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Common mistakes with conditionalss

Comments and questions to John Rowe.

Using the wrong expression

Using = not ==

Wrong Right 
if ( a = b )

  if ( a == b)
The expression on the left set a to equal b, the one on the right tests to see if a equals b.

Using a == b == c or a >b > c

Wrong Right 
if (a == b == c)

  if ( a == b && b == c )


if ( a > b > c )

  if ( a > b && b > c )

Not using else

Wrong Right 
  if ( a == b) {
    c = c; // Do nothing
  }
  else {
    c = c/(a-b);
  }


  if ( a != b) {
    c = c/(a-b);
  }

Comparing floating-point numbers for equality

Rounding errors mean that two floating-point expressions are unlikely to be equal even when mathematically they ought to be. Instead we use fabs() of the difference. Note fabs() not abs() which is the integer function.
Wrong Right 
  if ( x*x == 2.0 )           // Wrong!


  if (fabs(x*x - 2.0) < 1e-6)  // Right

Using abs() not fabs()

Remember that abs() is the integer so any floating-point expression whose absolute value is less than one will become zero. 
  if (abs(x*x - 2.0) < 1e-6)
    printf("Should have used fabs()!\n");


  if (fabs(x*x - 2.0) < 1e-6)  // Right
// (This is the same as the previous example.)

Longer example of comparing floating-point numbers for equality

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main() {
  double x;

  x = sqrt(2.0);

  if ( x*x == 2.0 )           // Wrong!
    printf("x*x is exactly 2\n");
  else
    printf("x*x - 2 is %g\n", x*x - 2.0);

  if (fabs(x*x - 2.0) < 1e-6)  // Right
    printf("Close enough!\n");

  // Now let's see what happens when we use abs() by mistake
  x = x + 0.1;
  if (abs(x*x - 2.0) < 1e-6)
    printf("Should have used fabs()!\n");

  return 0;
}

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