Select record uniformly

You need read a lot of records and you don’t know how many records here before you complete it. After you read all those records, you need select one record randomly. In another words, every record has same chance to be selected. The memory is limited, that means you cannot store all those records at one time.

01 int pickRecord(int* A, int size)
02 {
03     int tmp=A[0];
04     srand(time(0));
05     for(int i=1;i<size;i++){
07         float p=(float)1/(i+1);
08         float q=((float)rand())/((float)RAND_MAX);
09         if(q<p)
10             tmp=A[i];
11     }
12     return tmp;
13 }
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