Review Ch3/ch4

TEXTBOOK DEFINITION(C++)

• object: a region of memory
• variable: a named object
• type: the kind of values it can store

string box;

string is a type and box is a variable name.

Assignment vs initialization

Which one is assignment?

int a=3;

vs

a=5;

Type safety

C++ compiler cannot guarantee complete type safety.

double x;
double y=x;//?
double z=2.0+x;//?

safe conversion

• bool -> char, int or double
• char -> int or double
• int -> double
  
  

(narrow) unsafe conversion

• double -> int, char or bool
• int -> char
• int or char -> bool

Example Narrow Conversion

double x=2.7;
int y=x;
int z=5/2;
double w=2.5/2;

y is 2. z is 2. (round-down.) w is 1.25.

int a=1000;
char b=a;

b depends on machine.

How to check narrowing conversions?

Use { }-list notation (C++11)

double x {2.7};
int y {x}; //ERROR!

int a{1000};
int y {x}; //ERROR!

char b1 {1000};// ERROR!

Expression(4.3)

Expression computes a value.

• 10, 'a', 3.14 or "Hi"
• variable
• assignment

  length=9

Statement(4.4)

Statements Statement is the smallest standalone element that expresses some action.

a=b; 
++b;
a=b++b; //Syntax error:missing semicolon
int a=2;
cout<<a<<endl;
1+2; // Ok! Nothing.

Common mistake!

if

if (x==5);//empty statement!
{
    y=3;
}

In general,

if (expression) statement else statement

switch

switch (int, char or enum type){
    case (constant expression):
        statement
        ...
        break;// important!
        ...
    default:
        ...
        break;
}

Constant expression(4.3.1)

constexpr double pi=3.141592; // Compile time
int i=2;
const j=i; // running time

constexpr int max=100;
void use(int n){
    constexpr int c1=max+7;//OK!
    const int c2=n+7; //OK!

    constexpr int c3=n+7///Error!
    c2=3;// Error
}

Vector(4.6)

What is vector?

Vector is a sequence of elements that you can access by an index(iterator). Yes. It is a better array. We actually say it as a container. Vector also knows its.

How to declare or initialize

#include <vector>
#include <string>
vector<int> vi(6); // vector of 6 ints initialized to 0
vector<string> vs(4); // vector of 4 strings initialized to ""
vector<double> v; //empty vector
vector<int> v={1,2,3,4,5,6};// vector of 6 ints
vector<string> people={"Jobs","Gates","Larry"}
vector<int> {1,2,3}//{} initialization

size of a vector

vector<int> v={1,2,3,4};
for(int i=0;i<v.size();i++)  // Range is [0,v.size())
    cout<<v[i]<<endl;

or we can use Range-based for loop (c++11).

for(int x: v) // for each x in v
    cout<<x<<endl;

or

for(auto x: v) // auto keyword will find a correct type.(c++11)
    cout<<x<<endl;

Vector is dynamic.

You can increase the size of a vector by using push_back member function. push_back ad adds an element to the end.

vector<double> v; //empty
v.push_back(2.7);//Add 2.7
v.push_back(5.6);//Add 5.6

```c++
You can also delete the last element by `pop_back` member function.
```c++
v.pop_back();//Remove the last one, 5.6.

vector has many extra member functions.

Check 4 again.

CHECK 4. GET N INPUTS (ASSUME INTEGERS) AND STORE THEM INTO AN VECTOR. CALCULATE THE AVERAGE.

int sum(const vector<int>& v){
    //your code
}
int main(){
    //your code
}

Try

Again!

TYPE INTEGERS AND STOP UNTIL TYPE A NON-INTEGER like 'a'. STORE THEM INTO AN VECTOR. CALCULATE THE AVERAGE.

vector<int> v;
for(int t=0;cin>>t;){
    v.push_back(t);
}

If you type something which is not integer, it will stop since cin>>t will be converted to false. Otherwise it will be converted to true.