c - Filling array gives unexpected values -

i have serious problem when filling array. functions looks this:

int s_in[]; /* array filled lot of integer values */ for(frame=0; frame<maxframes; frame++){   left = 240*(frame-1) + 1;   right = 240*(frame +1);   int x[right-left+1]; /* 480 long */    for(i=left-1;i<right;i++){     x[i] = s_in[i];   } 

when try print out values stored in x[] each run, either empty x[] or random numbers not appear in s_in[]. can problem solved kind of memory management?

there 2 problems in line:

x[i] = s_in[i]; 

on first iteration, when aboce line executed, values of variables are:

frame:    0 left:  -239 i:     -240 

the code attempts read outside of bounds of s_in[] , write writes outside of bounds of x[].

this undefined behaviour.

it's difficult provide fix code without knowing how information laid out in s_in[]. assuming frame 240 bytes , on each iteration want copy values 2 consecutive frames s_in[] x[], code should along these lines:

int s_in[]; /* array filled lot of integer values */ (frame = 1; frame < maxframes; frame ++) {   left  = 240 * (frame - 1);        /* first value of previous frame */   right = 240 * (frame + 1);        /* first value of next frame */   int x[480];                       /* 480 long */    (i = 0; < 480; ++) {     x[i] = s_in[left + i];   } } 

there no point in computing right-left+1 on each iteration since know must 480. can see code above, value of right not needed.

the outer loop should start frame = 0 frame #0 offending line attempts read outside s_in[]; why starts frame #1.

i suppose more processing inside inner loop. otherwise can replace call memcpy(&x, &s_in[left], 480);. same current status of for loop runs faster.