Class Notes Week 3-4

There are only 10 different kinds of people in the world: 
those who can count in binary and those who can't. --Anon

Selection & Iteration

Iteration: About.com C++ Tutorial - Lesson 7: While statement

Selection: About.com C++ Tutorial - Lesson 5: If statement

Algorithm isPrime . "To understand a program, you must become both the machine and the program." ==> Hand-trace the algorithm/program.

Read about 122 pseudocode

Implement the algorithm as a C++ program: isPrime-v1.cpp

User Interface (UI): Interactive

//prompt and read
. . .

while(eof){
   //. . .
}

Exercise: modify pizza.cpp to use an EOF-controlled while loop

Exercise: modify glassvol.cpp to use an interactive UI

Exercise: modify sPrime-v1.cppto use an interactive UI: isPrime-eof.cpp

Standard Library Functions

Bottom-test do loop: Syntax

do{ 
   statement(s); 
}while(condition);

The do loop is a post-tested or bottom-tested loop (or a leap-before-you-look loop ;-). The statements in the body of the do loop will execute 1 or more times, depending on the value of the condition.

The do loop is a convenient control structure to use when a problem requires code to be executed at least once.

Problem: Count the number of times we must roll a die until we get a particular number (e.g., six). We're not going to hit our target without at least one roll, so this is a good problem for a do-while loop:

   //prompt and read n
   . . .
   //roll the die until we get n
   do{
      die = roll the die;
      ++rolls;
   }while(die != n);

Refining the code:

The above code segment has an instruction, roll the die , which is not a C++ statement. To simulate random events like tossing dice, the C++ standard runtime library includes a rand() function that returns a pseudorandom integer in the range [0 .. RAND_MAX].

pseu·do·ran·dom (su'do-ran'dom). adj.Of, relating to, or being random numbers generated by a definite, nonrandom computational process.

Since rand() returns a potentially large number, we can reduce it by using the remainder operator (%). This statement gives us a value in the range 0 .. 5:

die = rand() % 6;

If we just add 1 to this value (adding parens for readability), we've got our die roll, 1 .. 6:

die = (rand() % 6) + 1;

To use rand(), we include the stdlib header:

Implementation: rollDie.cpp

rand() always generates the same sequence of pseudorandom numbers. So, every time the rollDie.cpp executes it gets the same sequence of numbers from its calls to rand(). C++ offers another function, srand(), that addresses this limitation.

Calling predefined functions: standard library cmath

Functions in <cstdlib>

int abs(int aNum);
// return absolute value of integer parameter

int rand();
// returns a value between (inclusive) 0 and and RAND_MAX (defined by the compiler, often 32767):

#include 
#include 
using namespace std;

int main(){
   cout << "RAND_MAX = " << RAND_MAX    //32767
        << endl 
        << "Exiting.." << endl;
   return 0;
}

Functions in <cmath>

double ceil(double x);
//returns the smallest integer that is greater or equal to x

double floor(double x);
//returns the greatest integer that is less than or equal to x

double fabs(double number);
//returns absolute value of floating-point

double log2(double x);
//returns the base 2 logarithm for x > 0.0
double log(double x);
//returns the natural logarithm for x > 0.0
double log10(double x);
//returns the base 10 logarithm for x > 0.0

double pow(double b, double p);
//returns b raised to the p power.

Each standard function is like a mini-program: we call the function, passing any necessary data to it, the function computes a value and returns it to the point of call.

Here's how we called rand() to simulate the roll of a die:

die = rand() % 6 + 1;

Example: calling numeric functions

Next: Writing User-Defined Functions