Organizing string of int - using debugger - c

Question

as i stated before, i am learning C from home, and hitting some bumps on the way. Thanks to you all i am learning bit by bit.
I wrote something that made sense in my head and taking in account what i've learned so far:
int exercicio4a(int *s){
int tmp, i;
int nmax = strlen(s);
int z[nmax];
for(i=0; i!=nmax; i++){
if(s[i]>=s[i+1]){
z[i]=s[i+1];
}else{
z[i]=s[i];
}
}
return z;
}
main(){
int x, y, nmax;
int str[10];
printf("Organize a bunch of numbers:\n");
nmax = 9;
while(nmax!=0){
printf("Enter the random int number %d :\n", nmax);
scanf("%d", &str[nmax]);
nmax--;
}
printf("%s \n", exercicio4a(str) );
}
Needless to say that this is garbage as it doesn't take me anywhere!
One last thing, i would like to know if there is a way to use GDB in order to detect a malfunction in this code for example.
Cheers!

Answer 1

Well, you need to clarify your terms. You want to sort an array of integers, which you scan from stdin, I assume. Don't call this array a string, it is bad terminology. You also need to decide if exercicio4a should return a string of ascii characters, where the integers are inserted into a string, or an array of integers.
You really should be taking an integer array for your function and either sorting in place, or allocating a new one and returning it.
Now let's look at the problems with your code:
nmax = 9;
while(nmax!=0){
printf("Enter the random int number %d :\n", nmax);
scanf("%d", &str[nmax]);
nmax--;
}
You are exiting the loop when nmax == 0, but you never scan an integer into that location. To fix this:
nmax=10;
while (nmax--) {
//printf
scanf("%d", str + nmax); //or &str[nmax]; these are equal
}
Next: int exercicio4a(int *s){... This signature is incorrect. Assuming you don't want to sort in place, you want to return a new array so you should have int *exercicio4a(int *s){... You want to return a pointer to the first integer in the array.
Next int z[nmax]; This array is a local variable. It will go out of scope when you return from the function so the behavior of using it after you return is undefined! Instead you want to dynamically allocate space for the sorted array using int *z = calloc(nmax, sizeof(int)); You will need to free the returned array after the calling function is done using it to avoid memory leaks.
Next: printf("%s \n", exercicio4a(str) ); Here you are trying to print an integer as a string (with the code you have). Even if you fix it to return an int* you will still be trying to print an array of integers as a string. What you should do is use a loop to print the array:
nmax = 10;
int *ptr = exercicio4a(str); //only after you fix exercicio
while (nmax--)
printf("%d ", *ptr++);
Lastly, your loop does not perform a sort. It is an attempt to swap adjacent elements that are out of order. It does not perform this functionality correctly, but don't bother getting that working since it won't even implement a sorting algorithm. I suggest you read about insertion, bubble, and selection sort to get a feel for how to implement those algorithms if you want to learn how to do sorts on your own. Most of the time though, you will want to use a library function to sort an array. If you want to keep an un-sorted copy of your data, make a qsort wrapper:
int compare (void *one, void *two) {
return *(int *)one - *(int *)two;
}
int *my_sort(int *arr, int size) {
int *sorted = calloc(size, sizeof(int));
memcpy(sorted, arr, size * sizeof(int));
qsort(sorted, size, sizeof(int), compare);
return sorted;
}