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# Function checking a simple number

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Need to check whether the number is simple using the function.
I decided that way, but it works not to the end correctly. I can not catch the reason …

``````int prime (unsigned int num {
For (int i = 2; I * i & lt; = num; i ++) {
if (num% i == 0) {
Return 0;
}
}
Return 1;
}
``````

## Answer 1, Authority 100%

``````bool is_prime (unsigned p) {
If (P & LT; 2) Return False;
if (p == 2) Return True;
if (p% 2 == 0) Return False;
Double Limit = SQRT (P);
For (unsigned i = 3; i & lt; = limit; i + = 2) {
if ((p% i) == 0) Return False;
}
RETURN TRUE;
}
``````

Small optimization. First, check the `num `equal to 2 (simple), less than 2 and num even number (not simple). Then you can only sore odd:

``````uint32_t i, lim = sqrt (num) + 1;
for (i = 3; i & lt; lim; i + = 2)
if (num% i == 0)
Return 0;
Return 1;
``````

looked asm. code. Despite -o3 a little better:

``````uint32_t i, lim = ((uint32_t) SQRT (NUM)) + 1;
``````

@vladd , it’s not for this task. The sieve is the search all simple (specified them the number of ). Here you need to check only one number.

Here’s the solution on C++. You want to rewrite onto a Source: Akina Reshelo

``````# include & lt; iostream & gt;
#Include & lt; vector & gt;
Using Namespace STD;
BOOL IS_PRIME (unsigned int num) {
int limit = num;
INT SQR_LIM;
Vector & lt; bool & gt; IS_PRIME_F (Num + 10);
int x2, y2;
int i, j;
int n;
// Initialization of the sieve
SQR_LIM = (INT) SQRT ((Long Double) Limit);
for (i = 0; i & lt; = limit; i ++) is_prime_f [i] = false;
IS_PRIME_F [2] = TRUE;
is_prime_f [3] = True;
// Presumably simple - it is integer with an odd number
// Views in these square forms.
// X2 and Y2 are squares I and J (optimization).
x2 = 0;
For (i = 1; i & lt; = SQR_LIM; I ++) {
x2 + = 2 * i - 1;
y2 = 0;
For (j = 1; j & lt; = sqr_lim; j ++) {
y2 + = 2 * j - 1;
n = 4 * x2 + y2;
if ((N & LT; = Limit) & amp; & amp; (n% 12 == 1 || n% 12 == 5))
is_prime_f [n] =! is_prime_f [n];
// n = 3 * x2 + y2;
n - = x2; // Optimization
if ((N & LT; = Limit) & amp; & amp; (n% 12 == 7))
is_prime_f [n] =! is_prime_f [n];
// n = 3 * x2 - y2;
n - = 2 * y2; // Optimization
if ((i & gt; j) & amp; & amp; (n & lt; = limit) & amp; & amp; (n% 12 == 11))
is_prime_f [n] =! is_prime_f [n];
}
}
// Count the multiple squares of prime numbers in the interval [5, SQRT (Limit)].
// (the main stage can not cut them out)
For (i = 5; I & lt; = SQR_LIM; I ++) {
if (IS_PRime_F [i]) {
n = i * i;
For (j = n; j & lt; = limit; j + = n) {
IS_PRIME_F [J] = false;
}
}
}
// Conclusion of a list of prime numbers into the console.
// Printf ("2, 3, 5");
if ((i == 2) || (i == 3) || (i == 5))
RETURN TRUE;
For (i = 6; i & lt; = limit; I ++) {// Added a varying test at 3 and 5. In the original version of the algorithm, there is no need for it.
if ((IS_PRIME_F [i]) & amp; & amp; (i% 3! = 0) & amp; & amp; (i% 5! = 0)) {
// Printf (",% d", i);
if (i == num)
RETURN TRUE;
}
}
Return 0;
}
Int Main (int argc, char * argv []) {
if (IS_PRime (983)) {
COUT & LT; & LT; "Prime" & lt; & lt; Endl;
} else {
COUT & LT; & LT; "NOT PRIME" & LT; & LT; Endl;
}
Return 0;
}
``````

I agree with the Renegator. For VBA (perhaps someone needs) String function, Returning all squares of prime numbers

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