‘abs’ was not declared in this scope - c++

Question

I''m working on GNURadio. Included math.h in my program but still get an error saying that abs was not declared in this scope.
Here is my header file.
#ifndef INCLUDED_SLM_H
#define INCLUDED_SLM_H
#include "candidate_t.h"
namespace gr {
namespace uwspr {
class SLM {
public:
// returns frequency drift according to the straight line model
float slmFrequencyDrift(mode_nonlinear m_nl, float cf, float t);
// generator of trajectory parameters for the straight line model
bool slmGenerator(mode_nonlinear *m_nl);
// initialize the generator
void slmGeneratorInit();
// index of current instances
int current;
SLM(){};
~SLM(){};
};
} // namespace uwspr
} // namespace gr
#endif /* INCLUDED_SLM_H */
Here is my code snippet for main function.
#include "slm.h"
#include <math.h>
#ifndef DEBUG
#define DEBUG 1 // set debug mode
#endif
#include "debugmacro.h"
namespace gr {
namespace uwspr {
// Returns frequency drift (in Hz) due to the Doppler effect according to
// the straight line model, described in paper:
float SLM::slmFrequencyDrift(mode_nonlinear m_nl, float cf, float t)
// m_nl = trajectory parameters
// t = time, in seconds
{
const float c = 1500.0; // sound speed (m/s)
// sign of velocity vector
float Sign = (((m_nl.V1 * t + m_nl.p1) * m_nl.V1 +
(m_nl.V2 * t + m_nl.p2) * m_nl.V2) > 0) *
2 -
1;
//
double numerator = abs(m_nl.V1 * (m_nl.V1 * t + m_nl.p1) +
m_nl.V2 * (m_nl.V2 * t + m_nl.p2));
// norm of connectiong vector (Eq. 16)
double denominator =
sqrt(pow(m_nl.V1 * t + m_nl.p1, 2) + pow(m_nl.V2 * t + m_nl.p2, 2));
if (denominator == 0) {
return 0.0;
} else {
// return -Sign;
return -Sign * numerator / denominator * cf / c;
}
}
// Generators of trajectory parameters for the straight line model
bool SLM::slmGenerator(mode_nonlinear *m_nl) {
// control variables for init velocity
const double V1_min = -2, V1_max = 2, V1_step = 1;
const int nV1 = (V1_max - V1_min) / V1_step + 1;
const double V2_min = -2, V2_max = 2, V2_step = 1;
const int nV2 = (V2_max - V2_min) / V2_step + 1;
// control variables for init position on y-axis
const int p2_min = 50, p2_max = 850, p2_step = 200;
const int np2 = (p2_max - p2_min) / p2_step + 1;
// number of generated instances
const int last = nV1 * nV2 * np2;
// indices into the instances
static int ip2, iV1, iV2;
if (current == 0) {
ip2 = 0;
iV1 = 0;
iV2 = 0;
}
if (current < last) { // not generated all instances?
if (ip2 >= np2) {
ip2 = 0; // reset index into the list positions
iV1++; // next horizontal velocity
if (iV1 >= nV1) {
iV1 = 0; // reset index into the list horiz. velocities
iV2++; // next vertical velocity
}
}
// map horizontal velocity index to horizontal velocity (m/s)
m_nl->V1 = iV1 * V1_step + V1_min;
// map vertical velocity index to vertical velocity (m/s)
m_nl->V2 = iV2 * V2_step + V2_min;
// init coordinate on x-axis is always null
m_nl->p1 = 0;
// map y-axis coordinate index to y-axis coordinate (m)
m_nl->p2 = ip2 * p2_step + p2_min;
ip2++; // next position on y-axis
current++; // index of next instance
return true;
} else {
return false; // reach the end
}
}
void SLM::slmGeneratorInit() { current = 0; }
} /* namespace uwspr */
} /* namespace gr */

Answer 1

abs is not defined in <math.h>. It can be found in <stdlib.h> and it is also defined as std::abs in <cmath>.

Answer 2

Surprisingly, the abs function is defined in <stdlib.h> rather than <math.h>. (At least, that's the case since C++17). Try including <stdlib.h> or <cstdlib> and see if that fixes things.
Hope this helps!

Answer 3

The problem is that you were using the C header file, math.h. The C standard already defined (the global scope) int ::abs(int) to take an int and return an int, and this is defined in stdlib.h. The float version, double ::fabs(double) is defined in math.h, which is the one you need to use.
C++ has overloads, so std::abs has double std::abs(double) and int std::abs(int) overloads. So you could just use the C++ header, cmath, and use std::abs (or std::fabs to prevent conversions) instead.