4 ms·

At least once a quarter I'd have a student working on some programming assignment on one of our unix-ish systems where they named the executable "test" and coul

by ja27 14y ago

At least once a quarter I'd have a student working on some programming assignment on one of our unix-ish systems where they named the executable "test" and couldn't figure out why it wasn't doing anything they expected. I also got to the point with FORTRAN (yes, I'm old) students where I'd cut them off and tell them to count the columns and make sure each line started with 6 blanks, not 5. My favorite bug is still some piece of C code buried in a library that was like this: if (flag == TRUE) ... Or there's the old for loop typo (much easier to detect on today's high-DPI screens): for(float x=0.0; x < 3,4; x = x + 0.1) ...

viraptor 14y ago
I don't get what's wrong with the flag check apart from being a bit too verbose.
- mpyne 14y ago
  TRUE is going to be 1. The flag value might have the value 0x2, 0x4, 0xsomething_else, etc., which will not equal exactly 1 (TRUE). What you want varies but is probably something like: if (flag & FLAG_VALUE) ... if (flag) .... if ((flag & FLAG_VALUE) == TRUE) ... that kind of thing.
  - geon 14y ago
    if ((flag & FLAG_VALUE) == TRUE) ... That would still check if flag is 1, right?
    - azth 14y ago
      That is correct.
      - mpyne 14y ago
        Indeed, great catch (this is why I use things like QFlags now, as it has a simple "testFlag" method)
  - to3m 14y ago
    flag&FLAG_VALUE will be FLAG_VALUE or 0, so unless FLAG_VALUE==TRUE the answer could be wrong. Suggest (flag&FLAG_VALUE), or, if you need the expression to be of type bool, ((flag&FLAG_VALUE)!=0) or (!!(flag&FLAG_VALUE)).
    - mpyne 14y ago
      Very true, I just checked to see if I can edit but I can't, so mea culpa.
  - ja27 14y ago
    Boolean true / TRUE wasn't defined in any standard way before C99. Back in the "olden days", most programmers #defined it as 1, but not always. I ran into one of those "not always" cases in some inherited code.
anonymouz 14y ago
I'm curious about the test one. Do you have "." in your PATH? Or do students simply not realize that they should call their program as ./test anyway?
- ja27 14y ago
  Student accounts had "." in their PATH by default. I'm sure someone did it to keep students from asking why their program wouldn't run at all. Fun times.

by ja27 14y ago

viraptor 14y ago

I don't get what's wrong with the flag check apart from being a bit too verbose.

mpyne 14y ago
TRUE is going to be 1. The flag value might have the value 0x2, 0x4, 0xsomething_else, etc., which will not equal exactly 1 (TRUE). What you want varies but is probably something like: if (flag & FLAG_VALUE) ... if (flag) .... if ((flag & FLAG_VALUE) == TRUE) ... that kind of thing.
- geon 14y ago
  if ((flag & FLAG_VALUE) == TRUE) ... That would still check if flag is 1, right?
  - azth 14y ago
    That is correct.
    - mpyne 14y ago
      Indeed, great catch (this is why I use things like QFlags now, as it has a simple "testFlag" method)
- to3m 14y ago
  flag&FLAG_VALUE will be FLAG_VALUE or 0, so unless FLAG_VALUE==TRUE the answer could be wrong. Suggest (flag&FLAG_VALUE), or, if you need the expression to be of type bool, ((flag&FLAG_VALUE)!=0) or (!!(flag&FLAG_VALUE)).
  - mpyne 14y ago
    Very true, I just checked to see if I can edit but I can't, so mea culpa.
- ja27 14y ago
  Boolean true / TRUE wasn't defined in any standard way before C99. Back in the "olden days", most programmers #defined it as 1, but not always. I ran into one of those "not always" cases in some inherited code.

anonymouz 14y ago

I'm curious about the test one. Do you have "." in your PATH? Or do students simply not realize that they should call their program as ./test anyway?

ja27 14y ago
Student accounts had "." in their PATH by default. I'm sure someone did it to keep students from asking why their program wouldn't run at all. Fun times.