Key syntax elements for lab 4

Pass-by-reference

#include <iostream>
using std::cout;
using std::endl;
using std::cin;

void negate(float &orig);

int main()
{
    int x = 3;
    negate(x);
    // x has now been changed to -3
    cout << x << endl;
}

void negate(float &orig)
{
   // replace the original value with the opposite
   orig = - orig;
}

The boolean datatype

bool subtitlesOn; // will set to true if user wants subtitles, false otherwise

// use the variable to remember if a particular condition evaluated to true or false
char userChoice;
cout << "Do you wish to have subtitles turned on?  Enter Y for yes or N for no: ";
cin >> userChoice;
if (userChoice == 'Y') {
   subtitlesOn = true;
} else {
   subtitlesOn = false;
}

if (subtitlesOn) {
   // .... code here to turn on subtitles ...
} else {
   // .... code here to turn off subtitles ...
}

// return true if min < x and x < max
bool isXbetween(int x, int min, int max)
{
   if (x <= min) { // too small
      return false;
   } else if (x >= max) { // too big
      return false;
   } else {
      return true; // x is in between min and max
   }
}

Loops

• while loops
  these are top tested loops, and must include appropriate instructions inside the loop to update the condition being checked

  // print the values from 10 through 20, one per line int x = 10; while (x <= 20) // enter the body of the loop if x is less than or equal to 20, otherwise { // skip the body of the loop and go on with the rest of the program cout << x << endl; // do the loop actions (in this case just a single output statement) x++; // update x for the next potential pass through the loop } // upon reaching the bottom of the loop, go back up to the test condition // rest of the program would be down here

• do while loops
  these are bottom tested loops, so always run the body of the loop at least once (since you don't encounter the test condition until after the first pass through the loop)

  int x = 10; do { cout << x << endl; x++; } while (x <= 20); // if x is <= 20 go back and do the loop body again, // otherwise leave the loop now

• for loops
  for loops are top tested, but allow you to initiaze a loop counter as part of the loop header, as well as the counter update statement

  for loop	equivalent while loop
  for (int x = 1; x < 10; x++) { cout << x << endl; }	int x = 1; while (x < 10) { cout << x << endl; x++; }

  Note the condition is checked at the top of the for loop, and the update is applied at the bottom.

• nested loops
  If we put one loop inside another, each time we go through the "outer" loop once we might go through the "inner" loop many times. For example, in the code below the outer loop controls which row we're working on, the inner loop controls which column we're printing in the current row:

  code	output
  for (int row = 1; row < 8; row++) { cout << "Row " << row << ":"; for (int column = 1; column < 5; column++) { cout << column; } cout << endl;; }	Row 1: 1234 Row 2: 1234 Row 3: 1234 Row 4: 1234 Row 5: 1234 Row 6: 1234 Row 7: 1234

  ---

  Some notes on characters and ASCII

• Each char is represented by a unique integer, from 0 to 127. (If you're interested, a quick search for ASCII tables on any search engine will show you which values are associated with which specific characters.)

  When we want to, we can treat chars like ints to work with their ascii value, though it's safest to explicitly tell the compiler that we want to convert between the data types, e.g.

  char ch = 'Q'; int x = (int)(ch); // get integer (ascii) value of this char cout << "The ascii value for " << ch << " is " << x; // Q is 81 x = x - 10; ch = (char)(x); // get char equivalent of this integer value cout << "The char with ascii value " << x << " is " << ch; // 71 is G

Pseudo-random number generation

• For a real number in the range 0..1

     // take rand's result (as a float) and divide it by the biggest possible rand value
     float r;
     r = 
  

• For an integer in the range 0..N

     // take the remainder after dividing rand's result by N+1
     long r;
     r = rand() % (N+1);
  

• For an integer in the range M..N

     // generate a random value in the range 0..(1+N-M) then add M
     long r;
     r = M + (rand() % (1+N-M));
  

#include <iostream>
#include <cstdlib>
#include <ctime>

int main()
{
   // seed the rng with the current time, just needs to be done once
   srand(time(NULL));

   // generate the 50 random numbers in the range 1..100
   for (int i = 0; i < 50; i++) {
       long r = 1 + (rand() % 100);
       cout << r << endl;
   }

   return 0;
}