Doubt in pow function output

Projects Programming
1

Hi everyone, I executed pow() within for loop. while executing this output in integer data type. I get a output like resultant value -1.I do not know Why I am getting this output like this. Can anyone clarify my doubt. Below I attached my program.

void setup() {
  // put your setup code here, to run once:
Serial.begin(9600);

int powValue;
for(int i=1;i<8;i++){
  powValue=(int)pow(2,i);
  Serial.println(powValue);
}
}

void loop() {
  // put your main code here, to run repeatedly:

}

OUTPUT will be,
2
3
7
15
31
63
127

2

Why use a floating point function, to calculate integer powers of integers?

for(int i=1;i<8;i++){
  int powValue = 1 << i;
  Serial.println(powValue);
}
3 
void setup() {
  Serial.begin(250000);
  int powValue;
  float powValueF;
  for (int i = 1; i < 8; i++) {
    powValueF = pow(2, i);
    powValue = (int)powValueF;
    Serial.print(F("pow(2, "));
    Serial.print(i);
    Serial.print(F(") = "));
    Serial.print(powValueF, 6);
    Serial.print(F(", which gives after integer truncation "));
    Serial.println(powValue);
  }
}
void loop() {}

pow(2, 1) = 2.000000, which gives after integer truncation 2
pow(2, 2) = 4.000000, which gives after integer truncation 3
pow(2, 3) = 7.999998, which gives after integer truncation 7
pow(2, 4) = 15.999997, which gives after integer truncation 15
pow(2, 5) = 31.999988, which gives after integer truncation 31
pow(2, 6) = 63.999977, which gives after integer truncation 63
pow(2, 7) = 127.999954, which gives after integer truncation 127
4

Thank you for your clarification.

5

The other thing to consider is that in using int you're inviting negative numbers, as soon as you overflow 15 bits.

6

the built-in function round() is useful when converting floats to int - the default cast truncates towards
zero which is not always what you want.

7

Adding 0.5 before casting also avoids the truncate and rounds.

8

Whandall:
Adding 0.5 before casting also avoids the truncate and rounds.

No, it fails for negative values completely.

9

In the context of pow(2,x) there are no negative values.