if / else if / else statement not functioning properly

This is a discussion on if / else if / else statement not functioning properly within the C Programming forums, part of the General Programming Boards category; I'm having a very peculiar problem with my if / else if / else statement block. Certain inputs seems to ...

  1. #1
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    if / else if / else statement not functioning properly

    I'm having a very peculiar problem with my if / else if / else statement block. Certain inputs seems to be accessing multiple parts of the block, and some statements don't seem to be working at all. Here is my code:

    Code:
    int cc;
        ii = 0;
        for(cc = 0; cc < 1; cc--) { // infinite loop
            fgets(aa[ii], LINE_SZ, stdin);  // LINE_SZ = 90
    
            if(strlen(aa[ii]) == 1) // WORKS
                exit(0);
    
            if(strlen(aa[ii]) > 80) {
    
                fprintf(stderr, "Inside > 80\n");   
        
            } else if(aa[ii][0] == '<') {
                fprintf(stderr, "Inside <");
    
                // pop
                if(ii == 0) {
                    fprintf(stderr, "Empty stack.\n");
                } else {
                    printf("ii: &#37;d", ii);
                    fprintf(stdout, aa[ii-1]);
                }
    
            } else if(aa[ii][0] == '>') {
                fprintf(stderr, "Inside >\n");
    
                // push
                if(ii == stackSize) {
                    fprintf(stderr, "Full stack.\n");
                } else {
                    strcpy(aa[ii], aa[ii]+1);
                    ii++;
                }
    
            } else {
    
                // bad input
                fprintf(stderr, "Inside else - Bad input.\n");
    
            } 
                    
        }
    Even if my input line (aa[ii]) is over 80 chars long, it displays "Inside > 80" but then still goes to the else statement and displays "Inside else - Bad input."
    Last edited by Beowolf; 09-11-2007 at 07:47 PM.

  2. #2
    C++ Witch laserlight's Avatar
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    but then still goes to the else statement and displays "Inside else - Bad input."
    ... on the next iteration, perhaps?
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    What do you mean? I only enter one line of input in the test.

  4. #4
    C++ Witch laserlight's Avatar
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    What do you mean? I only enter one line of input in the test.
    Ah, but your loop is an infinite loop. Incidentally, since you never use cc, you could use for ( ;; ) if you really want an infinite loop.

    Still, why not print cc to check which iteration of the loop you are in?
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    Hmm between the first if statement for the 80 byte check and the else statement, CC is decreasing by 1, meaning it is going through the loop again. But why would it do that?

    That would explain why the else is executing next - because when it goes through again, it is picking up a null value, so it is below 80 strlen and is not < or >, so else executes.

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    I understand what it is doing now. If my input is greater than 80, the fgets line only takes the first 80 characters because that is what is specified by LINE_SZ. But say if my input is 91 chars long, that means 11 chars are left over. So when the if-else statements are done executing, fgets automatically retrieves those 11 chars, and since they do not begin with > or <, it labels them as bad input.

    I include fflush(stdin) inside my if statement checking for over 80 chars, and it seems to work now.

  7. #7
    C++ Witch laserlight's Avatar
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    I include fflush(stdin) inside my if statement checking for over 80 chars, and it seems to work now.
    You have the right idea, but the wrong implementation. Read the FAQ on Why fflush(stdin) is wrong.
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  8. #8
    and the hat of wrongness Salem's Avatar
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    > strcpy(aa[ii], aa[ii]+1);
    Very few standard C functions are guaranteed to work on overlapping memory.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  9. #9
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    And, this is not infinite - just VERY LONG:
    Code:
    for(cc = 0; cc < 1; cc--) { // infinite loop
    It may be infinite for the purposes of this code, but it will run out after it wraps around to positive numbers. If what you want is an infinite loop, there are four options, listed here in the order I would recommend using them:
    Code:
    // 1
      for(;;) ... 
    
    // 2
      while(1) ...
    
    // 3 
      do { ... } while(1);
    
    // 4
    Label:
      ... 
      goto Label;     // Not really a good option in most cases.
    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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