Why are double and void data types but long is not?

Question

Why are double and void data types but long is not?

Debojit Acharjee 455

In C double is considered a primary data type that can store floating-point numbers, but why double is also taken as a data type if it's the same as float?

Moreover, why long is not considered a data type?

If void means empty then why it's also considered as a data type?

Barry Schwarz 3,746

double and float are not the same. They are distinct data types. On most systems, they have different sizes. Why do you think they are the same?

void is treated as a data type in ONLY certain situations for conceptual consistency. What makes you think it means empty?

 If a function does not return a value, then it is declared as returning void.  Since functions that do return a value must declare the type of that value, void serves as the type for one that doesn't.  Historically, this is significant because in the old days if you omitted the function return type it defaulted to int.

 If a function can take a pointer to any type of data, then it is declared as accepting a pointer to void.  A pointer must point to a type and void is a convenient way of saying that the type is unspecified.  It is up to the function to figure out how to deal with the pointer.  Historically, this replaced the old practice of using a pointer to char as a generic pointer.

But you cannot declare a variable as having type void so that is not one of the situations. One reason for this is that the compiler would not know what the size of the variable is.

Debojit Acharjee 455 Reputation points

2023-06-13T02:12:15.3633333+00:00
I don't see any difference in digits of the output results between double and float.

Here is an example program:

int main() { float a = 1; double b = 1; printf("%f\n%lf", a, b); return 0; }

Output:

1.000000

1.000000

The output for both the variables have 6 digits in floating-point.

Why I think void means empty?

Check this program:

int main() { void a; printf("%d", a); return 0; }

Output:

tempCodeRunnerFile.c: In function 'main': tempCodeRunnerFile.c:5:10: error: variable or field 'a' declared void void a; ^

Why it shows error? Why we can't declare a variable as void?
Barry Schwarz 3,746 Reputation points

2023-06-13T08:08:04.35+00:00

The number of digits in the printed representation is not related to the number of significant digits the variable can hold. Since 1.0 can be represented exactly in both a float and a double, you will never see any difference. Try a value like 1122334455 to see the difference between a float and a double.
You cannot declare a variable to have type void because the language prohibits that. I gave ojne reason why in my previous response.

2 answers

Your answer

Barry Schwarz 3,746 Reputation points

2023-06-12T17:49:32.0666667+00:00

double and float are not the same. They are distinct data types. On most systems, they have different sizes. Why do you think they are the same?

void is treated as a data type in ONLY certain situations for conceptual consistency. What makes you think it means empty?

If a function does not return a value, then it is declared as returning void. Since functions that do return a value must declare the type of that value, void serves as the type for one that doesn't. Historically, this is significant because in the old days if you omitted the function return type it defaulted to int. If a function can take a pointer to any type of data, then it is declared as accepting a pointer to void. A pointer must point to a type and void is a convenient way of saying that the type is unspecified. It is up to the function to figure out how to deal with the pointer. Historically, this replaced the old practice of using a pointer to char as a generic pointer. But you cannot declare a variable as having type void so that is not one of the situations. One reason for this is that the compiler would not know what the size of the variable is.
Debojit Acharjee 455 Reputation points

2023-06-13T02:12:15.3633333+00:00

I don't see any difference in digits of the output results between double and float.

Here is an example program:

int main() { float a = 1; double b = 1; printf("%f\n%lf", a, b); return 0; }

Output:

1.000000

1.000000

The output for both the variables have 6 digits in floating-point.

Why I think void means empty?

Check this program:

int main() { void a; printf("%d", a); return 0; }

Output:

tempCodeRunnerFile.c: In function 'main': tempCodeRunnerFile.c:5:10: error: variable or field 'a' declared void void a; ^

Why it shows error? Why we can't declare a variable as void?
Barry Schwarz 3,746 Reputation points

2023-06-13T08:08:04.35+00:00

The number of digits in the printed representation is not related to the number of significant digits the variable can hold. Since 1.0 can be represented exactly in both a float and a double, you will never see any difference. Try a value like 1122334455 to see the difference between a float and a double.
You cannot declare a variable to have type void because the language prohibits that. I gave ojne reason why in my previous response.

Answer 1

Castorix31 90,686

Moreover, why long is not considered a data type?

long is a type.

See : Built-in types (C++)

Answer 2

Hi,

Since you're using C not C++, you'd better tell us which compiler you're using.

I don't see any difference in digits of the output results between double and float.

Check the sample:

#include <stdio.h>


int main() {
    float floatValue   = 3.1415926535897932384626433832795f;
    double doubleValue = 3.1415926535897932384626433832795;

    printf("Float: %.*f\n", 17,  floatValue);
    printf("Double: %.*lf\n", 17, doubleValue);
    return 0;
}

The result :

enter image description here

and your example uses 1 instead of the more typical decimal

 float a = 1.1f;
 double b = 1.1;

enter image description here

For the second question, I don't understand what you are struggling with. Read the usage of void in C: The void type in C.

Best regards,

Minxin Yu

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Minxin Yu 13,506 Reputation points Microsoft External Staff

2023-06-15T06:27:06.9333333+00:00

Hi, @Debojit Acharjee

May I know if you have got any chance to check my answer?

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Why are double and void data types but long is not?

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