[C&++] vector note－Edison.X. Blog

謠言指出，vector 效能操作比 C 之 malloc、memcpy、memset、memcmp 來得慢。還在謠傳這謠言的，麻煩開 Optimize 測一次，覺得結果有誤，多測幾次也無妨。

note 到後面愈來愈心虛，技巧概念就那一、二個而已。

零、變數假設

Code Snippet
typedef int  Type;    // Type can be class
const Type   Val=1;   // a value of Type
const size_t xCount=2;
const size_t yCount=3;

壹、一維初始化並給值

Code Snippet
// v1[xCount=2] = {0,0}
vector<Type> v1(xCount);
// v2[xCount=2] = {Val,Val}
vector<Type> v2(xCount, Val);

貮、二維初始配置並給值

Code Snippet
// Type v1[xCount][yCount] = {{0,0,0},{0,0,0}}
// Method 1
vector< vector<Type> > v1( xCount);
for(size_t i=0; i<xCount; ++i)
    v1[i].resize(yCount);
 
// Type v1[xCount][yCount] = {{0,0,0},{0,0,0}}
// Method 2
vector< vector<Type> > v2(xCount, vector<Type>(yCount) );
 
// Type v1[xCount][yCount] = {{val,val,val},{val,val,val}}
// Method 3
vector< vector<Type> > v3(xCount, vector<Type>(yCount, Val));

參、一維變更大小

Code Snippet
vector<Type> v1;    
v1.resize(xCount); /* v1[xCount=2] = {0,0}   */
v1.resize(3,Val) ; /* v1[3] = {0,0,Val}      */
v1.resize(5)     ; /* v1[5] = {0,0,Val,0,0}  */

肆、二維變更大小

Code Snippet
// resize v1[xCount][yCount] = {0}
vector<vector<Type> > v1;
v1.resize( xCount, vector<Type>(yCount) );
 
// resize v2[xCount][yCount] = {Val,Val,Val,...}
vector<vector<Type> > v2;
v2.resize( xCount, vector<Type>(yCount, Val));
 
//(!) be care  here
vector<vector<Type> > v3;
v3.resize( xCount, vector<Type>(yCount)); // v3 = {0.0,...}
v3.resize( xCount, vector<Type>(yCount,Val));
// v3 = {0,0,0,...} still.

伍、完整複製

Code Snippet
vector<Type> v2;
vector<Type> v1(3);
for(size_t i=0; i<v1.size(); ++i) v1[i] = i;
v2 = v1; // v2 = {0,1,2}

陸、部份複製

Code Snippet
// include <algorithm>
vector<Type> v2;
vector<Type> v1(10);
for(size_t i=0; i<v1.size(); ++i) v1[i] = i;
 
// give v2 size
v2.resize(10);
// v2[5:8] = v1[2:5], 也可初始化時複製
copy(v1.begin()+2, v1.begin()+5, v2.begin()+5);

柒、全部設定相同值

Code Snippet
// include <algorithm>
// for 1 dim
vector<Type> v1(10);
for(size_t i=0; i<v1.size(); ++i) v1[i] = i;
fill(v1.begin(), v1.end(), Val);
 
// for 2 dim
vector< vector<Type> > v2(xCount, vector<Type>(yCount));
fill(v2.begin(), v2.end(), vector<Type>(yCount, Val));