[c] Directly assigning values to C Pointers

I've just started learning C and I've been running some simple programs using MinGW for Windows to understand how pointers work. I tried the following:

#include <stdio.h>

int main(){
    int *ptr;
    *ptr = 20;
    printf("%d", *ptr);
    return 0;
}

which compiled properly but when I run the executable it doesn't work - the value isn't printed to the command line, instead I get an error message that says the .exe file has stopped working.

However when I tried storing the value in an int variable and assign *ptr to the memory address of that variable as shown below:

#include <stdio.h>

int main(){
    int *ptr;
    int q = 50;
    ptr = &q;
    printf("%d", *ptr);
    return 0;
}

it works fine.

My question is, why am I unable to directly set a literal value to the pointer? I've looked at tutorials online for pointers and most of them do it the same way as the second example.

Any help is appreciated.

First Program with comments

#include <stdio.h>

int main(){
    int *ptr;             //Create a pointer that points to random memory address

    *ptr = 20;            //Dereference that pointer, 
                          // and assign a value to random memory address.
                          //Depending on external (not inside your program) state
                          // this will either crash or SILENTLY CORRUPT another 
                          // data structure in your program.  

    printf("%d", *ptr);   //Print contents of same random memory address
                          // May or may not crash, depending on who owns this address

    return 0;             
}

Second Program with comments

#include <stdio.h>

int main(){
    int *ptr;              //Create pointer to random memory address

    int q = 50;            //Create local variable with contents int 50

    ptr = &q;              //Update address targeted by above created pointer to point
                           // to local variable your program properly created

    printf("%d", *ptr);    //Happily print the contents of said local variable (q)
    return 0;
}

The key is you cannot use a pointer until you know it is assigned to an address that you yourself have managed, either by pointing it at another variable you created or to the result of a malloc call.

Using it before is creating code that depends on uninitialized memory which will at best crash but at worst work sometimes, because the random memory address happens to be inside the memory space your program already owns. God help you if it overwrites a data structure you are using elsewhere in your program.

In the first example, ptr has not been initialized, so it points to an unspecified memory location. When you assign something to this unspecified location, your program blows up.

In the second example, the address is set when you say ptr = &q, so you're OK.