Test code:
#include <cmath>
#include <cstdio>
const int N = 4096;
const float PI = 3.1415926535897932384626;
float cosine[N][N];
float sine[N][N];
int main() {
printf("a\n");
for (int i = 0; i < N; i++) {
for (int j = 0; j < N; j++) {
cosine[i][j] = cos(i*j*2*PI/N);
sine[i][j] = sin(-i*j*2*PI/N);
}
}
printf("b\n");
}
Here is the time:
$ g++ main.cc -o main
$ time ./main
a
b
real 0m1.406s
user 0m1.370s
sys 0m0.030s
After adding using namespace std;
, the time is:
$ g++ main.cc -o main
$ time ./main
a
b
real 0m8.743s
user 0m8.680s
sys 0m0.030s
Compiler:
$ g++ --version
g++ (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2
Assembly:
Dump of assembler code for function sin@plt:
0x0000000000400500 <+0>: jmpq *0x200b12(%rip) # 0x601018 <_GLOBAL_OFFSET_TABLE_+48>
0x0000000000400506 <+6>: pushq $0x3
0x000000000040050b <+11>: jmpq 0x4004c0
End of assembler dump.
Dump of assembler code for function std::sin(float):
0x0000000000400702 <+0>: push %rbp
0x0000000000400703 <+1>: mov %rsp,%rbp
0x0000000000400706 <+4>: sub $0x10,%rsp
0x000000000040070a <+8>: movss %xmm0,-0x4(%rbp)
0x000000000040070f <+13>: movss -0x4(%rbp),%xmm0
0x0000000000400714 <+18>: callq 0x400500 <sinf@plt>
0x0000000000400719 <+23>: leaveq
0x000000000040071a <+24>: retq
End of assembler dump.
Dump of assembler code for function sinf@plt:
0x0000000000400500 <+0>: jmpq *0x200b12(%rip) # 0x601018 <_GLOBAL_OFFSET_TABLE_+48>
0x0000000000400506 <+6>: pushq $0x3
0x000000000040050b <+11>: jmpq 0x4004c0
End of assembler dump.
You're using a different overload:
Try
double angle = i*j*2*PI/N;
cosine[i][j] = cos(angle);
sine[i][j] = sin(angle);
it should perform the same with or without using namespace std;