在C中搜索数组类型struct的元素

Question

I have an array of structures that store some basic informations about a pc within a company (code,model,connected to internet).

typedef struct database{
    char code[5];
    char brand[20];
    char model[20];
    int lab;
    int connect;
}database;

Now I want to search through the array to find the pc coresponding to the code C01A

here is a little example of info stored in the struct:

C01A HP SJH1740 1 0
B02A HP SJ1290 3 1
A03B DELL PQ240 2 1
A02B DELL PQ240 3 1
C09H FUJITSU NP0001 1 0
A06D DELL PQ240 3 1
C00X FUJITSU LP1050 2 0
B89A HP SJ1290 3 1
A03F DELL PT1000 2 0
C12P HP AA0012 1 1
D01D DELL BB2300H 3 0

You can see that the first pc has the code C01A . here we see 11 pc's which I randomly picked from the file in which there can be millions of pc's (Imagine a big company, array is able to hold 10 million pc's just because I can).

I could just search every single element in the array until I find the right one. but (using a quick calculation dunno if it's OK). this means I have to scroll trough 10.5MB of memory, which is quite a lot if the searching process is not optimalized.

So I came up with this:

step 1: sort the array when the program is started.

step 2: start at the middle of the array and check that element

step 3: if the element I want to search is smaller than the element checked, check the middle of the first half, else check the middle of the second half

step 4: repeat until the element is found or until the same element is checked twice, in which case the element does not exist.

Here is a fast code that I tried:

database searchpc(database temp[],int n){
    int i=n/2;
    char code [5];
    printf("incerici codice da cercare: ");
    scanf("%s",code);
    getchar();
    while(strcmp(temp[i].code,code)!=0){
        if(strcmp(temp[i].code,code)>0)
            i=i/2;
        else
            i=i*2;
    }
    return temp[i];
}

this returns the element if found, else it gets stuck in a loop (This is work in process)

I tested it on the info above and I searched, just as in the example, for the C01A pc. but it got stuck as well.

What am I doing wrong? Can this be done even faster when I realy have to go trough a lot of pc's?

Answer 1

如果i为零，并且尝试将其减半，或者i * 2大于n ，则找不到该结构。

Answer 2

What you trying to implement is binary search. Your implementation is not correct. It should be similar to this:

/*
* searches for a value in sorted array
*   arr is an array to search in
*   value is searched value    
*   left is an index of left boundary
*   right is an index of right boundary
* returns position of searched value, if it presents in the array
* or -1, if it is absent
*/

int binarySearch(int arr[], int value, int left, int right) {
      while (left <= right) {
            int middle = (left + right) / 2;
            if (arr[middle] == value)
                  return middle;
            else if (arr[middle] > value)
                  right = middle - 1;
            else
                  left = middle + 1;
      }
      return -1;
}