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Deciding if a piece of code can be parallelized

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41 posts since Aug 2010
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Hello everyone, I'm having problems grasping some concepts of parallel programming; to be more precise, given some pieces of code, I have to determine which of them can be parallel and which cannot(this was in a test in a parallel programming class):

a) a=2; b=3;                    //yes-variables are independently initialised
b) a=3;b=a;                     //depends on the synchronisation
c) a=3;b=a;c=b;                 //depends on which gets executed first
d) b=a;c=b;a=4;                 //no-because some variables might get init to the new value while other to the old value
e) a=2;b=a+3;a=3;               //no-it's not clear to which value of 'a' the var. 'b' wil get calculated
f) for(i=0;i<100;i++) 
    b[i]=i;                     //yes- variable depends solely on the index
g) for(i=0;i<100;i++) 
    b[i]=f(b[i]);               //yes-variable doesn't depend on another variable
h) for(i=0;i<100;i++)
        b[i][j]= f(b[i][j-1]);  //no-current variable depends on previous 
i) a=f(x); b=g(x);              //variables get initialised independently
j) for(i=0;i<100;i++) 
    b[i] = f(b[index[i]]);      //same as i)

so my question is how far off am I from the correct answers ?
PS: if you like, you can contribute to the thread by posting questions(test questions, interviews etc.) of this type here as it might also help others

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3,824 posts since Aug 2010
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For lines 10-12 you can parallelize the outer loop, but not the inner. Also it will error on the first iteration as there is no index -1 (j = 0, j-1 = -1).

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3,989 posts since Mar 2010
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I think basically got it correct, with Momerath's comments applied. Actually, it may not error on line 12 if -1 is not a valid index, though it should; however, -1 IS a valid array index in theory. If this code was provided as part of the test, you might get extra credit if you point that out! :-)

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