What is wrong with my logic? - c++

Question

if(location[0] <= 'H' || location[0] >= 'A'
&& location[1] <= 8 || location[1] >= 1)
I am checking to see if the first indice is between 'A' and 'H' and the second indice is between 1 - 8.
so the argument would = 1 if the user entered { A, 1 }
-1 if the user entered { J, 1 }

Answer 1

And (&&) has higher precedence than or (||), so to express what you want you need:
if ((location[0] <= 'H' || location[0] >= 'A') && (location[1] <= 8 || location[1] >= 1))
Otherwise what's happening is that it does the equivalent of this:
if (location[0] <= 'H' || (location[0] >= 'A' && location[1] <= 8) || location[1] >= 1)
Which is clearly not what you intended.
You can put as many expressions in the condition as you want, but you must remember the precedence rules; I suggest always using extra parentheses for clarity.
As Jeremiah has pointed out in the comments, the condition still wouldn't be doing what you wanted -- you are asking if something is in a given range by checking if it's either less than the upper bound or greater than the lower bound (this covers all input), when you intended to check whether the input was both larger than the lower bound and smaller than the upper:
if (location[0] <= 'H' && location[0] >= 'A' && location[1] <= 8 && location[1] >= 1)
In this case, since the operators are all &&, no extra parentheses are needed, though you can add them for extra clarity.

Answer 2

You need &&s not ||s. For example: 'Z' >= 'A' || 'Z' <= 'H' is true.
if(location[0] <= 'H' && location[0] >= 'A' && location[1] <= 8 && location[1] >= 1)
While not necessary in this case, you should group your logic together with parenthesis (and if they were ||s you would have to for it to work as expected):
if((location[0] <= 'H' && location[0] >= 'A') && (location[1] <= 8 && location[1] >= 1))

Answer 3

Add parenthesis to group the conditions:
if( (location[0] <= 'H' || location[0] >= 'A') && (location[1] <= 8 || location[1] >= 1))

Answer 4

If location[0] was equal to 'J' (definitely NOT between 'A' and 'H'), your if-clause would be TRUE because even though location[0] <= 'H' is FALSE, location[0] >= 'A' is TRUE and you are using || (OR). The problem is that you are using || (OR) instead of && (AND). You need to use all && (AND) in the if-clause.

Answer 5

&& has higher precedence than ||. So, you may want to write it as
if((location[0] <= 'H' || location[0] >= 'A') && (location[1] <= 8 || location[1] >= 1))
If you already tried this, please tell what error you got

Answer 6

Sorry guys, new here. I should have posted my entire method. Thank you for explaining the precedence of the operators, but my issue was that I was comparing a char with an int. All I needed to do was put ' ' around my numbers.
void GameBoard::enterShips()
{
char location[2];
int ships = 0;
int count = 1;
while( ships < NUM_SHIPS )
{
cout << "Enter a location for Ship " << count << ": ";
cin >> location[0];
cin >> location[1];
cout << endl;
if((location[0] <= 'H' && location[0] >= 'A') && (location[1] <= '8' && location[1] >= '1'))
{
location[0] = location[0] - 64;
location[1]--;
Grid[location[0]][location[1]] = SHIP;
ships++;
count++;
}
else
{
cout << "Wrong coordinates entered. Try again." << endl << endl;
}
}
}

Answer 7

As you can see in this operator precedence table, && is #13. It's higher up than ||, which is #14.
Therefore:
if(location[0] <= 'H' || location[0] >= 'A'
&& location[1] <= 8 || location[1] >= 1)
Is equivalent to:
if(location[0] <= 'H' || (location[0] >= 'A'
&& location[1] <= 8) || location[1] >= 1)
You want:
if((location[0] <= 'H' || location[0] >= 'A') &&
(location[1] <= 8 || location[1] >= 1))
Or even better:
if(((location[0] <= 'H') || (location[0] >= 'A')) &&
((location[1] <= 8) || (location[1] >= 1)))
I always put brackets around everything except when */ and +- are concerned. Even if you memorize the table, you shouldn't assume others do.
It helps code readability and helps prevent bugs... even compiler bugs! Triple bonus. :)
EDIT: Ah, you want ALL of them to be true. Just use all &&s, then! This works:
if(((location[0] <= 'H') && (location[0] >= 'A')) &&
((location[1] <= 8) && (location[1] >= 1)))