Program Crashes

**astroboy739** · 08-19-2012

Hello,

I am a beginner for C Programming and is taking an introductory course over the summer. Right now I'm having some trouble with this programming assignment where it asks me to enter a resistance and voltage value for the input in order to calculate the min, max, and avg values for the voltage, current, and power across a network of resistors by using arrays and functions. But when I tried to compile my program, it crashes when I enter any input values. Here is my code below:

Code:

#include <stdio.h>

int ReadRes(int n, double *r, double *v);                                                // The function that will read the resistance and voltage. *r and *v are pointers while n is the resistor #.

void MinMaxAvg(double x[], int n, double *min, double *max, double *avg);                // The function that will calculate the quantities for each input as the pointers included in this function will end up being used to calculate the outputs.

int main ()    {
    
double R, V, I_min, P_min, V_min, I_max, P, P_max, V_max, I_avg, P_avg, V_avg;
    
double I; // Current values
    
int n; // # of elements in array
    

/* R and V are the resistance and voltage values to be entered by the user. 
    The other declared double variables will be used to calculate the min, max, and avg for current and power. */
    

for (n = 1; n <= 20; n++)    {
        printf(
"Enter R and V for resistor %d: ", n);
        ReadRes(n, &R, &V);
        
// Calculate min, max, and average values for current
        MinMaxAvg(&V, n, &V_min, &V_max, &V_avg);
        
        I = V / R;
        
        MinMaxAvg(&I, n, &I_min, &I_max, &I_avg);
        P = V * I;
        MinMaxAvg(&P, n, &P_min, &P_max, &P_avg);
                
        printf("    Voltage        Current        Power\n");
         printf("MIN       %.4lf   %.4lf   %.4lf\n", V_min, I_min, P_min);
        printf("MAX       %.4lf   %.4lf   %.4lf\n", V_max, I_max, P_max);
        printf("AVG       %.4lf   %.4lf   %.4lf\n", V_avg, I_avg, P_avg);
        }
    
return 0;
}


int ReadRes(int n, double *r, double *v)    {
    
int scanfRetVal = scanf("%lf %lf", *r, *v);
    
while (1)    {
        
if (*r > 0)    {
            
if (scanfRetVal < 2)    {
                
return -1;
            }
            
else
                
if (scanfRetVal == EOF)    {
                
return 0;                        // Indicates no valid data read
                
break;
                }
                
else
                    
return 1;
                    
break;
        }
        
else
    printf("Error: R must be greater than 0\n");
    }
}

void MinMaxAvg(double x[], int n, double *min, double *max, double *avg)    {
    
int i;
    
double sum;
    sum = 0;
    *min = x[0];
    
for (i = 0; i < n; i++)    {
        
if (x[i] < *min)
            *min = x[i];
    }
    *max = x[0];
    
for (i = 0; i < n; i++)    {
        
if (x[i] > *max)
            *max = x[i];
    }
    
    
for (i = 0; i < n; i++)    {
        sum += x[i];
    }
    *avg = sum / n;
}

Another thing to note about this program is that when the user enters CTRL-Z to end the loop, the main program should look at the return value from ReadRes() function and use it to determine what to do next.

In case you need to know what the output of this program should turn out to be it should look something like this:

Attachment 11910

So what's causing my program to crash? Any help would be greatly appreciated. Thanks.

**std10093** · 08-19-2012

Program crashes when input is given,so you should check the part where the input is handled by your code.Indeed,in function redRes where input is handled,the scanf is not called as it ought to be.When scanf if called we tend to pass the addresses of the arguments.Here the variables you want to pass to scnaf are with pointers(*).So when you write *r you refer to the value of r.So how are you going to refer to the adress of it?By typing r.Try this.If you have any question please ask!
Also i saw that n is not used in redRes function.So why do you pass it as an argument?

PS-Welcome to the forum

**astroboy739** · 08-19-2012

Hi! So I followed your procedure (not sure I did it correctly) and now I'm stuck at a runtime error #3 where the variable scanfRetVal is uninitialized. My question is, am I supposed to put the scanf in the main function or in the ReadRes function? What happened was I put the scanf in the main function and then I followed it up by using the ReadRes function after it.

**std10093** · 08-19-2012

I think that it very elegant to have a function that fully handles input.So my suggestion is to put the scanf inside the function.If you wish ,you may post your code here in order to find the problem

**astroboy739** · 08-19-2012

Okay so here's what I've come up with so far. The code below only deals with the main function.

Code:

for
 (n = 1; n <= 20; n++)    {

        printf(
"Enter R and V for resistor %d: ", n);

        scanfRetVal = scanf(
"%lf %lf", &R, &V);

        ReadRes(&R, &V);

The other code below deals with the ReadRes function.

Code:

int
 ReadRes(double *r, double *v)    {

    
int scanfRetVal;

    scanfRetVal = scanf(
"%lf %lf", *r, *v);


    
while (1)    {

        
if (*r > 0)    {

            
if (scanfRetVal < 2)    {

                
return -1;

            }

            
else

                
if (scanfRetVal == EOF)    {

                
return 0;                        // Indicates no valid data read

                
break;

                }

                
else

                    
return 1;

                    
break;

        }

        
else

            printf(
"Error: R must be greater than 0\n");

    }

}

**std10093** · 08-19-2012

You mean you call scanf twice per resistor?That's wrong.You should call it once per resistor(either in main or in function(i recommend it)).

**astroboy739** · 08-19-2012

Ah my bad. So I removed the scanf from the main function, but now when I compile, the program crashes again. Do you know what the problem is?

**std10093** · 08-19-2012

The problem is what i posted you from start.Inside the function you do not call the scanf as you ought to do!You should pass the adrresses of the variables,not the values of them

Here the program crashes

Code:

  scanfRetVal = scanf(
"%lf %lf", *r, *v);

Useful tip : Use printf functions where you think your code might has errors.This way you find at exactly which line your program crashes.
e.g.

Code:

..
printf("here\n");
a line of code with no problem
printf("here1\n");
the line where your program crashes!!!
printf("here2\n");
....

the output will be

Code:

here
here1
-then the program crashes(this will not be shown in screen of course)

This way you are able to spot the line that causes your program to crash.
Have questions?Ask

**astroboy739** · 08-19-2012

From my understanding of what you meant, I got rid of the pointers (*) from the ReadRes function. But by doing that now I got another runtime error where the local variable V is being uninitialized. Here's what I have so far.

Code:

for
 (n = 1; n <= 20; n++)    {

        printf(
"Enter R and V for resistor %d: ", n);

        ReadRes(R, V); // the error occurs here

And here's the revised code of the function.

Code:


int
 
 
 
 ReadRes(double r, double v)    {

 
    
 
 
int scanfRetVal;

 
    scanfRetVal = scanf(
 
 
"%lf %lf", &r, &v);

**std10093** · 08-19-2012

You have to remember two important things

scanf(because of her prototype(The C Library Reference Guide at 2.12.4.2 ) you have to pass the arguments by their addresses,not by their value.
e.g.

Code:

#include <stdio.h>

int main(void)
{
    int number;
    printf("Please enter a number\n");
    scanf("%d",&number);
    printf("Number given is %d\n",number);
    return 0;
}

Output

Code:

Please enter a number : 5
Number given is 5

Mind that * and & when written together deactivate each other.
e.g. You have a function print which needs the argument by value.See the code
Every human programmer ( ) would write something like this
Code:
```
#include <stdio.h>

void print(int n);

int main(void)
{
    int number;
    printf("Please enter a number : ");
    scanf("%d",&number);
    print(number);
    return 0;
}

void print(int n)
{
    printf("Number given is %d\n",n);
}
```
However you could call function print like this(not recommended)
Code:
```
print(*&number);
```
The point is that print(*&number) is equivalent to print(number).So the postscript is that operators * and & kill each other!

When you want a function to maintain the changes in values of arguments that are passed to her,even after the function is terminated you have to pass the argument by reference.Otherwise the modifications made to argument are lost when the function terminates
e.g.

Code:

#include <stdio.h>

void test(int val,int arg);
void modificationsRemain(int *val,int *arg);

int main(void)
{
    int a,b;
    a=5;
    b=6;
    test(a,b);
    printf("After function test : a= %d and b= %d\n",a,b);
    modificationsRemain(&a,&b);
    printf("After function modificationsRemain : a= %d and b= %d\n",a,b);
    return 0;
}

void test(int val,int arg)
{
    val=1000;
    arg=2727;
}

void modificationsRemain(int *val,int *arg)
{
    *val = 55;
    *arg = 66;
}

and the output is

Code:

After function test : a= 5 and b= 6
After function modificationsRemain : a= 55 and b= 66

Back to first part of code i wrote,we are going to use scanf inside a function.Let's see the code
Code:
```
#include <stdio.h>

void print(int n);
/*Argument passed by reference in order 
 * to maintain modifications*/
void input(int *n);

int main(void)
{
    int number;
    printf("Please enter a number : ");
    input(&number);
    print(number);
    return 0;
}

void input(int *n)
{
    scanf("%d",n);
}

void print(int n)
{
    printf("Number given is %d\n",n);
}
```
and the output
Code:
```
Please enter a number : 5
Number given is 5
```
So why do we call scanf with just n and not &n?Because inside the body of the function input,the value of n is accessed by
Code:
```
*n
```
so we need the address of n,so actually what we are doing is to request for the address by writting
Code:
```
scanf("%d",&*n);
```
but because & and * kill each other(and vice versa) we simply write
Code:
```
scanf("%d",n);
```
Better now?Try to understand what i said and apply these to your code.If questions are in your head do not hesitate to post back

**astroboy739** · 08-19-2012

That helps A LOT! Thank you so much! That explanation right there helped me to finally fix those crash issues as well as the errors I keep getting. I really appreciate it.

And my apologies for misunderstanding your previous messages, I'm still in the learning process of understanding how pointers work exactly because the topic is probably one of the most difficult to learn in C Programming.

**std10093** · 08-20-2012

No need to apology astroboy! Keep improving yourself

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