1. Copyright 1998-2014 Curt Hill Arrays in C/C++ What? Why? How?

2. Copyright 1998-2014 Curt Hill Motivating Example Suppose we have an application that collects votes We have one office with four candidates We want to read in a number and then increment the vote count for that candidate

3. Copyright 1998-2014 Curt Hill Use the following first cut Set variables to zero While input do –Read in a variable –Test value –If in 0..3 –Then increment appropriate vote total –Else reject When done show results

4. Copyright 1998-2014 Curt Hill Voting Machine int count,v0=0,v1=0,v2=0,v3=0, vote; while(cin>>vote) { switch (vote) { case 0: v0++; break; case 1: v1++; break; case 2: v2++; break; case 3: v3++; break; default: cout << "Invalid vote\n"; }} cout << "Totals " << v0 << v1 << v2 << v3 << "\n";

5. Copyright 1998-2014 Curt Hill Explanation cin >> vote also returns a value that can drive the if If the cin fails to find an int, because it: –hit end of file –hit something that did not conform to the specifications of an integer Then the while ends –Classic C way of doing two things in one construct

6. Copyright 1998-2014 Curt Hill What is wrong with this? This will work fine for 4 candidates What about 20? What is the significant difference between v0 and v1? The candidate to which the count applied They all have the same type and the same meaning

7. Copyright 1998-2014 Curt Hill Array Solution int v[4], vote; for (int i=0;i<4;i++) v[i]=0; while(cin>>vote) { if (vote 3) cout << "Invalid vote\n"; else v[vote]++; } for (i=0;i<4;i++) cout << "Candidate " << i << " received " << v[i] << " votes\n";

8. Copyright 1998-2014 Curt Hill What is an array? Multiple occurrences of values of one type –One name many values Item is selected at run time by an integer index The mark of arrays is a pair of square brackets: [ ] Arrays are not a type in themselves –Rather an array name is a constant pointer

9. Copyright 1998-2014 Curt Hill Why do we need arrays? We often need to select a value by an index rather than just name Awkward to have 20 names to hold the vote totals of 20 candidates –More awkward to use Instead we find one name and use an integer subscript to choose which of the twenty we want

10. Copyright 1998-2014 Curt Hill Arrays One name, many values Arrays elements demonstrate identical form and function The form is that each element has exactly the same type –Polymorphism can come into play here, but not considered until later The function is that they are there for the same purpose

11. Copyright 1998-2014 Curt Hill Voting Machine Again The form and function each v variable was the same Each was an integer (identical form) Each recorded votes cast for one candidate (identical function or purpose) The only thing that distinguished the values is the numeric value of each candidate

12. Copyright 1998-2014 Curt Hill Explanations still needed Declaration –Marked by the [] Access and reference –Indexing or subscripting –In languages other than COBOL index and subscript are synonyms Answers to these questions: –What can we do to an array element? –What can we do to the whole array?

13. Copyright 1998-2014 Curt Hill Array declaration General form: type name [constant]; Example: –int a[5], b, c[20]; –float d[50]; Size must be enclosed in brackets Size must be a constant: –Literal –Defined –constant integer

14. Copyright 1998-2014 Curt Hill More on declaration Brackets are attached to variable name Example: int a[5], b, c[4]; The variables a and c are arrays, but b is a simple integer Bracketed expression does two things: –Declares this to be an array –Specifies the size of the array

15. Copyright 1998-2014 Curt Hill Index Range The index or subscript range is always zero to one less than the declared size Example: –int a[4]; There are four elements: a[0], a[1], a[2], a[3]

16. Copyright 1998-2014 Curt Hill Initialization Generally done with a for loop: for(int i = 0;i<5;i++) a[i] = 0; The [i] chooses which integer to set to zero This is an example of a subscript or index Starts at zero

17. Copyright 1998-2014 Curt Hill Alternate Initialization Initialization may be done with a brace list at declaration time –Several variations to this First example –int b[4] = {0,-1,-2,-3}; – Only general form of whole array assignment Leave out size: –int b[] = {0,-1,-2,-3}; –Counts list for size –This is same as above

18. Copyright 1998-2014 Curt Hill This allows problems What if the size of the brace list does not match the size of the array? Two possibilities: Too long: –int a[3] = {0,1,2,3} –This is a syntax error - what to do with 3? Too short: –int a[10] = {0,1,2,3} –First four items are initialized and rest are not –No syntax error, no warning

19. Copyright 1998-2014 Curt Hill Initialization Again What can be in the braces? Most anything that may be on right hand side of an assignment operator –Constants –Variables –Expressions –Function calls –Constructors for classes

20. Copyright 1998-2014 Curt Hill Constant Array Bounds We use defined constants to parameterize arrays and the for loops that process them Thus: #define MAX 10 const int SMax 50 … int counts[MAX], states[SMax]; … for(int i = 0;i<MAX;i++) {…} Easy to change the constant if we need a different size

21. Copyright 1998-2014 Curt Hill Brackets [ ] We can think of brackets as an address modification operator in code and a length modification in declarations As such they have very high precedence They can only be applied to pointer like operands An array name is a constant pointer

22. Copyright 1998-2014 Curt Hill Alternate Initialization Again No parameterized way to initialize with braces FORTRAN allows an initializer: 10*0 –which generates 10 zeros Nothing like that in C/C++ Thus arrays that have ranges that may change are usually initialized with a for

23. Copyright 1998-2014 Curt Hill Subscripting Using an index obtains an element –a[2] = 5; –a[k] = a[j+2]-2; Anywhere an int can be used a[k] can be used –Even nested a[a[k+1]] = a[a[a[j]+2]]-2; First element has index zero Last element has subscript array size minus one

24. Copyright 1998-2014 Curt Hill A Subscript Chooses the element of the array The item may be a –integral constant –integral variable –integral expression No doubles or float subscripts –There is no element between [2] and [3] –They will be truncated to an integer Must be in the correct range –0 to length of array-1

25. Copyright 1998-2014 Curt Hill Errors What if the value presented for a subscript is not valid? –It is not in proper range Oops - by default there is no checking –In many cases checking is not possible Presumably if you go after the array element that is larger than the bounds then you damage nearby variables This is not necessarily caught at compile time or runtime

26. Copyright 1998-2014 Curt Hill Subscript error example Suppose: int first, v[10], last; Thus v[-1] or v[10] will not cause a compile error However first and last will be accessed rather than v The compiler is under no obligation to lay out the variables in any way other than for its own convenience –Thus v[10] could be the same as first or last – we cannot tell which At runtime the OS may intervene if the subscript is wildly out of bounds

27. Copyright 1998-2014 Curt Hill Discussion Since an array is one name with many values, an array element has a two piece name –The array name and the subscript We may assign to this name or use in an expression Processing an array sequentially is almost always done with a for statement

28. Copyright 1998-2014 Curt Hill Address Modification The address calculation needs some explanation Consider the following declaration Item var[MAX]; Suppose the following expression: var[ndx] How does the compiler compute the needed address? address(var) + sizeof(Item) * ndx

29. Copyright 1998-2014 Curt Hill What was that again? address(var) + sizeof(Item) * ndx The first thing it needs is the base address of the array This is actually the constant pointer –The name of the variable To this it must add the product of –The length of one Item thing –The subscript In this case ndx

30. Copyright 1998-2014 Curt Hill Examples int x[4],y[4]; x[i] = y[j]; To find x[i] –Take the base address of x –Add the product of i the length of an int (4) No assignments of the this form are allowed: x = y; because x is a constant pointer

31. Using whole arrays Suppose the following declaration –int a[50], b[50]; Without subscript an array name is just a constant pointer Thus –b = a; –Is not legal Only place to use an array name without an index is when passing as a parameter Copyright 1998-2014 Curt Hill

32. Arrays as parameters Array elements by value –Nothing odd: cos(z[3]) –An element of an array may be used anywhere that type may be used Array elements by reference –Nothing odd: void doit(int & x); … doit(z[i]);

33. Copyright 1998-2014 Curt Hill Whole array parameters Two things that are different –No size needed –Passed by reference only

34. Copyright 1998-2014 Curt Hill No Size Needed The formal parameter list does not need the size: int sum(int ar[], int size); –The brackets mark it as an array –It is already allocated so no size needed –Any array of right type may be passed There is no way for sum to discover the size on its own Thus most functions take the array and a separate parameter to indicate size to use

35. Copyright 1998-2014 Curt Hill Reference Only Since an array name is a constant pointer we do not need to use the & in the formal parameter list A value parameter makes a copy and passes it Since arrays could be large, making a copy could be expensive –Thus disallowed

36. Copyright 1998-2014 Curt Hill Example formal parameters Consider: int average(int ar[], int size){…} … int large[1000], small[10]; … x = average(large,1000); y = average(small,10); Creating generalized array processing libraries in C/C++ is easy

37. Copyright 1998-2014 Curt Hill Arrays as input parameters Since we only use call by reference, how do we prevent a function from changing an array? Use const prefix: int cc(const int ar[], int size); If you attempt to assign to ar then errors occur –Unfortunately you can get around it and change a constant array –But I am not going to say how

38. Copyright 1998-2014 Curt Hill Specifying the size You may specify a size in a formal parameter list –But it is ignored Thus: int doit(int ar[], int size); int doit(int ar[9],int size); are exactly the same