constant integeral expression?

This is a discussion on constant integeral expression? within the C Programming forums, part of the General Programming Boards category; i'v been reading the tutorial about the switch statement on this website( http://www.cprogramming.com/tutorial/c/lesson5.html ) and it mentions that the switch ...

  1. #1
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    constant integeral expression?

    i'v been reading the tutorial about the switch statement on this website(http://www.cprogramming.com/tutorial/c/lesson5.html) and it mentions that the switch statement can't be used with "constant integeral expressions". exactly what is a "constant integeral expressions"?

  2. #2
    The larch
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    It says exactly the opposite and gives an example of non-working code. For cases you need literals: case 10: or case 'A': etc

  3. #3
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    sorry, typing error. so what are literals?

  4. #4
    Jaguar
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    They are compile time constants. You can't switch constants defined in your program.
    #define etc.

  5. #5
    Frequently Quite Prolix dwks's Avatar
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    They are compile time constants. You can't switch constants defined in your program.
    #define etc.
    Yes, you can, if the #defines are literals themselves. The compiler never sees the pre-preprocessor code so it doesn't care.
    Code:
    #define YES 0
    #define NO 1
    
    /* ... */
    
    switch(answer) {
    case YES: break;
    case NO: break;
    }
    You just can't use an expression that must be evaluated at runtime, eg
    Code:
    int x;
    switch(y) {
    case x:
    }
    You can use a constant expression, as long as that expression can be evaluated by the compiler (I think, not sure on this one):
    Code:
    int array[BUFSIZ];
    switch(size) {
    case sizeof(array)/sizeof(*array):
    }
    It's like initializing a global/static variable.
    dwk

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  6. #6
    and the hat of wrongness Salem's Avatar
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    > exactly what is a "constant integeral expressions"?
    const - as in not a variable
    So you can't have case myVariable:

    integral - as in being an integer, and not a pointer or a float
    So you can't have case "hello": or case PI:

    expression - as in using simple arithmetic operators.
    Eg
    case 10:
    case '\n':
    case 5+5:
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  7. #7
    Jaguar
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    Quote Originally Posted by dwks
    Code:
    #define YES 0
    #define NO 1
    
    /* ... */
    
    switch(answer) {
    case YES: break;
    case NO: break;
    }
    but you can't have the defines in switch expression.
    Code:
    switch(YES){
    }

  8. #8
    and the hat of wrongness Salem's Avatar
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    > but you can't have the defines in switch expression.
    Of course you can.
    It's dumb, not illegal - that's all.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  9. #9
    Jaguar
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    Yep, that's right.
    it's useless.

  10. #10
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    Unless you're trying to implement an 11-bit ALU in the preprocessor or something similiarly insane...
    Code:
    #include <stdio.h>
    
    void J(char*a){int f,i=0,c='1';for(;a[i]!='0';++i)if(i==81){
    puts(a);return;}for(;c<='9';++c){for(f=0;f<9;++f)if(a[i-i%27+i%9
    /3*3+f/3*9+f%3]==c||a[i%9+f*9]==c||a[i-i%9+f]==c)goto e;a[i]=c;J(a);a[i]
    ='0';e:;}}int main(int c,char**v){int t=0;if(c>1){for(;v[1][
    t];++t);if(t==81){J(v[1]);return 0;}}puts("sudoku [0-9]{81}");return 1;}

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