Checking if a number is an integer in C++ [duplicate]

Question

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Testing if given number is integer (9 answers)

Closed 9 years ago.

I am searching for a way to check if a certain variable holds an integer or no. My program asks the user for the values of a set of variable then it computes the variable k needed to make another variable e an integer. The code for calculating k is essentially a for loop that increments k from zero until it reaches a value making e an integer. But I can't figure out how to check that e is an integer. Here is the code I am thinking of:

for (int k = 0; k!=wk; k++) 
 { 
  e = (1+k*f_n)/d;
 if()
 }

The variable wk is just there so that when e is an integer, wk is to be equal to k, and so the loop ends.

Answer 1

You don't want to check if e is an integer. You want to check if 1 + k*f_n is a multiple of d. Use the modulo operator for that.

if (((1 + k*f_n) % d) == 0)

Answer 2

Assuming that e is a floating-point type,

int u = (int) e;
if (e > (float)u || e < (float)u)
  continue;
break;

This solution within the confines of what is otherwise modular arithmetic will suffer from rounding errors. Please consider using the modulus operator instead.