Clarify the difference in representation of numbers

[vensriram]

void main()
{
  float me = 0.7;
if(me<0.7)
printf("C");
else
printf("C++");
}

For the above code my compiler prints as C

However for the code

void main()
{
  float me = 1.7;
if(me<1.7)
printf("C");
else
printf("C++");
 

my compile prints as C++.

Hence kindly clarify why is the above difference?

[whodgson]

and where do you change the values of me? ......to trigger the print op?

[vensriram]

I am not changing the value of me anywhere.
In the first program the if statement compares whether me(i.e.0.7) is lesser than 0.7 and the above decision turns true.

In my next code the value of me is 1.7 and when i compared me(i.e. 1.7) with 1.7 the decision shows that me is greater than 1.7.

At first i though me is float and if we speify a number with decimal point explicitly compiler will treat that as double and hence in the first case me<0.7. But this doesn't works for my second code. Hence kindly clarify

[donbock]

Take a look at what every computer scientist should know about floating point.

When assigning a value to me, the value is effectively cast to a float. When comparing me to a value, me is effectively cast to a double. When does changing from one floating point type to another change the value? It depends on implementation-dependent features of the underlying representation.

[haicp]

In this kind of comaprisions, 99% of the time the else part of the if condition gets executed.

In this example, when you try to compare 0.7 with 0.7, what actually happens internally is, one of the 0.7 is interpreted as 0.6999999999.

Floating point representation is machine dependent.

Even though there is not much of a difference between 0.7 and 0.6999999999, the if condition fails.

You execute the same code on another machine, probably you will get another result.

So, never use floating numbers for comparision.