Lecture 20: Arrays

1. CSC 107 – Programming For Science Lecture 20: Arrays

2. Today’s Goal • Become familiar with simple arrays • Declaring an array variable • Assigning data to array entries • Using values stored in an array • Know connection between arrays and loops

3. Variables • Variable name location to store data • Only for humans; 0x7E8A2410 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

4. 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

5. Could Have Spent Time

6. Can Make Stronger, Bigger • Arrays are variables that can hold many items • Creates range of locations in which to store data • Locations are numbered sequentially from 0 • 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

7. Declaring Array Variables • Like all variables, must declare before use • Type, name, & sizeneeded 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

8. Declaring Array Variables • Like all variables, must declare before use • Type, name, & sizeneeded 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

9. Declaring Array Size • When declaring array, best use constant for size • Changing is easy if larger size needed later • Have simple way to find array's size during program • Explain reasoning for size using good constant name • Type, name, & sizeneeded for array declaration intplanetsWeight[NOT_PLUTO];float armada[MAX_SHIPS];double annualIncomes[MAX_LIFETIME];char responses[17];long timeToWait[35];

10. Initializing an Array • Can set initial values when declaring array • Will need to specify value for every entry in array • Initialize all entries to same value, if desired • Or the value of each entry specified separately • Additional way of specifying size also provided double taxrate[LEVELS] ={0.15, 0.25, 0.3};intvector[100]={0}; // all 100 entries set to 0integer[]={5,0,-5};// eger's size will be 3

11. 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 as1

12. 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 as1 • Stupidity defense legal, if array declared with size 0

13. Guns (& C++) Don't Kill • C++ make arrays easy, but mistakes easy also • Code can access any index within an array • No problems compiling, as long as index an int • Includes indices like -1 or 1029374729192 that stupid • 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

14. Using An Array • Each array entry behaves like variable • Accessed via array variable is only difference • To use or assign entry, specify index inside brackets • Example code snippet computing powers of 2: long loserArray[10];loserArray[0] = 1;for (long i=0; i < sizeof(loserArray);i++){loserArray[i] = loserArray[i-1] * 2;cout << loserArray[i] << endl;}

15. Using An Array • Each array entry behaves like variable • Accessed via array variable is only difference • To use or assign entry, specify index inside brackets • Example code snippet computing powers of 2: long loserArray[10];loserArray[0] = 1;for (longi=0; i < sizeof(loserArray);i++){loserArray[i] = loserArray[i-1] * 2;cout << loserArray[i] << endl;}

16. Using An Array • Each array entry behaves like variable • Accessed via array variable is only difference • To use or assign entry, specify index inside brackets • Example code snippet computing powers of 2: long loserArray[10];loserArray[0] = 1;for (inti=0; i < sizeof(loserArray); i++){loserArray[i] = loserArray[i-1] * 2;cout << loserArray[i] << endl;}

17. Using An Array • Each array entry behaves like variable • Accessed via array variable is only difference • To use or assign entry, specify index inside brackets • Example code snippet computing powers of 2: const int BORED_NOW = 10;long loserArray[BORED_NOW];loserArray[0] = 1;for (inti=0; i < BORED_NOW; i++){loserArray[i] = loserArray[i-1] * 2;cout << loserArray[i] << endl;}

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

19. Your Turn • Get into your groups and try this assignment

20. For Next Lecture • Read 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 #7 out & due Wednesday • Avoid the rush by start working on it now • Programming Assignment #2 now on Angel • This is a larger assignment; start it now!