C linked list search function [closed]

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

/*Define custom functions */
void insertElement();
bool elementExists();
int getNumElements();


/*Create linked list */
struct node {
    int number;
    int occurence;
    struct node *next;
};

/*Call our linked list freqTable */
struct node *freqTable = NULL;

unsigned int numElements = 0;

int main(){
    int readNumElements = 0;
    int i = 0;
    int newNum, status;

    status = scanf("%d", &readNumElements);
    if(status == -1){
        fprintf(stderr, "%d is not a number\n", readNumElements);
        exit(-1);
    }


    for (i = 0; i < readNumElements;i++) {
        status = scanf("%d", &newNum);
        if(status == -1){
            fprintf(stderr, "%d is not a number\n", newNum);
            exit(-1);
        }
        if(elementExists(newNum)){
            printf("%d exists\n", newNum);
        }else{
            insertElement(&freqTable, newNum);
        }

    }

return 0;
}

void insertElement(struct node **list, int n){
    struct node *new_input; 

    new_input = malloc(sizeof(struct node));

    if(new_input == NU开发者_如何学运维LL){
        fprintf(stderr,"Error: Failed to create memory for new node\n");
        exit(EXIT_FAILURE);
    }

    new_input->number = n;
    new_input->occurence = 1;
    new_input->next = *list;
    numElements++;

    *list = new_input;
}

bool elementExists(int n){
    printf("%d\n", freqTable->number);
return false;
}

int getNumElements(){
    return numElements;
}

Ok heres what i got. This should compile.

Problem comes at

    if(elementExists(newNum)){
        printf("%d exists\n", newNum);
    }else{
        insertElement(&freqTable, newNum);
    }

I get segmentation error and i am not sure why.

---

In the function elementExists you need to ensure that freqTable is not NULL :

bool elementExists(int n){
 if(freqTable) {  // add this check
   printf("%d\n", freqTable->number);
 }
}

Also your elementExists does not do what its supposed to do(check for existence of a node with value n), you should do something like:

bool elementExists(int n) {

 if(!freqTale) { // table does not exist..return false.
  return false;
 }
 // table exists..iterate node by node and check.
 struct node *tmp = freqTable;
 while(tmp) { // loop till tmp becomes NULL
  if(*tmp == n) { // it node contains n..return false.
    return true;
   }
  tmp = tmp->next; // move on
 }
 return false; // n does not exist in the list..return false.
}

---

There's a problem with the call to elementExists:

if(elementExists(&freqTable, newNum)){

You pass the address of freqTable (i.e. a pointer to where the pointer to the first element of the list is stored) instead of its value. Yet in elementExists you dereference the argument as though it were a pointer to a list element:

printf("Compare with: %d\n", list->number);

and

else list = list->next;

Remove the & from the call to elementExists. And don't reference the global freqTable inside elementExists if you are passing the list as an argument.

---

As suggested in the comments, you're dereferencing freqTable in elementExists before any nodes are allocated. Thus, freqTable is always NULL, so you invoke undefined behavior. You could just add a check for NULL in elementExists, and return false in that case.