Looping through linked list - endless loop

Question

Teaching myself to program, I decided to make a simple wattage calculator. Each state has a different (average) price/KWh. My problem starts a line 34 in the code. The curr->next is never null.

What am I doing wrong?

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


float calculate(float w, float cents, float h);

struct states 
{
    char *name;
    char *price;
    struct states *next;
};

int main()
{
    float watts,hours,cents;
    int a = 0;
    struct states *curr, dict[52];
    char st_ab[52][3] = {"ID","ND","WA","AR","UT","LA","WY","SD","NE","KY","WV","OK","OR","MT","MS","TN","IN","NC","MS","IA","KS","VA","NM","MN","SC","AZ","GA","AL","CO","OH","TX","IL","FL","NV","MI","WI","PA","MD","DC","DE","MA","RI","CA","ME","VT","NJ","NH","AK","CT","NY","HI"};
    char st_pr[52][5] = {"8.0","8.1","8.2","8.7","8.8","8.9","8.9","9.0","9.0","9.0","9.2","9.2","9.4","9.6","9.7","9.8","10.0","10.2","10.3","10.5","10.5","10.5","10.7","10.9","11.0","11.1","11.1","11.1","11.2","11.2","11.3","11.7","11.7","11.7","13.0","13.0","13.2","13.7","13.7","13.7","14.8","14.9","15.2","15.5","16.1","16.3","16.5","17.5","18.1","18.1","33.2"};
    char state[2];

    for(a=0;a<=52;a++)
    {
        memset(&dict[a],0,sizeof(struct states)); /* zero out structure */
        if(a==52)
        {
            break;  /* if this is the last struct, leave it NULL so we can loop through linked list */
        }
        dict[a].next=&dict[a+1];        
    }

    for(curr = dict;curr ->next != NULL;curr = curr->next)  /* fill our list with state abbr. and cost/KWh */
    {
        curr->name = st_ab[a];
        curr->price = st_pr[a];
        a++;
    }

    puts("Dictionary loaded!");
    printf(" Two letter state code: ");                 /* get info from user */
    scanf("%s",state);

    printf(" Wattage of appliance: ");
    scanf("%f",&watts);

    printf(" Hours of use per day: ");
    scanf("%f",&hours);

    for(curr = dict; curr->next != NULL; curr=curr->next)
    {
        printf("comparing %s with %s\n", curr->name,state);  /* search for our state, set cents respectively */
        if((strcmp(curr->name,state)) == 0)
        {
            printf("State found! -- %s\n",state);
            cents=atof(curr->price);
            printf("%f cents\n",cents);
            break;
        } else {
            printf("state didn't match\n");
            continue;
        }
    }

    printf("\nAverage cost per day: %.2f\n",  calculate(watts,cents,hours));
    printf("Average cost per year: %.2f\n\n", (calculate(watts,cents,hours)) * 365);    

    return 0;

}

float calculate(float w, float c, float h)
{   
    float kwh = (w/1000) * h;
    float cos = kwh*(c/100);
    return(cos);
}

for(a=0;a<=52;a++) The last valid index is 51.

Daniel Fischer
– Daniel Fischer

2013-03-28 14:43:35 +00:00
Commented Mar 28, 2013 at 14:43 — Daniel Fischer
– Daniel Fischer, Commented Mar 28, 2013 at 14:43

Fred Foo · Accepted Answer · 2013-03-28 14:49:34Z

2

The problem is in

dict[a].next = &dict[a+1];

The C standard guarantees that &dict[a+1] is never NULL, even though it is one past the end of the array. This causes the next loop to go past the end of the list, with undefined behavior resulting (the program might do anything). After the first loop, do

dict[51].next = NULL;

to end the linked list.

EDIT: with for(a=0;a<=52;a++) you actually walk two elements past the end of the array, so the behavior is undefined in two places. To fix that, you need to loop until a<52.

edited Mar 28, 2013 at 14:49

answered Mar 28, 2013 at 14:43

Fred Foo

365k80 gold badges765 silver badges852 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

elixxir Over a year ago

Thanks so much for the fast response! After some other minor tweaking it's working wonderfully now!

SomeWittyUsername · Accepted Answer · 2013-03-28 14:43:43Z

0

"curr->next is never null" - well, how do you expect your for loop to stop if your stopping condition is curr->next that equals to null?

answered Mar 28, 2013 at 14:43

SomeWittyUsername

18.4k3 gold badges46 silver badges87 bronze badges

Collectives™ on Stack Overflow

Looping through linked list - endless loop

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related