Thread: Pointer problem I think

Hey guys, I am having a problem using pointers. In function process_current, I call the function assign_vals, at the end of assign vals, we have values for v1 and v2, but when printed out the values in process_current, we're back at zero?

Please help, here is the code:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

const char VOLTAGE[] = "voltage";
const char CURRENT[] = "current";
const char RESISTANCE[] = "resistance";

void process_voltage(double current, double resist);
void process_resistance(double current, double voltage);
void process_current(double voltage, double resist);
void assign_vals(const char s1[], double *v1, const char s2[],
                 double *v2);
void display_result(const char var1[], double *v1, const char var2[],
                    double *v2, const char var3[],double *v3);

int main()
{
        double voltage = 0;
        double resist = 0;
        double current = 0;
        char unknownval[12];
        int read;

        printf("What do you wish to know:  ");
        read = scanf("%s", unknownval);

        while(read == 1)
        {
           if(!strcmp (unknownval, VOLTAGE))
           {
              process_voltage(resist, current);
           }

           else if(!strcmp (unknownval, RESISTANCE))
           {
              process_resistance(voltage, current);
           }

           else if(!strcmp (unknownval, CURRENT))
           {
{
        double voltage = 0;
        double resist = 0;
        double current = 0;
        char unknownval[12];
        int read;

        printf("What do you wish to know:  ");
        read = scanf("%s", unknownval);

        while(read == 1)
        {
           if(!strcmp (unknownval, VOLTAGE))
           {
              process_voltage(resist, current);
           }

           else if(!strcmp (unknownval, RESISTANCE))
           {
              process_resistance(voltage, current);
           }

           else if(!strcmp (unknownval, CURRENT))
           {
              process_current(voltage, resist);
           }

           else
           {
              printf("Incorrect unknown value %s. ", unknownval);
              printf(" Exiting.\n");
              exit(1);
           }

           read = scanf("%s", unknownval);
        }
         
        printf("\n");
            
        return 0;
}

void process_voltage(double resist, double current) 
{
        double voltage;
            
        assign_vals(RESISTANCE, &resist, CURRENT, &current);

        voltage = resist * current;
           }
        
           else
           {
              printf("Incorrect unknown value %s. ", unknownval);
              printf(" Exiting.\n");
              exit(1);
           }
        
           read = scanf("%s", unknownval);
        }
         
        printf("\n");
            
        return 0;
}

void process_voltage(double resist, double current) 
{
        double voltage;
            
        assign_vals(RESISTANCE, &resist, CURRENT, &current);

        voltage = resist * current;
              
        display_result(RESISTANCE, &resist, CURRENT, &current,
                       VOLTAGE, &voltage);
}
            
void process_resistance(double current, double voltage)
{
        double resist;
            
        assign_vals(VOLTAGE, &voltage, CURRENT, &current);

        resist = voltage / current;
         
        display_result(VOLTAGE, &voltage, CURRENT, &current,
                       RESISTANCE, &resist);
}
 
void process_current(double voltage, double resist)
{
        double current;

        assign_vals(VOLTAGE, &voltage, RESISTANCE, &resist);

    //Values in voltage and resist are zero, not what they were in assign_vals
        printf("%lf", voltage);
        printf("%lf", resist);
        current = voltage / resist;

        display_result(VOLTAGE, &voltage, RESISTANCE, &resist,
                       CURRENT, &current);
}
         
void assign_vals(const char var1[], double *v1, const char var2[],
                 double *v2)
{
        double value1;
        double value2;
        char str1[12];
        char str2[12];  

        printf("First variable:  ");
        scanf("%s", str1);
        printf("Value:  ");
        scanf("%lf", &value1);
        printf("Second variable:  ");
        scanf("%s", str2);
        printf("Value:  ");
        scanf("%lf", &value2);
 
        if(!strcmp(str1, str2))
        {
           printf("not enough unique variables. exiting\n");
           exit(1);
        }
            
        else if((!strcmp(str1, var1) && !strcmp(str2, var2)))
        {
           v1 = &value1;
           v2 = &value2;
        }

        else if((!strcmp(str1, var2) && !strcmp(str2, var1)))
        {
           v1 = &value2;
           v2 = &value1;
        }
 
        else
        {
           printf("variables are not valid for this option.");
           printf("  exiting\n");
           exit(1);
        }

   //Prints out the values in v1 and v2 correctly, now go back to process_current   //function
        printf("%lf", *v1);
        printf("%lf", *v2);
}

void display_result(const char str1[], double *v1, const char str2[],
                    double *v2, const char str3[], double *v3)
{
        printf("\n");
        printf("%s = %.2lf", str1, *v1);
        printf("%s = %.2lf", str2, *v2);
        printf("%s = %.2lf", str3, *v3);
}

Let me know if I am not making sense. Thanks for the time.

The following, in assign_vals():

Code:

v1 = &value2;

sets the local variable (parameters are local to a function) v1 to have the value &value2. What you want is:

Code:

*v1 = value2;

This says to store value2 in the location that v1 points to.

Originally Posted by cas

The following, in assign_vals():

Code:

v1 = &value2;

sets the local variable (parameters are local to a function) v1 to have the value &value2. What you want is:

Code:

*v1 = value2;

This says to store value2 in the location that v1 points to.

IT WORKS! Thank you so much and thanks for the reasoning behind it. Greatly appreciated.