If statement should be true, but isn't?

This is a discussion on If statement should be true, but isn't? within the C++ Programming forums, part of the General Programming Boards category; Hi all, I seem to be having a silly issue with an IF statement that I can't figure out. I'm ...

  1. #1
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    If statement should be true, but isn't?

    Hi all,

    I seem to be having a silly issue with an IF statement that I can't figure out. I'm quite new to C++ but not to code in general. I understand how logical statements work but I might be missing something here.

    Just so there's no confusion. I'm trying to create a basic draughts/checkers game and the function in question is being called to move the correct piece as referrence by a grid:

    1 2 3 4 5 6 7 8
    a
    b
    c
    d
    e
    f
    g
    h

    The code checks the first element of the "position_from" array and looks for a letter (A-H). If it finds any of these it will then continue in to a nested loop and check for 1-8 to complete the grid referrence.

    I've only just begun so only have code which checks for 'A', then continues in to a loop and checks for 1-8. My problem is, the nested IF statement I am using within the loop, has no other option but to come true, but for some reason won't.

    The following code will output:
    Quote Originally Posted by Code output
    True - 'position_from[0] does equal 'A'
    0 = A <- element zero contains 'A'.
    1 = 1 <- element one contains '1'.
    As you can see from the output, element[1] does indeed contain the number 1. With that in mind, why is the following code not working? The for loop checks 'position_from[1]' (which equals '1') against 'I', which equals 0 on the first loop then 1 on the second loop, then stops looping.

    My code:
    Code:
    void move_piece(char *pieces, char *position_from, char *position_to)
    {
         if(position_from[0] == 'A')
         {
               cout << "True - 'position_from[0]' does equal 'A'" << endl;
               for(int i = 0; i < 2; i++)
               {
                      if(position_from[1] == i) /* This statement must come true once */
                      {
                           cout << "True - 'position_from[1]' does equal 1";
                      }
                      else
                      {
                           cout << i << " = ";
                           cout << position_from[i] << endl;
                      }
               }
          }
          else
          {
                cout << "False - 'position_from[0] does not equal 'A'";
          }
    }

    Help is much appreciated. Thanks.
    Last edited by George88; 10-08-2011 at 07:54 PM.

  2. #2
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    You are assuming, mistakenly, that the test if ('1' == 1) will be true. It will not. In the ASCII character set for example, the character '1' has a numeric value of 49.
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Sunshine, and read this, this, and this before posting again.

  3. #3
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    I had a cruder form of this code earlier on today, which was working as I wanted it to.

    I was testing to see the result of the loop by using a temporary "num" integer variable. I would assign the value of 'i' to num (num = i) and would then print out the result as '1' after the loop had ended, so the comparison has already shown to be working using the same format.

    I don't know why it has stopped working now, though.

    I can't even remember if I may have changed some minor line of code or re-wrote the if statement differently to that which I had originally.

    I understand what you're saying about the ASCII code value for '1', but why would the IF statement check the ASCII value of '1' and not the actual value? I am checking the position_from[] array against an integer after all.

    I hope I am making a really simple mistake here.

  4. #4
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    Comparing characters with an integer compares the numeric value.

    You are saying that position_from[i] contains the value of '0'. You are then comparing that against the value of i (which, in your code) can only have the value of 0 or 1. '0' will not compare equal with either 0 or 1. That is what the test "if (position_from[1] == i)" does.
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Sunshine, and read this, this, and this before posting again.

  5. #5
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    Quote Originally Posted by grumpy
    Comparing characters with an integer compares the numeric value.
    That's what I want.

    Quote Originally Posted by grumpy
    '0' will not compare equal with either 0 or 1.
    Sorry, I don't understand. Would you please be able to explain why? Aren't '0' and '0' the same, which would make the comparison true? Also, this does not explain how I actually had this code working today, comparing it in the same manner.

    Please excuse my newby ignorance . Thanks.

  6. #6
    Registered User manasij7479's Avatar
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    >Aren't '0' and '0' the same ...
    Yes, but '0' and 0 are not the same.
    '0' is..as explained above.. the numerical value of the character looking like a 0.
    Manasij Mukherjee | gcc-4.9.2 @Arch Linux
    Slow and Steady wins the race... if and only if :
    1.None of the other participants are fast and steady.
    2.The fast and unsteady suddenly falls asleep while running !



  7. #7
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    Thanks for your replies. The problem has been solved. The above code needs to subtract '0' from the char array during the comparison, so it becomes:

    if((position_from[1] - '0') == i)
    instead of
    if(position_from[1] == i)

    Again, thanks. Would never have figured that out alone. I can't seem to change the title to [SOLVED], would a mod be able to? This thread may help someone in the future.

    Code:
    void move_piece(char *pieces, char *position_from, char *position_to)
    {
         if(position_from[0] == 'A')
         {
               cout << "True - 'position_from[0]' does equal 'A'" << endl;
               for(int i = 0; i < 2; i++)
               {
                      if((position_from[1] - '0') == i) /* This statement must come true once */
                      {
                           cout << "True - 'position_from[1]' does equal 1";
                      }
                      else
                      {
                           cout << i << " = ";
                           cout << position_from[i] << endl;
                      }
               }
          }
          else
          {
                cout << "False - 'position_from[0] does not equal 'A'";
          }
    }
    Last edited by George88; 10-09-2011 at 09:52 AM.

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