Strings in C++

Character Arrays

Applying the first approach:

#include <iostream>
using namespace std;

void printString(char arr[], int n);
int main() {
	char greet[] = "Hello world!";
	cout << greet << endl;

	return 0;
}

	Hello world!

Notice the use of square bracket syntax to denote an array of char. An important point to notice about character arrays is their size. Recalling the fact that a char type takes 1 byte, consider this output:

#include <iostream>
using namespace std;

void printString(char arr[], int n);
int main() {
	char greet[] = "Hello world!";
	int sizeOf_greet = sizeof(greet);
	cout << "size of greet = " << sizeOf_greet << " bytes" << endl;

	return 0;
}

size of greet = 13 bytes

The array greet consists of 12 elements (there are 12 characters in “Hello world!”). Why are we seeing 13 bytes as the size of greet? Because every character array, denoted as a string, takes one extra byte, reserved for \0. This value, \0, is called the null byte__, or more specifically, the __string delimiter. It is the byte at the very end of the array, placed there to indicate the end of an array of characters, a string.¹

Reading & Writing Character Arrays.

We can read and write char arrays with cin and cout. The catch, however, is that we must indicate the size of the character array. This can lead to wasted space, but we will address this problem later.

#include <iostream>
using namespace std;

void printString(char arr[], int n);
int main() {
	char name[20];
	cout << "What's your name?" << endl;
	cin >> name;
	cout << "Hello, " << name << "." << endl;

	return 0;
}

What's your name?
	Dorian
	Hello, Dorian.

The problem with the approach above, however, is that it will not read anything after a whitespace:

#include <iostream>
using namespace std;

void printString(char arr[], int n);
int main() {
	char name[20];
	cout << "What's your name?" << endl;
	cin >> name;
	cout << "Hello, " << name << "." << endl;

	return 0;
}

What's your name?
	Dorian Gray
	Hello, Dorian.

To fix this, we need to use cin.get:

#include <iostream>
using namespace std;

void printString(char arr[], int n);
int main() {
	char name[20];
	cout << "What's your name?" << endl;
	cin.get(name, 20);
	cout << "Hello, " << name << "." << endl;

	return 0;
}

What's your name?
Dorian Gray
Hello, Dorian Gray.

Notice the syntax for cin.get(). We must pass into it the variable we want to store the input in, and the number of characters to be read. With cin.get(), if we pass in 20 for the number of characters, the number of characters to be read is actually 19, with 1 for the null byte.

If we want multiple string inputs from the user, we use cin.getline(). This is a separate function for reading strings because cin.get() does not differentiate between different enter key inputs. If we do want to use cin.get(), we should separate the different cin.get() calls with a cin.ignore(). Of course, it is much easier to just use cin.getline() whenever we seek multiple inputs from the user.

Functions on Strings

C++ provides several string functions through string.h, a header file, or alternatively, through cstring. These are both libraries providing various operations we can perform on strings.

#include <iostream>
using namespace std;

int main() {
	char greet[] = "Foo bar baz bang";
	int sizeOfGreet = sizeof(greet) - 1;
	cout << "size of greet = " << sizeOfGreet << endl;
	return 0;
}

	size of greet = 16

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

int main() {
	char greet[] = "Foo bar baz bang";
	int sizeOfGreet = strlen(greet);
	cout << "size of greet = " << sizeOfGreet << endl;
	return 0;
}

size of greet = 16

Concatenating Strings

When we concatenate two strings, we merge the strings together. We can concatenate strings with the strcat() method:

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

int main() {
	char foo[] = "John ";
	char bar[] = "Kim";
	strcat(foo, bar);
	cout << foo << endl;
	return 0;
}

John Kim

The syntax for strcat() is as follows:

strcat(destination, source)

Where destination is the string to be merged on to (here foo), and source is the string to merge (here bar).

To understand how concatenation works, examine the array of characters:

array1 = ['J', 'o', 'h', 'n', ' ', '\0'];
array2 = ['K', 'i', 'm', '\0'];

When we concatenate Kim onto John, we take the first character of Kim — K — and use it to replace the null byte of John. If we want to concatenate only some characters in the source string, we use strncat():

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

int main() {
	char x[] = "Big ";
	char y[] = "Apple";
	strncat(x, y, 3);
	cout << x << endl;

	return 0;
}

Big App

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

int main() {
	char x[] = "Hello";
	int sizeOfx = sizeof(x);
	char z[sizeOfx];
	char y[] = " world!";
	strcpy(z, x);
	strcat(z, y);
	cout << "x: " << x << endl;
	cout << "z: " << z << endl;

	return 0;
}

x: Hello
z: Hello world!

Substrings

Another useful feature to have when working with strings is determining whether a given string is a substring of another string. For example, the string “corn” is a substring of the string “acorn.” We can make this determination with strstr(). The general syntax:

strstr(main, substring)

In the syntax above, main is the existing string, and substring is the substring we want to check for. An example:

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

int main() {
	char x[] = "Apple pie";
	bool result = strstr(x, "Apple");
	cout << result << endl;

	return 0;
}

	1

We get back 1, indicating "Apple" is a substring of the string "Apple pie". Remember that upper and lowercase characters are different. Checking for the substring "apple" will return 0.

If we want to find an instance of a char, we use strchar(). The general syntax:

strchr(string, char)

The function strchr() will search for a given character starting from the left. If we want to find a given character from the right, we use strrchar().

Comparing Strings

In some languages, we can compare strings with the < or > operators. These operations output which of the given strings appears first in the alphabet. C++ provides a similar function, strcmp().² The general syntax:

strcmp(s1, s2)

Where s1 and s2 are the strings to be compared. For example:

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

int main() {
	char apple[] = "apple";
	char kiwi[] = "kiwi";
	char peach[] = "peach";

	int result1 = strcmp(apple, kiwi);
	int result2 = strcmp(peach, apple);
	int result3 = strcmp(peach, peach);

	cout << "result1 = " << result1 << endl;
	cout << "result2 = " << result2 << endl;
	cout << "result3 = " << result3 << endl;

	return 0;
}

result1 = -10
result2 = 15
result3 = 0

Let's examine this output. First, strcmp(s1, s2) returns either a negative, 0, or positive value. If ${s_1}$ comes before ${s_2}$ in the alphabet, the value returned is negative. If ${s_1}$ comes after ${s_2}$ in the alphabet, the value returns positive. Finally, if both ${s_1}$ and ${s_2}$ are the same, the value returned is 0.

How does strcmp() work? It compares the ACII integer values of the first non-matching character. For example, with apple and kiwi, the first non-matching characters are a and k. The character a has an ASCII value of 97, and the character k has an ASCII value of 107. Thus, apple is different from kiwi by -10 in terms in ASCII value.

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

	int main() {
		char num1[] = "157";
		char num2[] = "2.49";

		return 0;
	}

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

int main() {
	char num1[] = "157";
	char num2[] = "2.49";
	long int x = strtol(num1, NULL, 10);
	float y = strtof(num2, NULL);

	cout << typeid(num1).name() << " num1 : " << num1 << endl;
	cout << typeid(num2).name() << " num2 : " << num2 << endl;
	cout << typeid(x).name() << " x : " << x << endl;
	cout << typeid(y).name() << " y : " << y << endl;

	return 0;
}

A4_c num1 : 157 A5_c num2 : 2.49 l x : 157 f y : 2.49

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

int main() {
	char userInput[] = "Hello world";

	return 0;
}

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

int main() {
	char userInput[] = "Hello world";
	char *substr = strtok(userInput, " ");
	cout << substr << endl;

	return 0;
}

Hello

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

void printStringArray(char *arr[], int n);

int main() {
	char str[] = "Hello world";
	char *token = strtok(str, " ");
	char *tokenArr[2];
	int i = 0;
	while (token != NULL) {
		tokenArr[i] = token;
		token = strtok(NULL, " ");
		i++;
	}

	printStringArray(tokenArr, 2);
	// we can then index into the array
	cout << tokenArr[0] << endl;
	cout << tokenArr[1] << endl;

	return 0;
}

void printStringArray(char *arr[], int n) {
	cout << "[ ";
	for (int i = 0; i < n; i++) {
		cout << arr[i] << "  ";
	}
	cout << "]" << endl;
}

[ Hello  world ]
	Hello
	world