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Since there are 9592 prime numbers between 1 to n , I have declared an array a having 9592 elements. We know that 2 and 3 are primes so I have stored them in the array directly. Starting from 4 till 100000 , each number i is checked for divisibility starting from j = i / 2 till we find a number which divides i . When such a number is found, we know that i is not a prime and so a[n] (starting from n = 2) is assigned a value of -1 and n is incremented by 1. If i % j != 0, variable u is incremented by 1. For each i , if (u == (i / 2) - 1) , i is stored in the array a because it's a prime. (not divisible by any number in the range i / 2 to 2)