Cleaning a text field

How can I optimize the if condition in this snippet? The only one difference is && [self isCurrentPosition:i]. How can I make it a single if, including the condition val?

Note: self is a category of NSArray.

- (void) cleanTextfieldExcluding:(int)current checkPosition:(BOOL)val {
  for ( int i=0; i<[self count]; i++ ) {

    // -----------
    if ( val ) {  // this IF block is very bad
      if ( i != current && [self isCurrentPosition:i] )
        [self replaceObjectAtIndex:i withObject:@""];
    } else {
      if ( i != current ) 
        [self replaceObjectAtIndex:i withObject:@""];
    }
    // -----------

  }
}

the (val · ¬cur · pos) | (¬val · ¬cur) = ¬cur (val·pos | ¬val) = ¬cur (pos | ¬val). So, the condition is

if ( (i != current) && (!val || [self isCurrentPosition:i]) ) {
        [self replaceObjectAtIndex:i withObject:@""];
    }

You just must practise factoring out the common subexpressions and subprograms and learn how to use the Carnot maps for boolean minimization.

StackExchange Code Review Q#19101, answer score: 5