6.087 Practical Programming in C, lec7
Pointers to pointers, pointer and string arrays, multidimensional arrays. Stacks and queues.
Pointer pointers
• What does this function do?
void swap (int ∗∗a , int ∗∗b){
int ∗temp = ∗a;
∗ a = ∗b;
∗b = temp;
}
• How does it compare to the familiar version of swap?
void swap ( int ∗a , int ∗b ){
int temp = ∗a;
∗a = ∗b;
∗b = temp;
}
第一个是交换了两个指针，内存中的数据并没有改变；第二个指针没变，但内存中的数据变了。
Pointer array
• Have an array int arr [100]; that contains some numbers
• Want to have a sorted version of the array, but not modify arr
• Can declare a pointer array int ∗ sorted_array[100];containing pointers to elements of arr and sort the pointers insteadof the numbers themselves
• Good approach for sorting arrays whose elements are very large(like strings)
在struct的排序中，pointer会发挥很大的作用。
String arrays
• An array of strings, each stored as a pointer to an array ofchars
• Each string may be of different length
char str1[] = "hello"; /∗length = 6 ∗/
char str2[] = "goodbye";/∗ length = 8 */
char str3[] = "ciao"; /∗length = 5 ∗/
char ∗ strArray[ ] = { str1,str2, str3};
• Note that strArray contains only pointers, not the charactersthemselves
数组存储的不是字符，而是指针，因此要注意自动变量的作用域，如果是用malloc分配的堆内存，则需要注意free的时机。
Multidimensional arrays
• C also permits multidimensional arrays specified using []brackets notation:
int world [20][30]; is a 20x30 2-Darray of int’s
• Higher dimensions possible:
char bigcharmatrix [15][7][35][4]; –what are the dimensions of this?
• Multidimensional arrays are rectangular; pointer arrays can bearbitrary shaped
二维char数组和char*数组的区别？我想二维char数组在内存中是一块连续的空间，通过指针代数来获取其中元素的地址，char*数组则是一维数组，每个元素都是指针，两者最大的不同在于二维char数组中是存储对象的，而char*数组则不存储任何对象。