Why does the variable x of the double type always print the same WRONG value in scientific notation?

Question

In this first example I would like to know why the variable x read in double, when it is formatted by the function "printf()" does not display the value correctly, and every time the value is the same.

#include <stdio.h>

int main(){
    double x;

    scanf("%f", &x);

    printf("%e", x);

    return 0;
}

2) A variable x of the type float, when printed on the screen in the whole format will always give 1?

#include <stdio.h>

int main(){
    float x;

    scanf("%f", &x);

    printf("%d", x);

    return 0;
}

3) Why the variable x of the whole type when printed in double format is 0 ?

#include <stdio.h>

int main(){
    int x;

    scanf("%d", &x);

    printf("%f", x);

    return 0;
}

Answer 1

So - as it is in the comments: these functions printf and scanf has not how to know what type of argument you are passing to them from the declared type of the variable.

They just "guess" what kind of data was passed, or what they should copy to the variable (in the case of scanf), from precisely the formatting string.

So when you, in case 3, say to the printf that will pass a float type number, it will take the corresponding bytes of the parameter stack and treat them as a double. (See below for a double and not a float). And the internal values of these bytes are very different for a float and an integer of 4 bytes.

The same way for the double, when using the scanf you have to say if the variable you will fill is a "double" or a "float" - the second case uses the formats %f, %e or %g but always fills only 4 bytes in target memory with the bytes corresponding to a float. To the scanf work with a double, it is necessary to use the modifier l in format: %lf, %le, %lg.

What confuses is that the compiler itself, due to the specification of the C language will always turn a float in a double automatically when a variable parameter function is called.

So in fact the %f (and %e) in the printf will always handle doubles (and use 8 bytes of argument stack), even if in your code you write that you are passing a float. (and yes, the codes are therefore different from those used by scanf).

In C it is necessary to understand well how the different types of data are represented in memory, and there can be even an error. For example, if instead of the examples you passed, you tried to print a variable int using the formatting %f, your program could end with a segmentation failure - since the printf will want to consume 8 bytes of argument where there is only 4. In dynamic languages with a higher abstraction level such as Javascript, Python, PHP and Ruby, these exact data conversion problems for your internal representation in general are handled by the language itself. Other static languages like C#, go, Java, etc... will have better mechanisms to detect this type of problem at the time of compilation. But in C, it’s just you and the CPU. :-)