CSci 125

Lecture 21

Martin van Bommel

Memory Allocation• Variable declarations cause compiler to reserve

memory to hold values - allocation• Global variables

– allocated when program begins execution, remain until program exits

• Local variables in function– allocated upon each call to function– placed in area of memory assigned to this particular

invocation of function - frame– frame is deallocated when function returns

Size of Allocation

• Compiler must reserve number of bytes corresponding to size of object– e.g. char - 1 byte int - 2 float - 4 double - 8

• sizeof operator gives number of bytes– sizeof (int) is 2– sizeof x is number of bytes to store x

Allocating Arrays

double temp[5];

• requires 8 bytes/element * 5 elements = 40• Assume stored in frame with base address b• Takes bytes b to b + 40• Addresses of elements are if b = 1000

temp[0] = b 1000

temp[1] = b + 8 1008

temp[i] = b + i*8 1000 + i*8

Exceeding Array Bounds• What if we access temp[7]?

1000 + 8 * 7 = 1056

• But this is outside of bounds of array

• Most systems do not detect error unless outside bounds of program space (e.g. in OS)

• Just go ahead and access address 1056

• May be another variable or array

• May even be different data type

Preventing Bounds Errors

• To debug possible error in bounds, use printf– e.g. before accessing temp[i]

use printf(”% d\n”, i);

• To prevent possible error in bounds, use if– e.g. before accessing temp[i]

use if (0 > i || i >= MaxElement) printf(”Error in bounds.\n”); else

...

Array Uses

• Arrays prove useful because they are ordered structures which allow both– sequential access

for (i=0; i<MaxElement; i++) temp[i] = 0;

– direct accesstemp[index] = value;

• Allows operations on entire array while permitting access based on some index value

Array Application

• Count frequency of occurrence of each decile in a list of course marks

• Create array to store countsint freq[10];

• Initialize arrayfor (i=0; i<10; i++) freq[i] = 0;

• Increment count for each markfreq[mark / 10]++;