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c++decimaltruncate

Truncate a decimal value in C++

What's the easiest way to truncate a C++ float variable that has a value of 0.6000002 to a value of 0.6000 and store it back in the variable?

Solution

  • First it is important to know that floating point numbers are approximated. See the link provided by @Greg Hewgill to understand why this problem is not fully solvable.

    But here are a couple of solutions to the problem that will probably meet your need:

    Probably the better method but less efficient:

    char sz[64];
double lf = 0.600000002;
sprintf(sz, "%.4lf\n", lf); //sz contains 0.6000

double lf2 = atof(sz);

//lf == 0.600000002;
//lf2 == 0.6000

printf("%.4lf", lf2); //print 0.6000

    The more efficient way, but probably less precise:

    double lf = 0.600000002;
int iSigned = lf > 0? 1: -1;
unsigned int uiTemp = (lf*pow(10, 4)) * iSigned; //Note I'm using unsigned int so that I can increase the precision of the truncate
lf = (((double)uiTemp)/pow(10,4) * iSigned);