C++

1B CUED C++ crib

Here's a "cheat sheet" showing how to use some C++ features. I suggest you build up your own collection of reusable code.

Variables

// Some examples of creating and setting variables
int i;
int i=5;
int i,j;
int i=5,j;
unsigned int i;
const int j=99;
float f;
double g=99.9;
string s="hello"; // needs #include <string>
char c='h';
bool b=true;

Arrays

int month[12]; // An array of 12 integers. month[0] ...  month[11]

float grid[3][3]; // A 3 by 3 grid of real numbers

Decisions

// if
if (i==3) {
  // do this if i is 3
}

// if ... else
if  (i==3) {
   // do this if i is 3
}
else {
   // do this if i isn't 3 
}

// Boolean logic
if (3<num and num<5) {
   // do this if num is between 3 and 5
}

if (num1!=0 or num2>=5) {
   // do this if num1 isn't 0 or num2 is greater than or equal to 5
}

switch

switch (i) {
    case 0: cout << "i is zero" ;
            break;
    case 1: cout << "i is one" ;
            break;
    case 2: cout << "i is two" ;
            break;
    default: cout << "i isn't 0, 1, or 2";
            break;
    }

Loops

// while loop
int num=1;
while (num<11) {
   cout << num  << endl;
   num=num+1;
}

// for loop
for(int num=1;num<11; num=num+1) {
  cout << num  << endl;
}

// continue  and  break 
int num=1;
while (num<11) {
   num=num+1;
   if (num == 5)
      continue; // start the next cycle 
   if (num == 9)
      break;    // break out of the loop
   cout << num  << endl;
}

Input from keyboard

int num;
cin >> num;

Output to screen

cout << i;
cout << "hello!";
cout << endl; // start a new line

cout << i << "hello!" << endl;

Program structure

#include <iostream> // We want to use input/output functions
#include <string>   // We want to use strings
using namespace std;      // We want to use the standard versions of
                          // the above functions.

// This is the main function
int main() {
   int i=3; // Create an integer variable and set it to 3
    // print the value of i and end the line 
   cout << "i has the value " << i  << endl;
}

Functions

// Prototype of a function called 'fun' that has no inputs or outputs
void fun();

// Prototype of a function called 'fun' that needs
// 2 inputs (an integer and a real) and produces an integer as output
int fun(int x, float y);

// The following complete program shows the prototype, call and code of a function
int double_i(int x);      // prototype 

int main() {
   int i= double_i(8);    // function call
}

int double_i(int x) {     // function code
    return x*2;
}

If you want a C++ function to modify an argument, pass it by reference (add '&' to the argument type):

   void value_func(int param) { param += 2; }
   void ref_func(int& param) { param += 2; }
   int main() {
      int x = 2;
      value_func(x); // x is still 2
      ref_func(x);   // x is now 4
   }

Files

#include <fstream>

// Reading
string message;
ifstream fin; // a variable used for storing info about a file
fin.open("secretmessage"); // trying to open the file for reading
if(fin.good()) {
      // we've managed to open the file. Now we'll read a line 
      // from the file into the string 
      getline(fin, message);
      // If num is of type int, read an integer from the file
      fin >> num;   
}


// Writing
ofstream fout;  // create a variable to store info about a file
fout.open("myData"); // try to open the file for writing
if(fout.good()) {
  fout << x << endl;   //write the contexts of variable x to the file
}

Here's a complete list. You'll never use many of these operators. The Associativity column indicates (for example) whether 8/4/2 is evaluated as (8/4)/2 (left to right associativity, resulting in 1) or 8/(4/2) (resulting in 4). Each row is a level of precedence, the highest first.

Associativity	Operator
left to right	::
left to right	() [], ->, ., typeid, casts,
right to left	! (negation), ~ (bit-not)) new, delete ++, --, - (unary) , * (unary), & (unary), sizeof
right to left	(type)
left to right	*, /, % (modulus)
left to right	-, +
left to right	<<, >>
left to right	<, <=, >, >=
left to right	==, !=
left to right	& (bit-and), \| (bit-or)
left to right	^ (bit-xor)
left to right	&&, and
left to right	\|\|, or
right to left	?:
right to left	=, +=, -=, /=, %=, >>=, <<=, &=
left to right	throw
left to right	,

Maths

#include <cmath>
using namespace std;    

float x,y;

sqrt(x);
pow(x,y); // x to the power y;
sin(x);   // radians
cos(x);   // radians
tan(x);   // radians
exp(x);   // exponential
log(x);   // natural log
log10(x); // base 10 log
fabs(x);  // absolute value
ceil(x);  // round up 
floor(x); // round down

#include <cstdlib>

using namespace std;    

srandom(time(0)); // intialize randomiser
random();         // random integer

Classes

// Create and use a new class
class Cat {
public:
   string name;
   float weight;
};

Cat c;

c.name="kitty";

Vectors

To declare a vector that can contain elements of type double use

   #include <vector>
   ...
   using namespace std;

   vector<double> v;

The type of element that the vector holds is written between the angle brackets. The vector declared above is initially empty. You can...

get the number of elements in the vector with the size() member function:
```
   cout << v.size() << endl;
```

append an element onto the back of the vector using the push_back(...) member function:


   v.push_back(3.14); // increases v.size() by 1
   v.push_back(2.71); // increases v.size() by 1

access the elements of the vector using '[]' (the index of first element is 0):
```
   cout << v[0] << endl;
   v[1] = 1.68;
```

declare a vector with a known size:

   vector<int> some_ints(10);  // 10 int elements, uninitialized

specify a default value in the declaration:


   vector<double> some_reals(10, 0.0);  // 10 real numbers, all zero

loop through the vector forwards...

   for (unsigned int i=0; i<v.size(); ++i) {
     cout << v[i] << " ";
  }

...or backwards:

   for (int i=v.size()-1; i>=0; --i) {
     cout << v[i] << " ";
   }

Department of Engineering

1B C++ Computing