Why is this code experiencing an exception on the scanf line

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C

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>

double input1 = 1.1;

int main()
{
    double celsius1, kelvin1, rankine1;
    scanf ("%g", input1);
    celsius1 = (input1 - 32) * (5 / 9);
    kelvin1 = (input1 + 459.67) * (5 / 9);
    rankine1 = (input1 + 459.67);
    printf("%g degrees Fahrenheit\n%g degrees Celsius\n%g degrees Kelvin\n%g degrees Rankine", input1, celsius1, kelvin1, rankine1);
    return 0;
}

This code is saying "Exception thrown at 0x7AB06582 (ucrtbased.dll) in ConsoleApplication1.exe: 0xC0000005: Access violation writing location 0x9999999A."when I attempt to enter an input at the scanf line. I have to use %g for the assignment but cannot tell why it is not working within this code.

What I have tried:

I've tried adding & in front of the variables akin to some examples online, however more build errors began to show up.

Posted 13-Jul-20 18:02pm

Ryan Thomsen

Updated 13-Jul-20 20:03pm

CPallini

v2

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Peter_in_2780 · Accepted Answer · 2020-07-13T18:37:00

Solution 1

You need to write

scanf ("%g", &input1);

to pass the address of your variable to scanf. Elsewhere, you're referencing values, so no & is needed or wanted.

Posted 13-Jul-20 18:37pm

Peter_in_2780

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CPallini 14-Jul-20 2:02am

5.

Jon McKee · Accepted Answer · 2020-07-13T18:41:00

scanf - C++ Reference[^]

Depending on the format string, the function may expect a sequence of additional arguments, each containing a pointer to allocated storage where the interpretation of the extracted characters is stored with the appropriate type.
There should be at least as many of these arguments as the number of values stored by the format specifiers. Additional arguments are ignored by the function.
These arguments are expected to be pointers: to store the result of a scanf operation on a regular variable, its name should be preceded by the reference operator (&)

You only need the '&' in front of input1 on scanf because it takes memory addresses not values like printf. Adding it anywhere else in this code would cause errors.

Some more material on pointers below. It's an important topic to understand in C++.
Pointers - C++ Tutorials[^]
Pointers in C/C++ with Examples - GeeksforGeeks[^]

EDIT:
The reason you get that error is because scanf is trying to use the value of input1 (1.1) as a memory address (0x9999999A). Your code doesn't own that memory so is getting an access violation.