Casting of Variables

 

 When to Cast

 

When you cast a variable from one type to another, all you are doing is telling the computer to use a different type to store the variable. Why would you need (or want) to do this? Let's say you declared a variable of type short. In most cases, that would mean that the largest positive value you could store would be 32,767. But somewhere in your program, you realize that you're going to have to do a calculation which could increase the value over this maximum. Perhaps you are computing very large triplets. To calculate the value of c , you need to take the square root of the quantity a2 + b2. But what if a or b is very large? Then squaring that number will make it much, much larger -- and if the value becomes bigger than 32,767 your values will not be what you expected (if you had used a short to store a or b. Remember, a short can only store the values between -32,768 and +32,767, so if you try to store a number out of this range, your data will be incorrect!

 

So, the solution is to cast. We can cast the numbers to a larger data type, such as an int or a long, for the purposes of the calculation -- and then we can cast them back to a short when we are done, since the final value for c will probably be small enough to be stored in a short.

 

This is a somewhat trivial example, since in this case you could store the numbers in ints or longs from the beginning and not worry about it! A more useful example might be if you have a number which represents an average. You'll probably want to represent the number with a floating-point type like a float or a double so that it is accurate while you are computing it (otherwise you'd only be able to store a value like "26" instead of "26.3141885"). Let's say that you want to display the value in a table, yet the table would look cluttered if you displayed "26.3141885", so you decide to simply display the integer portion, 26. You can cast the float to an int and then display the int in the table -- since ints can't store floating-point numbers, the decimal portion of "26.3141885" will be truncated and you will be left with "26".

 

 How to Cast

 

Casting in C++ is easy. Let's say that you have a float storing a number like "26.3141885", and you want to have an int storing the integer portion instead. Here's how to do it:

 

int GetAverage() {

// assume that regularAverage and specialAverage store two floats
  float totalAverage = regularAverage + specialAverage;

// cast totalAverage to an int
  int truncatedAverage = (int) totalAverage;

// return the truncated value
  return truncatedAverage;
}
 

There's a little bit of syntax that you haven't seen before, but the key part to notice is the line of code that reads int truncatedAverage = (int) totalAverage. What we're doing here is taking a float, totalAverage, which stores some kind of decimal number (like 82.41832), and getting rid of the ".41832" part by casting it to an int. That works because the int is only capable of storing integers, so it simply stores the integer portion of totalAverage.

 

#include<stdio.h>
int main()
{
float a=12.445;
int b=a;
float c=b;

printf("%f\n",a);
printf("%d\n",b);
printf("%f\n",c);
}

Output:

12.445500

12

12.000000

 

You can cast from from int to short if the integer is not greater than 32767.