if (num < 0) {
  //negative
}
if (num > 0) {
  //positive
}
if (num == 0) {
  //neither positive or negative,
}

或者使用else if

if (num < 0) {
  //negative
}
if (num > 0) {
  //positive
}
if (num == 0) {
  //neither positive or negative,
}

或者使用else if

bool positive = number > 0;
bool negative = number < 0;

你只需要比较是否值和它的绝对值相等:

if (value == Math.abs(value))
    return "Positif"
else return "Negatif"

当然没有人给出正确答案，

num != 0   // num is positive *or* negative!

这是行业标准:

int is_negative(float num)
{
  char *p = (char*) malloc(20);
  sprintf(p, "%f", num);
  return p[0] == '-';
}