Question

So here’s my code… My understanding is that I’m supposed to create a function “map” that takes a function as an argument. It’s not going as planned. Any help would be completely amazing.

Here’s a compilable (well not compilable, but scaled down) version of the code:

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
int main(int argc, const char *argv[])
{
//it should be apparent that I am quite new to C, I have some java experience.
struct linkedList {
   int count;
   int num;
   struct linkedList *next;
};
struct linkedList *head, *tail, *curr;
int count1=0;
int i=0;

int square(int v) {return v=v*v;}

void map(int (*func)(int v), struct linkedList){
    struct linkedList2 *head, *tail, *curr;
    for(curr=head; curr!=NULL; curr=curr->next){
        curr->num=func(curr->num);
        printList();
    }
}

void start(){
    printf("This program will ONLY accept integer input.\n");
    head=NULL;
    for(i=1;i<=4;i++) {
        count1++;
        curr=malloc(sizeof(struct linkedList));
        curr->count=count1;
        printf("Enter a number: ");
        scanf("%d", &curr->num);
        if(head==NULL) { head=curr; }
        else { tail->next=curr; }
        tail=curr;
        tail->next=NULL;
    }
    printf("A list has been created.\n");
}

void printList(){
    printf("The list now contains these numbers: ");
    for(curr=head;curr!=NULL;curr=curr->next){
        printf("%d, ", curr->num);
    }
    printf("\n");
}

 start();
 printList();

 map(square, linkedList);
 printList();
 system("PAUSE");   
 return 0;
}

Answer 1

Defining all of those structs and functions inside of main is not how you’re supposed to write C. Move the int main(int argc, const char *argv[]) { to right after the definition of printList so that main only contains the actual code for main.

Moreover, your definition of map appears to have an unfinished prototype. Instead of void map(int (*func)(int v), struct linkedList), in which the second parameter is unused, you want void map(int (*func)(int v), struct linkedList* head) (and then get rid of the declaration of head on the next line). Moreover, linkedList2 here presumably should be changed to linkedList. In addition, your attempt to call map in main with map(square, linkedList) is nonsensical; you want to use map(square, head).