Syntax for lab 2

Key syntax elements for lab 2

The C++ syntax we will focus on for lab 2 includes function definitions (including prototypes), calls, returns, if/else if/else statements, and basic error checking/handling for scanf and cin.

Functions and prototypes

#include <cstdio>

// prototype for a function that takes no parameters and returns no value
void printSomething();

// prototype for a function that takes no parameters 
//    and returns a value of type integer
int  getAValue();

// prototype for a function that takes two parameters (an int and a float)
//    and returns a value of type double
double multiply(int x, float y);

// a main routine calling each of the three functions listed above
int main()
{
   int a = 1;
   float b = 2.0;
   double c;
   printSomething();  
   a = getAValue();
   c = multiply(a, b);
   return 0;
}

//  ===== implementations of the three functions =====

// prototype for a function that takes no parameters and returns no value
void printSomething()
{
   printf("Something!\n");
}

// prototype for a function that takes no parameters 
//    and returns a value of type integer
int  getAValue()
{
   // getting the user to type in a value, and return whatever it is
   int v;
   printf("Please enter an integer value\n");
   scanf("%d", &v);
   return v;
}

// prototype for a function that takes two parameters (an int and a float)
//    and returns a value of type double
double multiply(int x, float y)
{
   // compute and return the product of x and y
   double result = x * y;
   return result;
}

if/else statments

   // an if by itself
   if (x == 3) {
      // do this if x is 3, otherwise move on to whatever is next
      printf("x is 3!\n"); 
   }
   
   // an if with an else
   if (x < 0) {
      // things to do if the expression is true, i.e. x < 0
      printf("x is negative\n");
   } else {
      // things to do if the expression is false, i.e. x >= 0
      printf("x is non-negative\n");
   }
   
   // an if with an else-if and an else
   if (x < 0) {
      // things to do if the expression is true, i.e. x < 0
      printf("x is negative\n");
   } else if (x == 0) {
      // things to do if the first expression was false, but the second one is true
      printf("x is zero\n");
   } else {
      // things to do if all of the expressions above were false
      printf("x is positive\n");
   }
   
   // single-statement bodies, you can leave off the { }
   if (x < 0) 
      printf("x is negative\n");
   else if (x == 0)
      printf("x is zero\n");
   else
      printf("x is positive\n");

testing the results of scanf

// variables to hold two numbers the user will enter,
// here we have initialized both to negative values
float x = -1;
float y = -1;

// variable to count how many variables were successfully read
int scanCount;

// prompt the user to enter two non-negative numbers
print("Please enter two non-negative numbers\n");

// read the numbers into x and y, use scanCount to record how many variables were filled
//   i.e. scanCount is 0 if scanf couldn't read anything,
//                     1 if scanf could read the value for x but not y
//                     2 if scanf successfully read both x and y values
scanCount = scanf("%f %f", &x, &y);

// check for non-numeric input
if (scanCount == 0) {
   printf("Sorry, the first value entered was not a number, we are discarding both values\n");
   scanf("%*s"); // discards the first value
   scanf("%*s"); // discards the second value
}

else if (scanCount == 1) {
   printf("Sorry, the second value entered was not a number, we are discarding it\n");
   scanf("%*s"); // discards the second value, the first value is already in x
}

// if we reach this case we know they entered numbers,
//    but we still need to check for negative values
else if ((x < 0) || (y < 0)) {
   if (x < 0) {
      printf("You entered a negative value for the first number\n");
   }
   if (y < 0) {
      printf("You entered a negative value for the second number\n");
   }
}

// if we reach this case then they actually entered valid data!
else {
   printf("Your valid non-negative values were %g and %g\n", x, y);
}

testing the results of cin

// presumed maximum length of a typed line of user input
const int LINELEN = 256;

// variables to hold the numbers th user will enter,
// here we have initialized it to a negative value
float x = -1;

// prompt the user to enter a non-negative number
cout << "Please enter a non-negative number" << endl;

// read the number into x, use cin.fail() to check if it succeeded
cin >> x;
if (cin.fail()) {
   cout << "Sorry, the value entered was not a number, we are discarding the value" << endl;
   cin.clear();
   cin.ignore(LINELEN,'\n'); // flush to the end of line or 80 characters, whichever comes first
}

// if we reach this case we know they entered numbers,
//    but we still need to check for negative values
else if (x < 0) {
   cout << "You entered a negative value for the number" << endl;
}

// if we reach this case then they actually entered valid data!
else {
   cout << "Your valid non-negative value was " << x << endl;
}