Why this program shows error while initializing with float value?

Question

Why this program shows error while initializing with float value?

Walkbitterwyrm Lightwarg 20

Why the following program shows error (incompatible types when initializing type 'float *' using type 'double') when a float value is initialized with a float pointer?

#include <stdio.h>

int main()
{
    float *a = 2.5;

    printf("%f", a);

    return 0;
}

VS Code Error

David Lowndes 2,745 Reputation points MVP

2023-09-03T22:50:28.53+00:00

float * is a pointer to a float not a float itself. Remove the *.

1 answer

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David Lowndes 2,745 Reputation points MVP

2023-09-03T22:50:28.53+00:00

float * is a pointer to a float not a float itself. Remove the *.

Answer 1

WayneAKing 4,931

>when a float value is initialized with a float pointer?

Where exactly in your posted code do you think that is happening?

float *a = 2.5;

This line creates a variable named "a" of type pointer to float, and attempts to initialize it with the double value 2.5 which clearly is an illegal memory address.

Wayne

Walkbitterwyrm Lightwarg 20 Reputation points

2023-09-04T11:38:04.69+00:00
David Lowndes---that's not what you are thinking but it's something else. Check the following program and you will know why it's working now.

#include <stdio.h> int main() { float f, *a = &f; *a = 2.5; printf("%.2f", *a); return 0; }

WayneAKing---I don't know what you are asking or saying but the main reason for the bug was not initializing the pointer with an address.
David Lowndes 2,745 Reputation points MVP

2023-09-04T11:47:33.8633333+00:00

The main problem is that you don't need a pointer anyway!
P a u l 10,761 Reputation points

2023-09-04T13:02:14.0433333+00:00

I think you might be getting confused because the asterisk (*) is used for two purposes in C, in regard to pointers. In your original snippet it was a part of the declared type of a, in which you're communicating to the compiler that it's a pointer variable. However you're assigning the constant float value 2.5 to it, which isn't a valid memory address.

In the second snippet you're using the asterisk to dereference the a pointer in order to set it's value. May just be my opinion but I think it was a bad language design choice for this reason (i.e. people may assume that you can both declare a pointer and dereference it to assign it's value to a constant in one line).
Walkbitterwyrm Lightwarg 20 Reputation points

2023-09-05T10:58:13.9933333+00:00

David Lowndes---that's not a problem because it's my requirement. Whether you travel to San Francisco by bus or flight, it's your choice. Even though, people prefer travelling through flight. Here you are suppose to answer the question but not to question about the question.

P a u l---I know what's the use of asterisk with a pointer but why do you think assigning 2.5 to the pointer is not having valid memory address? 2.5 will be placed in the memory address of f because it's address was assigned to the pointer during declaration.

You can't assign 2.5 at the time of declaration because you need to assign the address of a variable to the pointer first. However, 2.5 can be assigned to the variable f at the declaration but I didn't do that because I want to see whether the pointer can be updated with new values or not.

Anyway, I came to know that the bug was the I didn't assign the address to the pointer in the first program and that's why it was not working. So we can't declare like float *a = 2.5 but this is possible float f = 2.5, *a = &f.
Bruce (SqlWork.com) 81,976 Reputation points Volunteer Moderator

2023-09-05T17:07:00.27+00:00

you seem to confuse initialization and assignment of a variable:

float f; // declare uninitiated float variable
float f2 = 3.0; // declare and initialize;
float* fp = &f; // declare fp as pointer, initialization as address of f;
float* f2p; // declare f2p as pointer, uninitialized
*fp = 2.5; // assignment, 2.5 is copied to the address fp points to
f2p = &f2; // assignment, address of f2 is copied to f2p pointer;

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Why this program shows error while initializing with float value?

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