Using for loop to find powers

[Noone99]

Hi all. I am trying to implement a code in a textbook but it is giving me an error
[code]
unsigned int
pow(int val,int exp)
{
for (unsigned int res=1;exp>0;--exp)
res=res*val;
return res;
}




#include <iostream>
extern unsigned int pow(int,int);
using namespace std;

int main()
{
int val=2;
int exp=15;

cout<<"The powers of 2\n";
for (int i=0;i<=exp;++i)
cout<<i<<": "
<< pow(val,i)<<endl;


return 0;
}
[code]

The error message I'm getting is

error: ‘res’ was not declared in this scope
14 | return int(res);
The output is supposed to look like

The Powers of 2

0:1
1:2
2:4
3:8
.....

Any help would be appreciated
Thank you

[seeplus]

That line is saying that the function pow() is defined in another compilation unit - which it isn't.

Replace that line with:

#include <cmath> 

Which textbook are you using?

[PS when I replied, the pow() function hadn't been provided]

[jonnin]

its /code in the ending block.
pow is the name used in cmath, perhaps that is a conflict. I highly advise using another name.

res is only scoped in the for loop. you can't return it because it was destroyed.
fix it by saying
int res{1};
for(...)
...
return res; //now its ok, it still exists here.

for fun (its not significantly better than the for loop) (the commented out chunks allow bigger exponents, but its rare to need more than ^16 at least in my code..). It does a lot less work for large exponents, which it turned out that I almost never use.. /facepalm

long long ipow(long long p, unsigned long long e) 
{ 
  const long long one = 1;
  const long long *lut[2] = {&p,&one};
  long long result = 1;
    result *= lut[!(e&1)][0]; p *= p;
	result *= lut[!(e&2)][0]; p *= p;
	result *= lut[!(e&4)][0]; p *= p;
	result *= lut[!(e&8)][0]; //p *= p;
	//result *= lut[!(e&16)][0]; p *= p;
	//result *= lut[!(e&32)][0]; p *= p;
	//result *= lut[!(e&64)][0]; 	
	return result;
}

[deleted account xyzzy]

@Noone99, you almost had it correct with code tags. You didn't use the proper closing tag.

PLEASE learn to use code tags, they make reading and commenting on source code MUCH easier.

http://www.cplusplus.com/articles/jEywvCM9/
http://www.cplusplus.com/articles/z13hAqkS/

HINT: you can edit your post and add code tags.

Some formatting & indentation would not hurt either

There are more tags you can (and should use), so do read the links above.

[JLBorges]

> The error message I'm getting is error: ‘res’ was not declared in this scope

In a for loop, the names declared by the init-statement (and the names declared in the condition) are within the scope of the for loop statement.

for( unsigned int res=1; exp>0 ; --exp ) { /* body of loop */ }
// res is out of scope now; it is no longer declared  

Rewrite as:

long long pow( int val, unsigned int exp )
{
    long long res = 1 ; // declared outside the for statement
    for( ; exp>0 ; --exp ) res *= val ;
    return res; // ok: res is in scope now
}

and you should be ok.