Thread: printf output makes no sense. Is it a Pointer problem?

Firstly a quick hello and thanks to the people here for such a helpful resource.

I am very new to C and have run into a problem with program I'm experimenting with. My guess is it is a very simple issue, but I just cannot figure it out.

The gist of the program is to create a very simple stats program which has a struct containing the raw data and variables to hold basic descriptive stats (mean, SD, N). And a series of functions to initialise the data, modify and print.

I'm just learning about pointers and thought I'd try implementing this using pointers as return values.

In the code below, I wanted to test whether my "initialse struct" function was working so added some printf statements into the main function to check. It seems to print the "n" value from the struct, but all other values were completely bizarre. Rather than copy the whole thing, I reduced the programme down to the code below. It still replicates the problem (the mean value is plain wrong).

I have tried various work arounds. Somewhat bizarrely (i.e. I have no idea why it works) if I insert a printf statement into the body of the initialisation function, it seems to print OK.

Here is the out put I got from running that code. Am working using XCode (standard tool)

New dataset, n = 101
New dataset, mean = -12978830180352.000000
The Debugger has exited with status 0.

Any help would be really appreciated....

Dave

Code:

#include <stdio.h>

struct dataSet
{
    struct dataSet * pdataSet;
    int n;
    float mean;
    float sd;
    int data[100];
} ;

struct dataSet * initDataSet(int n);


int main () 
{
    int n = 5;
    struct dataSet * pNewDataSet =  initDataSet(n);
    
    printf("\nNew dataset, n = %d", (*pNewDataSet).n);
    printf("\nNew dataset, mean = %f", (*pNewDataSet).mean);
    
    return 0;
}


struct dataSet * initDataSet(int n)
{
    struct dataSet dataSet1;
    
    dataSet1.pdataSet = &dataSet1;  //own address    
    dataSet1.n = 101;
    dataSet1.mean = 8;
    
    return dataSet1.pdataSet;  // return a pointer 
}

dataSet1 only lives as long as the function initDataSet is still running; once it stops, the dataSet1 variable dies with it. However, you're still holding a pointer to it, which now points into the deepest abyss. It is luck (good or bad, you decide) that the n printed correctly.

If you need a piece of memory to still be around after a function exits, you would need to acquire that piece of memory with malloc.

That is a very helpful reply. Thanks