Array and String

What is an Array

An array is a data format that store several value of the same type. For example, an array can store 12 integers indicating day number of every month.

Compiler won’t check if the index is valid. If we write a statement month[100] = 0, the code may be compiled successfully. However this statement will break other data in the program or even stop the program when it’s running.

Array Initialization

The most common form of array initializer is a list of constant expressions enclosed in braces and separated by commas:

int arr[6] = {1, 2, 3, 4, 5, 6};
// initialize value of arr is {1, 2, 3, 4, 5, 6}

int arr[6] = {1, 2}
// initialize value of arr is {1, 2, 0, 0, 0, 0}S

Using this feature, we can use {0} to initialize an array to all zeros:

int arr[6] = {0}
// initialize value of arr is {0, 0, 0, 0, 0, 0}

int arr[6] = {1}
// initialize value of arr is {1, 0, 0, 0, 0, 0}

For char array, it’s better to put null character (‘\0’) into the last element. So that IO functions like cout, printf can process it normally. Characters after the first null character will be ignored by cout and printf.

char str1[6] = {'h', 'e', 'l', 'l', 'o', '!'};
// cout << str1; may cause unpredictable behavior since it has no '\0' at the end

char str2[7] = {'h', 'e', 'l', 'l', 'o', '!', '\0'}; // a string with null character at the end

char str3[10] = "hello"
// initialize value of arr is {'h', 'e', 'l', 'l', 'o', '!', '\0', '\0', '\0', '\0'}

Use sizeof() with Arrays

#include<iostream>
using namespace std;
int main() {
  int arr[21];
  cout << sizeof(arr) << endl;
  /* get the size of entire array*/
  cout << sizeof(arr[0]) << endl;
  /* get the size of one element in the array*/
  return 0;
}

84
4

Multidimensional Arrays

C++ stores arrays in row-major order shown as below:

Multidimensional Array Initialization

int arr[3][4] = { {1, 2, 3, 4}, {4, 3, 2, 1}, {5, 6, 7, 8} };

Higher dimensional arrays are constructed similarly.

String Object

In ISO/ANSI C++98, string is added in C++ library. Though string is quite simple to manipulate, it is not a basic type but an object wrapped in class, the later will be explained in another post. Here are some comparisons of string and char array.

String use tha same initialize statement as char array
string str1 = "string";
char ch1[20] = "char";
String can be assigned to another string, but char array not
string str2 = str1;
//valid assignment, the value of str2 becomes "string"
char ch2[20];
ch2 = ch1; //invalid, no array assignment
Two strings can be connected by operator + and +=
string str3 = str1 + str2;
//valid operation, the value of str3 becomes "stringstring"
char ch3[40] = ch1 + ch2 //invalid
str1 += "hello"
//valid operation, the value of str1 becomes "stringhello"
String use both [] operator and at() method to get char in specific index
cout << str1[3]; //this statement shows 'i'
cout << str1.at(3) //this also shows 'i'

Notice that [] operator checks if the index is out of the range of string, at() method not. So we should use at() carefully.

Some Useful String Methods

str1.size();
//return the length of string (no '\0' at the end)
//in this case it returns 11

str1.erase(5,3);
//erase some words from some index
//in this case, it erases 3 chars from index str1[5]
//so str1 becomes "strinllo"

string str4 = str3.substr(6,6);
//return a new string storing some words from some index
//in this case, it copies 6 chars from index str3[6]
//so str4 becomes "string"

There are still many more methods. cplusplus.com has more explanations about string. Make good use of it : )