Erle Robotics C++ GitBook

Strings basics

String literals such as "Hello, world!" are actually represented by C++ as a sequence of characters in memory. In other words, a string is simply a character array and can be manipulated as such. Consider the following program:

#include <iostream>
using namespace std;

int main() {

  char helloworld[] = { 'H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd', '!', '\0' };
  cout << helloworld << endl;
  return 0;

}

This program prints Hello, world!. Note that the character array helloworld ends with a special character known as the null character. This character is used to indicate the end of the string.

Character arrays can also be initialized using string literals. In this case, no null character is needed, as the compiler will automatically insert one:

char helloworld[] = "Hello, world!";

The individual characters in a string can be manipulated either directly by the programmer or by using special functions provided by the C/C++ libraries. These can be included in a program through the use of the #includedirective. Of particular note are the following:

• cctype (ctype.h): character handling
• cstdio (stdio.h): input/output operations
• cstdlib (stdlib.h): general utilities
• cstring (string.h): string manipulation

Here is an example to illustrate the cctype library:

#include <iostream>
#include <cctype>
using namespace std;

int main() {

  char messyString[] = "t6H0I9s6.iS.999a9.STRING";
  char current = messyString[0];

  for(int i = 0; current != '\0'; current = messyString[++i]) {

    if(isalpha(current))

      cout << (char)(isupper(current) ? tolower(current) : current);

    else if(ispunct(current))

      cout << ' ';

  }
  cout << endl;

  return 0;

 }

This example uses the isalpha, isupper, ispunct, and tolower functions from the cctype library. The is- functions check whether a given character is an alphabetic character, an uppercase letter, or a punctuation character, respectively. These functions return a Boolean value of either true or false. The tolower function converts a given character to lowercase.

The for loop beginning at line 9 takes each successive character from messyString until it reaches the null character. On each iteration, if the current character is alphabetic and uppercase, it is converted to lowercase and then displayed. If it is already lowercase it is simply displayed. If the character is a punctuation mark, a space is displayed. All other characters are ignored. The resulting output is this is a string. For now, ignore the (char) on line 11; we will cover later.

Here is an example to illustrate the cstring library:

#include <iostream>
#include <cstring>
using namespace std;

int main() {

  char fragment1[] = "I'm a s";

  char fragment2[] = "tring!";

  char fragment3[20];

  char finalString[20] = "";

  strcpy(fragment3, fragment1);

  strcat(finalString, fragment3);

  strcat(finalString, fragment2);

  cout << finalString;

  return 0;

}

This example creates and initializes two strings, fragment1 and fragment2. fragment3 is declared but not initialized. finalString is partially initialized (with just the null character). fragment1 is copied into fragment3 using strcpy, in effect initializing fragment3 to "I'm a s". strcat is then used to concatenate fragment3 onto finalString (the function overwrites the existing null character), thereby giving finalString the same contents as fragment3. Then strcat is used again to concatenate fragment2 onto finalString. finalString is displayed, giving "I'm a string!".