Why is the difference in running time so long between these two codes?

Question

For both codes l is initially 1 , all variables except the array are long long , array is global int of size N. N is to the order of 10^6. But code 1 exceeds time limit of 5 seconds but code 2 runs in milliseconds. The only difference is in the last line. a,b,c are integer variables less than 50.

Compiler used :- g++

test case :- 3000000 50 50 50 1 (N,a,b,c,l)

Code 1:-

for(i=1;i<N;i++) { j=l; k=(j*j)%1000000; k=(a*k)%1000000; j=(b*j)%1000000; l=(j+k+c)%1000000; x=(int)l; ar[i]=x; }

Code 2:-

for(i=1;i<N;i++) { j=l; k=(j*j)%1000000; k=(a*k)%1000000; j=(b*j)%1000000; l=(j+k+c)%1000000; x=(int)l; ar[i]=1; }

Is these timings an average for a number of runs or just a one off? — Ed Heal
– Ed Heal, Commented Mar 8, 2014 at 12:24
You should post a test-case. There are too many unknowns to reach any meaningful conclusions. — juanchopanza
– juanchopanza, Commented Mar 8, 2014 at 12:25

Avt · Accepted Answer · 2014-03-08 12:25:30Z

2

It is compiler's optimisation. Probably second code is just replaced with

for(i=1;i<N;i++) { ar[i]=1; }

answered Mar 8, 2014 at 12:25

Avt

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1 Comment

Ed Heal Over a year ago

May not even bother to compute l

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