Sorting an array by second string

Question

I have a textfile with name followed by a number that is the priority of the name, now I'm trying to sort the textfile by priority and write a new file.

old
name1 1
name2 2
name3 3
name4 1
name5 1
name6 2 
name7 1
name8 3

new
name 1 1
name4 1
name5 1
name2 2
name6 2
name3 3
name8 3

I have achieved to get the old textfile in an array, but I'm stuck with sorting that array by priority. I shouldn't loop through the file again, just wanna sort the array then write that new sorted array to a new text file. How should I proceed??

The code

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


typedef struct{
    int p;
    char *name;
}names;

int main(void){
    FILE *old= fopen("old.txt", "r");
    FILE *new = fopen("new.txt", "w");

    char n[10];
    int i =0;

    names *name= malloc(sizeof(names));

    for(i; i<count; i++){
        int p;

        char *n= malloc(sizeof(char) * 4); 
        fscanf(old, "%s %i", n, &p); 

        names[i].name= n; 
        names[i].p= p;

    }
    int j=0;

    for(i=0; i < count;i++){

    }

    return 0;
}

You've done good work. You've tested it in pieces and you have something that solves part of the puzzle. The next step is to describe in words how you want to sort the data. Are elements one and two sorted -- yes. Are element two and three sorted yes. Are elements 3 and sorted -- no. Then do what, ... So in words describe what you want to program next. Then put that in the question and make an attempt to do it. — dcaswell
– dcaswell, Commented Aug 27, 2013 at 7:34
fscanf(eerste, "%s %f", name, &prio);, be careful, prio is an int and you are scanning a float — David Ranieri
– David Ranieri, Commented Aug 27, 2013 at 7:42
You only seem to have allocated space for one job, yet you're trying to read in 8 jobs and store them in unallocated memory, — Paul R
– Paul R, Commented Aug 27, 2013 at 7:44
It would be simpler to implement this if you declared job as { int prio; char jobName[8]; }; not having to deal with a separate malloc() just for the (very short) string will make it both easier and faster. — unwind
– unwind, Commented Aug 27, 2013 at 7:55

verbose · Accepted Answer · 2013-08-27 09:17:16Z

Assuming that you cannot use qsort() because one of your constraints is to write your own sorting algorithm, here goes.

What do you need to sort? An array ofjob.

What is the key by which you need to sort? job.prio.

So how to sort? Any common selection, insertion, or (eek) bubble sort will do. (Though if you go down the bubble sort route, at least make it sexy and do a cocktail shaker sort.) Just compare two job.prios, both are ints so that's not hard, and swap the places of their respective job structures in the array as needed.

Here is a selection sort algorithm that will work. You can find plenty of others on Google.

void selectionSort (job* jobs, int size) {

    int smallest;
    job temp;

    for (int i = 0; i < size - 1; i++) {
        smallest = i;
        for (int walk = i + 1; walk <= size - 1; walk++) {
            if (jobs[walk].prio < jobs[smallest].prio)
                smallest = walk;
        }  // end inner loop
        temp = jobs[i];
        jobs[i] = jobs[smallest];
        jobs[smallest] = temp;
    }   // end outer loop
    return;
}

Fairly simple; it's just like any old selection sort. But selection sort is boring. Try to do an insertion sort, now that the selection sort gives you the general idea of how to swap the elements in the array.

Note that there are other problems with your code, as folks have pointed out: you are allocating space for only one job in your array, but you need eight; you have undefined variables like eerste; you have name declared twice, once as a char* and once as an array of char. So there's plenty of cleanup, but hopefully you have enough idea now to be able to finish your assignment.

unwind · Accepted Answer · 2013-08-27 08:28:41Z

1

You should sort using the qsort() standard library function.

It will require you to implement a function that compares two job structures, in order to provide the sorting.

Here's one attempt:

static int compare_jobs(const void *a, const void *b)
{
  const job *ja = a, *jb = b;

  return ja->prio < jb->prio ? -1 : ja->prio > jb->prio;
}

You will need to call this properly (read the documentation!), then loop over your array and write the contents to a new file.

edited Aug 27, 2013 at 8:28

answered Aug 27, 2013 at 7:56

unwind

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1 Comment

fhahn Over a year ago

Qsort is part of libc (stdlib.h) and you are already using malloc, which is also part of stdlib.h, so you won't need an extra library

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