C program outputting different values with different numbers? [duplicate] - c++

Question

This question already has answers here:
strange output in comparison of float with float literal
(8 answers)
Closed 6 years ago.
See the program below
#include<stdio.h>
int main()
{
float x = 0.1;
if (x == 0.1)
printf("IF");
else if (x == 0.1f)
printf("ELSE IF");
else
printf("ELSE");
}
And another program here
#include<stdio.h>
int main()
{
float x = 0.5;
if (x == 0.5)
printf("IF");
else if (x == 0.5f)
printf("ELSE IF");
else
printf("ELSE");
}
From the both programs we expect similar results because nothing has literally changed in both changed, everything is same and also comparison terms are changed correspondingly.
BUT 2 above programs produce different results
1st Program
ELSE
2nd Program
IF
Why is this 2 programs behaving differently

Answer 1

The behavior of these two programs will vary between computers and operating systems - you are testing for exact equality of floats.
In memory, floats are stored as a string of bits in binary - i.e. 0.1 in binary (0.1b) represents 0.5 in decimal (0.5d).
Similarly,
Binary | Decimal
0.1 | 2^-1 = 1/2
0.01 | 2^-2 = 1/4
0.001 | 2^-3 = 1/8
0.11 | 2^-1 + 2^-2 = 3/4
The problem is that some decimals don't have nice floating point representations.
0.1d = 0.0001100110011001100110011...
which is infinitely long.
So, 0.5 is really nice in binary
0.5d = 0.1000000000000000...b
but 0.1 is really nasty
0.1d = 0.00011001100110011...
Now depending on your compiler, it may assume that 0.1f is a double type, which stores more of the infinite sequence of 0.0001100110011001100110011001100110011...
so it is not equal to the float version, which truncates the sequence much earlier.
On the other hand, 0.5f is the same regardless of how many decimal places are stored, since it has all zeroes after the first place.
The accepted way to compare floats or doubles in C++ or C is to #define a very small number (I like to call it EPS, short for EPSILON) and replace
float a = 0.1f
if (a == 0.1f) {
printf("IF\n")
} else {
printf("ELSE\n")
}
with
#include <math.h>
#define EPS 0.0000001f
float a = 0.1f
if (abs(a - 0.1f) < EPS) {
printf("IF\n")
} else {
printf("ELSE\n")
}
Effectively, this tests if a is 'close enough' to 0.1f instead of exact equality. For 99% of applications, this approach works just fine, but for super-sensitive calculations some stranger tricks are needed that involve using long double, or defining a custom data type.

Answer 2

You are using two data types: double,automaticly in if(x=0.1))(0.1 is double) and x is float. these types differ how they store the value. 0.1 is not 0.1f, it is 0.100000000001 (double) or 0.09388383(something)