just the basic methods).

// Create a stream with str as string
stringstream SS(str);

//string to get the words
string word;

//Loop to read all the words
while( SS>>word)
{
    cout<<word<<endl;
}

Exercice

Write a function that receives a string that contains of numbers separated by comma, and print those number (each number in its line.