CSC 107 – Programming For Science

Today’s Goal

Variables  Variable  Variable name location to store data  Only for humans; 0 x 7E8A2410 harder to remember  Assignments update memory location with new value  Memory location updated by assignment ONLY  When variable is used in program…  …uses current value at that memory location  Variable can store only one value  Often want multiple values, like adjusting for interest

Getting an “A”

Adjusting For Interest  First call, computed result for every year  Then all but last year computed in second call  Third time we called function, computed all but last 3 … and so on…  Only adjusted for 1 year last time we called function  Interest rate, amount, and results were constant  Unless save in variables, we have to recompute  Using variables required knowing years when coding

Could Have Spent Time

Can Make Stronger, Bigger  Arrays  Arrays are variables that can hold many items  Creates range of locations in which to store data  Locations are numbered sequentially from 0 entries  Array entries like variables in their own right  To be able to use them, must declare array (& entries)  Value unknown until assigned in the program  But not a true variable, entries depend on array  Access only via array using the entry's index

Declaring Array Variables  Like all variables, must declare before use size  Type, name, & size needed for array declaration  Still variables, so names follow usual rules  Variable is array of the type, so use any legal type  Each of the array's entries hold value of that type  Size must be integer since ½ a value hard to use

Declaring Array Variables  Like all variables, must declare before use size  Type, name, & size needed for array declaration  Still variables, so names follow usual rules  Variable is array of the type, so use any legal type  Each of the array's entries hold value of that type  Size must be integer since ½ a value hard to use

Declaring Array Size

Initializing an Array

Legal Array Entries  Access array's entries indexed from 0 to size-1  0, 1, 2, 3, 4 legal if size of 5 used to declare array  Array created with size of 8: 0, 1, 2, 3, 4, 5, 6, 7 legal  0 only legal index if size declared as 1

Legal Array Entries

Guns (& C++) Don't Kill  C++ make arrays easy, but mistakes easy also  Code can access any index within an array int  No problems compiling, as long as index an int  Includes ridiculous indices like -1 or  To find size, could try using sizeof( array variable )  Entry outside array bounds accessed by program  Program may crash with “Segmentation Fault”  Other variable's value used and updated  Program may be able to complete normally

Using An Array  Within an array, each entry behaves like variable  But need array variable to access the entry  To use or assign entry, specify index inside brackets  Example code snippet computing powers of 2: long lA[10]; lA[0] = 1; for (long i=0; i < sizeof(lA)/sizeof(long);i++){ lA[i] = lA[i-1] * 2; cout << lA[i] << endl; }

Using An Array  Within an array, each entry behaves like variable  But need array variable to access the entry  To use or assign entry, specify index inside brackets  Example code snippet computing powers of 2: long lA[10]; lA[0] = 1; for (long i=0; i < sizeof(lA)/sizeof(long);i++){ lA[i] = lA[i-1] * 2; cout << lA[i] << endl; }

Using An Array  Within an array, each entry behaves like variable  But need array variable to access the entry  To use or assign entry, specify index inside brackets  Example code snippet computing powers of 2: long lA[10]; lA[0] = 1; for (int i=0; i < sizeof(lA)/sizeof(long);i++){ lA[i] = lA[i-1] * 2; cout << lA[i] << endl; }

Using An Array  Within an array, each entry behaves like variable  But need array variable to access the entry  To use or assign entry, specify index inside brackets  Example code snippet computing powers of 2: const int BORED_NOW = 10; long lA[BORED_NOW]; lA[0] = 1; for (int i=0; i < BORED_NOW;i++){ lA[i] = lA[i-1] * 2; cout << lA[i] << endl; }

Let's Trace This Code int main() { const int BORED_NOW = 4; long loserArray[BORED_NOW]; loserArray[0] = 1; for (int i=1; i < BORED_NOW; i++){ loserArray[i] = loserArray[i-1] * 2; cout << loserArray[i] << endl; } cout << "Sorry its stupid!" << endl; return 0; }

Your Turn  Get into your groups and try this assignment

For Next Lecture  Read more about arrays in Section 10.5  How can we pass arrays as parameters?  Can values be changed in the array no matter what?  Why couldn't they be consistent about params?  Weekly Assignment #8 out & due Tuesday  Avoid the rush by start working on it now  Programming Assignment #2 now on Angel  Could start working on this now – know all you need