(define x (read))
(begin (display x)
       3
       (display "Hurry, lab is about to start.")
       (reverse (list 2 'x 'y))
       17)

By the way, Dr. Scheme can also display pictures.

Comp210 Lab 13: Vectors and Strings in C

Table of Contents

Vectors in C

(define size 3)                         const int size = 3;
(define nums (make-vector size))        int nums[ size ];
(define nums (vector 8 9 10))           int nums[] = { 8, 9, 10 };
(vector-ref nums 2)                     nums[2];
(vector-set! nums 2 1000)               nums[2] = 1000;
(vector-length nums)                    (no equivalent)

for loops are idiomatic for accessing each element of a vector.

const int size = 100;       // "const" means you can't assign to this var
int   vec[size];            // a global array, indexed 0..size-1.

// init()
// Initialize the vector "vec" to be all zeroes.
//
void init() {

  for ( int i = 0;  i < size;  i = i + 1 ) {
    vec[ i ] = 0;
    }
  return;
  }

Exercise: Create an array of size booleans, and initialize them to true, except for entries 0 and 1, which should be false

If you want, write a function to print out your vector, so you can confirm your initialization. Does your code work even if size==1?!

Index-out-of-bound Errors

However, the Scheme and C handle this error in very different ways. Scheme lets you know immediately that there is an error; C doesn't! Instead, C might:

Here is a correct program; in a moment you'll introduce an error to see what happens. For the moment, understand, compile and run it. (Remember to copy it into a file whose names in .cc.)

#include <iostream.h>

int main() {
  const int arrSize = 3;
  int before = 0;
  int nums[ arrSize ] = { 97, 98, 99 };
  int after = 1;

  // In a moment, you'll add some code here.

  cout <<  "Before should be 0: " << before << ".\n";

  for ( int i = 0;  i < arrSize;  i++ ) {
    cout << "nums[" << i << "] is " << nums[i] << ".\n";
    }

  cout << "After should be 1: " << after << ".\n";
}

One word of warning about using ++ to increment an int: Feel free to use this by itself, but do not combine ++ with other statements. For instance, j = i++; should be re-written as two seperate statements, j = i; i++;. (Just because something is legal C doesn't mean it's good C.) There are common, hard-to-find errors which can occur when combining expressions.

Now, add the following code at the indicated place:

  // ILLEGAL, OUT-OF-BOUND INDICES!
  nums[ arrSize+1 ] = 17;
  nums[ -1 ] = 19;

Passing Arrays as Arguments

Passing a vector to a function is straightforward, except for one thing: keeping track of the size of the vector. (Remember that C doesn't have a vector-length function.) You can write a function taking in a vector, but it is up to that function to never index past the array bounds. The usual solution is to pass the function an additional argument, which tells the function how big the vector is.

Example:

#include <iostream.h>

// sumVec
// A function to add the first "size" entries
// of the vector "data".
//
double sumVec( double data[], int size ) {
  double sum = 0;

  for ( int i = 0;  i < size;  i++ ) {
    sum += data[ i ];      // this is same as: sum = sum + data[i]
    }

  return sum;
  }


int main() {
  const int N = 30;
  double myVec[ N ];

  cout << "The sum of entries 0 through " << N-1 << " is "
       << sumVec( myVec, N ) << ".\n";
  }

Exercise: Copy the above into a file whose name ends in .cc, and compile it. What is the result? Is it the same as your neighbor's?

Exercise: Write the function readVec, which (like sumVec) takes a vector of doubles and a number N, and uses cin to read a number from the keyboard into the first N elements fo the vector. Call readVec from main and run your program again.

(You may wish to set N = 4 or so, to save typing. Or, use UNIX's input redirection to have your program take input from a file instead of the keyboard. For example,

  owl%   readVec < ~comp210/Labs/lab13/30nums.txt

Functions that Modify Their Arguments

But what if readVec had assigned to size--would main's variable N be modified? No! That's because there is a new rule: When passing structures or vectors to a function, any changes made to them are seen by the caller, which is not hte case for simple ints or bools, etc. (This same difference occurs in Scheme as well as C--try it!)

If interested in why, ask a labbie after class, or see last lab's digression on hat variables for structures. Or, think about, in Jam2000, how would you effect passing an array to a function? Hint: if vec[0] is in memory location 2000, where is vec[1], or vec[17]? What about the general case of vec[i], for any i? So if you need to tell a function how to access elements of some vector vec, what is the only piece of information you really need to give to the function? Now, what if the function tries to change an element of the vector?

Two-dimensional Arrays

In Scheme, you can use vectors-of-vectors to serve as two-dimensional vectors, writing your own functions make-array, array-ref, and array-set!.

You could take this same approach in C, but the functionality is also built-in (at least, for arrays of a fixed size). Here is some C syntax for declaring such arrays.

const int numRows = 20;
const int numCols = 10;

const char plains = '.';
const char water = '~';
const char mountain = '^';

void flood( char map[ numRows ][ numCols ] ) {

  for ( int r = 0; r < numRows; r++ ) {
    for ( int c = 0; c < numCols; c++ ) {
      map[r][c] = water;
      }
    }

  // build Mt. Ararat?
  map [ numRows/2 ][ numCols/2 ] = mountain;
  // Is this valid for all possible numRows, numCols?
  // No, not if either are zero.  A bizarre case, but good code would check!
                                     
  return;
  }


int main() {
  char earth[ numRows ][ numCols ];
  flood( earth );
  }

Why the inconsistency between one- and two-dimensional arrays? Well actually the unifying rule is that when passing an array, you must include all but the first dimension. (If you do provide a number for that dimesion in the formal parameter, it is ignored.) This is a limitation of C.

Why does C have this limitation? Ask your labbie, or again go back to the Jam2000 model, and think about what's happening. How is the two-dimensional array stored in memory? If you tell the function where v[0][0] resides in memory, but not the dimensions of v, how can the function deduce where v[0][17] resides? But what about v[2][17]? Exercise: Write a function printMap which takes a numRows$\times$numCols array of char, and prints its contents (returning void). Use this function to display the earth.

Strings in Extended C

If you are using Extended C (well, actually, C++) then we'll show you a nice simple way to use strings. However, it's worth mentioning that in plain old non-extended C, strings are handled as vectors of characters. This approach has its own ways of handling strings, which you should learn if and only if you plan on using non-extended C.

If you want to use regular old strings, you'll #include <Strings.h>, and then pretend that the type String is built-in just like int and char, etc.

#include <String.h>
#include <iostream.h>

void threaten( String target ) {
  cout << "Death to " << target << ".\n";
  }

int main() {
  String myName = "Santa Claus";
  String nemesisName;
  nemesisName = "The Easter Bunny";

  threaten( nemesisName );

  cout << "My name has " << myName.length() << " letters.\n";
  return 0;
  }

This is all okay, except that there are now some fancy things we can do. In particular, go back and add the following line before the call to threaten:

  nemesisName[11] = 'f';

Finally, what's with this myName.length()? It turns out myName is an object which not only has a string value, but also knows how to respond to certain messages, like length()! That's your preview of C++ proper (or Java); take Comp212! :-)