Syntax for lab 4

Key syntax elements for lab 4

The C++ feature we will focus on for lab 4 covers pass-by-reference, the bool datatype, and loops. (Refer back to the course notes or to the lab 3 syntax examples for refreshers on the syntax used earlier.)

Pass-by-reference

Remember that regular parameter passing simply passes a copy of a value, whereas pass-by-reference allows the called function to access/change the original variable passed to it.

This is done by adding the & symbol before the parameter name in the formal parameter list (in the prototype and declaration of the function).

For example, in the program below the negate function switches the passed parameter between positive and negative.

#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

Here we introduce a new datatype, bool, that can be used to hold true/false values, e.g.

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;
}

We can test if the variable currently holds value 'true' using if (thevariablename) or we can test if it holds value false using if (!thevariablename), e.g.:

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

Functions can have bool as a return type, e.g.

// 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

The other C++ syntax we will focus on covers while loops, do-while loops, and for 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

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++; }