Programming Forums > Application Development > C++

JD-Salinger · May 4th, 2008, 12:26 AM

in line 26
assert(maxIdFun==sizeof FunctionArray/sizeof FunctionArray[0])
what is the size of functionArray and size of functionArray[0] i cant understand that... and how come it would equal to maxIdFun... and the point of dividing it

 c++ Syntax (Toggle Plain Text) 
 
const int maxIdFun=14;
 
FunctionEntry FunctionArray [] =
{
    log,  "log",
    log10,"log10",
    exp,  "exp",
    sqrt, "sqrt",
    sin,  "sin",
    cos,  "cos",
    tan,  "tan",
    CoTan,"cotan",
    sinh, "sinh",
    cosh, "cosh",
    tanh, "tanh",
    asin, "asin",
    acos, "acos",
    atan, "atan",
};
 
FunctionTable::FunctionTable(SymbolTable& symTab,FunctionEntry* funArr)
    :_size(0)
{
 
    assert(maxIdFun==sizeof FunctionArray/sizeof FunctionArray[0])
 
    for(int i=0; i<maxIdFun; ++i)
    {
        int len=strlen(funArr[i].strFun);
        if(len==0)
            break;
        _pFun[i]=funArr[i].pFun;
        cout<< funArr[i].strFun<<endl;
        int j=symTab.ForceAdd(funArr[i].strFun,len);
        assert(i==j);
        ++_size;
    }
}
const int maxIdFun=14;

FunctionEntry FunctionArray [] =
{
    log,  "log",
    log10,"log10",
    exp,  "exp",
    sqrt, "sqrt",
    sin,  "sin",
    cos,  "cos",
    tan,  "tan",
    CoTan,"cotan",
    sinh, "sinh",
    cosh, "cosh",
    tanh, "tanh",
    asin, "asin",
    acos, "acos",
    atan, "atan",
};

FunctionTable::FunctionTable(SymbolTable& symTab,FunctionEntry* funArr)
    :_size(0)
{
    
    assert(maxIdFun==sizeof FunctionArray/sizeof FunctionArray[0])
    
    for(int i=0; i<maxIdFun; ++i)
    {
        int len=strlen(funArr[i].strFun);
        if(len==0)
            break;
        _pFun[i]=funArr[i].pFun;
        cout<< funArr[i].strFun<<endl;
        int j=symTab.ForceAdd(funArr[i].strFun,len);
        assert(i==j);
        ++_size;
    }
}

thanks a lot

Sane · May 4th, 2008, 12:40 AM

The purpose of that statement is to assert that there are actually 14 elements in the array "FunctionArray". It accomplishes this by dividing the total number of bytes in the array (sizeof FunctionArray) by the number of bytes in the first element of the array (sizeof FunctionArray[0]). Since the size of the first element is the same as every other element, the resulting quotient is the number of elements in the array. If this doesn't equal maxIdFun, we have a problem, since maxIdFun is used to determine which elements in the array are accessable.

But beware, this kind of sizeof trick can only be used on arrays where the compiler knows the size of it in advance.

Sorrofix · May 4th, 2008, 12:48 AM

>But beware, this kind of sizeof trick can only be used on
>arrays where the compiler knows the size of it in advance.

And of course, don't use it on a pointer. Even if they do seem a lot like arrays...

JD-Salinger · May 4th, 2008, 5:35 AM

Quote:

Originally Posted by Sorrofix

And of course, don't use it on a pointer. Even if they do seem a lot like arrays...

it returns 4bytes as always? even in a pointer? and y cant i use it in a pointer?
anyway tnx sane

Sane · May 4th, 2008, 11:19 AM

Yes. You can't "use it on a pointer" because the compiler doesn't know the size of its array in advance. If you need the size of a dynamically allocated array, or a pointer to an array, you need to keep track of the size yourself.

JD-Salinger · May 4th, 2008, 7:22 PM

ok... thnx sane

May 4th, 2008, 12:26 AM	#1
JD-Salinger Unknown Join Date: Apr 2008 Location: unknown Posts: 21 Rep Power: 0	sizeof question in line 26 `assert(maxIdFun==sizeof FunctionArray/sizeof FunctionArray[0])` what is the size of functionArray and size of functionArray[0] i cant understand that... and how come it would equal to maxIdFun... and the point of dividing it c++ Syntax (Toggle Plain Text) const int maxIdFun=14; FunctionEntry FunctionArray [] = { log, "log", log10,"log10", exp, "exp", sqrt, "sqrt", sin, "sin", cos, "cos", tan, "tan", CoTan,"cotan", sinh, "sinh", cosh, "cosh", tanh, "tanh", asin, "asin", acos, "acos", atan, "atan", }; FunctionTable::FunctionTable(SymbolTable& symTab,FunctionEntry* funArr) :_size(0) { assert(maxIdFun==sizeof FunctionArray/sizeof FunctionArray[0]) for(int i=0; i<maxIdFun; ++i) { int len=strlen(funArr[i].strFun); if(len==0) break; _pFun[i]=funArr[i].pFun; cout<< funArr[i].strFun<<endl; int j=symTab.ForceAdd(funArr[i].strFun,len); assert(i==j); ++_size; } } const int maxIdFun=14; FunctionEntry FunctionArray [] = { log, "log", log10,"log10", exp, "exp", sqrt, "sqrt", sin, "sin", cos, "cos", tan, "tan", CoTan,"cotan", sinh, "sinh", cosh, "cosh", tanh, "tanh", asin, "asin", acos, "acos", atan, "atan", }; FunctionTable::FunctionTable(SymbolTable& symTab,FunctionEntry* funArr) :_size(0) { assert(maxIdFun==sizeof FunctionArray/sizeof FunctionArray[0]) for(int i=0; i<maxIdFun; ++i) { int len=strlen(funArr[i].strFun); if(len==0) break; _pFun[i]=funArr[i].pFun; cout<< funArr[i].strFun<<endl; int j=symTab.ForceAdd(funArr[i].strFun,len); assert(i==j); ++_size; } } thanks a lot __________________ ------------------------------------------------------------------------- I thought what I'd Do was, I'd pretend to be one of those deaf mutes ------------------------------------------------------------------------

May 4th, 2008, 12:40 AM	#2
Sane Programming Guru Join Date: Apr 2005 Posts: 1,729 Rep Power: 5	Re: sizeof question The purpose of that statement is to assert that there are actually 14 elements in the array "FunctionArray". It accomplishes this by dividing the total number of bytes in the array (sizeof FunctionArray) by the number of bytes in the first element of the array (sizeof FunctionArray[0]). Since the size of the first element is the same as every other element, the resulting quotient is the number of elements in the array. If this doesn't equal maxIdFun, we have a problem, since maxIdFun is used to determine which elements in the array are accessable. But beware, this kind of sizeof trick can only be used on arrays where the compiler knows the size of it in advance. __________________ Waterloo's Canadian Computing Competition (CCC) - Stage 2 Problems, Solutions, and Test Data

May 4th, 2008, 12:48 AM	#3
Sorrofix Bug developer Join Date: Apr 2008 Posts: 6 Rep Power: 0	Re: sizeof question >But beware, this kind of sizeof trick can only be used on >arrays where the compiler knows the size of it in advance. And of course, don't use it on a pointer. Even if they do seem a lot like arrays...

May 4th, 2008, 11:19 AM	#5
Sane Programming Guru Join Date: Apr 2005 Posts: 1,729 Rep Power: 5	Re: sizeof question Yes. You can't "use it on a pointer" because the compiler doesn't know the size of its array in advance. If you need the size of a dynamically allocated array, or a pointer to an array, you need to keep track of the size yourself. __________________ Waterloo's Canadian Computing Competition (CCC) - Stage 2 Problems, Solutions, and Test Data

May 4th, 2008, 7:22 PM	#6
JD-Salinger Unknown Join Date: Apr 2008 Location: unknown Posts: 21 Rep Power: 0	Re: sizeof question ok... thnx sane __________________ ------------------------------------------------------------------------- I thought what I'd Do was, I'd pretend to be one of those deaf mutes ------------------------------------------------------------------------

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Programming Forums > Application Development > C++

sizeof question