C + + pen questions common STL and other small operation use summary __c++

Strings String substr Action:

String S ("Lewis");
Returns a string containing the copy default 0-s.size ()
s.substr (pos,n) of n characters starting from Pos in S;

ways to change string:

String S ("Lewis");
S.insert (Pos,args); Insert before POS note here POS uses subscript, and an iterator will make an error.
s.erase (Pos,len);   Delete
s.assign (args) from Pos;     Using Args's content to replace s original string returns the reference
S.append (args) of S;
S.replace (Range,args)//range is a subscript and a length, or a pair of iterators of S,
//args can be a string str 
//     or can be a 
str,pos,len//     CP, (len)
//     n,c n characters c
//     b,e  pair of iterators
//Can be said to be very flexible

conversion between string and numeric value:

To_string (val);//Converts the value Val to a string, Val can be any arithmetic type
stoi (s,p,b);//Returns the number of the starting substring of s (representing the integer content), B is the base for the conversion, and the default is 10,p is the size_t pointer, Used to hold the subscript of the first digit in S, p default 0 is not to save subscript
STOL (s,p,b);
Stof (S,P,B); L:long  f:float d:double
stod (s,p,b);

Association container set Map
Determines whether the element to be processed already exists in the set:

C.find (k); Returns an iterator that points to the element with the first keyword K and, if K is not in the container, returns the iterator ==c.end ()
C.count (k);//Returns the number of elements that have a keyword equal to K ==0 the set is empty
c.equal _range (k);/returns the pair of an iterator that represents the range of elements that have a keyword equal to K, and if K does not exist, the two members of the pair are equal to C.end ()

Flexible selection of map operation subscript and find

map<string,int>word_cnt;
word_cnt["Hello"];//use the subscript operator, if "Hello" is not in the map, the value initialization is used to give the int an initial 0
word_cnt.find ("Hello"); Hello map is not generated automatically

control output bit number

#include <iomanip>  //need to include this header file
cout << "a=" << setprecision (2) << a <<endl;//a= 0.2124  will output 0.2
//If you want to keep the decimal point after several:
cout.setf (ios::fixed);  0
cout << "a=" <<fixed<< setprecision (2) << a <<endl;//output a=0.20  
for insufficient digits COUT.UNSETF (ios::fixed); Cancel the collar.

flexible use of ceil () floor () to take the whole up and down

int a=2;
cout<<a/3<<endl;
Cout<<ceil (A/3) <<endl;
Cout<<ceil ((a+0.0)/3) <<endl;
Cout<<floor ((a+0.0)/3) <<endl;

>>>0
>>>0
>>>1
>>>0

keep in mind that you want to maintain the type uniformity between the values of the operations. Especially when designing int and long

Vector<long long>value;
When using accumulate summation
if (Accumulate (Value.cbegin (), Value.cend (), 0))
    cout<< "wrong!" <<endl;
Because value is a long long and the sum of the initial value 0 is the int type, you must use 0LL common 0L 0F 0D
if (accmulate (Value.cbegin), Value.cend (), 0LL))
cout<< "right" <<endl;