advanced cis15 programming techniques, using c++ summer 2008 lecture # V.1 topics: arrays pointers arrays of objects resources: some of this lecture is covered in parts of Pohl, chapter 3
arrays review
a data structure consisting of related elements of the same data type an array is a regular grouping or ordering example: a string is an array of characters an array has a length associated with it arrays need: data type name length length can be determined: statically at compile time e.g., char str1[10]; dynamically at run time e.g., char *str2; (well talk about how to do this in our next lecture)
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arrays and memory
dening a variable is called allocating memory to store that variable dening an array means allocating memory for a group of bytes, i.e., assigning a label to the rst byte in the group individual array elements are indexed starting with 0 ending with length 1 indeces follow array name, enclosed in square brackets ([ ]) e.g., arr[25]
arrays: character array example
// example: arrays0c.cpp #include <iostream> using namespace std; const int MAX = 6; int main( void ) { char str[MAX] = "ABCDE"; int i; for ( i=0; i<MAX-1; i++ ) { cout << str[i] << " "; } cout << endl; } /* end of main() */
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arrays: integer array example
// example: arrays0i.cpp #include <iostream> using namespace std; const int MAX = 6; int main() { int arr[MAX] = { -45, 6, 0, 72, 1543, 62 }; int i; for ( i=0; i<MAX; i++ ) { cout << arr[i] << " "; } cout << endl; } /* end of main() */
pointers overview
a pointer contains the address of an element allows one to access the element indirectly & = unary operator that gives address of its argument * = unary operator that fetches contents of its argument (i.e., its argument is an address) note that & and * bind more tightly than arithmetic operators you can print the value of a pointer using cout with the pointer or using C-style printing (e.g., printf()) and the formatting character %p
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pointers: memory addresses (1)
declaring pointers: pointers indirectly address memory where data of the types weve already discussed is stored (e.g., int, char, oat, etc.even classes) declaration uses asterisks (*) to indicate a pointer to a memory location storing a particular data type example: int *count; float *avg;
pointers: memory addresses (2)
ampersand & is used to get the address of a variable example: int count = 12; int *countPtr = &count; &count returns the address of count and stores it in the pointer variable countPtr a picture: countPtr count 12
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pointers: memory addresses (3)
heres another example: int i = 3, j = -99; int count = 12; int *countPtr = &count;
pointers: address arithmetic
an array is some number of contiguous memory locations an array denition is really a pointer to the starting memory location of the array and pointers are really integers so you can perform integer arithmetic on them e.g., +1 increments a pointer, -1 decrements you can use this to move from one array element to another
and heres what the memory looks like: variable name memory location value count 0xb4f0 12 i 0xb4f4 3 j 0xb4f8 -99 ... countPtr 0xb600 0xb4f0 ...
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pointers: example
// pointers0.cpp #include <iostream> using namespace std; int main() { int i, *j, arr[5]; for ( i=0; i<5; i++ ) { arr[i] = i; } cout << "arr=" << arr << endl; cout << endl; for ( i=0; i<5; i++ ) {
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cout cout cout cout
<< << << <<
"i=" << i; " arr[i]=" << arr[i]; " &arr[i]=" << &arr[i]; endl;
} cout << endl; j = &arr[0]; cout << "j=" << j; cout << " *j=" << *j; cout << endl << endl;; j++; cout << "after adding 1 to j: j=" << j; cout << " *j=" << *j << endl; }
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and the output is... arr=0xbffff864 i=0 i=1 i=2 i=3 i=4 arr[i]=0 arr[i]=1 arr[i]=2 arr[i]=3 arr[i]=4 &arr[i]=0xbffff864 &arr[i]=0xbffff868 &arr[i]=0xbffff86c &arr[i]=0xbffff870 &arr[i]=0xbffff874 // pointers1.cpp #include <iostream> using namespace std; int main() { int x, y; int *px; x = 3; px = &x;
pointers: another example
j=0xbffff864 *j=0 after adding 1 to j: j=0xbffff868 *j=1 NOTE that the absolute pointer values can change each time you run the program! BUT the relative values will stay the same.
// declare two ints // declare a pointer to an int // initialize x // set px to the value of the address of x; // i.e., to point to x // set y to the value stored at the address pointed // to by px; in other words, the value of x
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y = *px;
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printf( "x=%d px=%p y=%d\n",x,px,y ); // printing them (above) produces something like: // x=3 px=0xbffffce0 y=3 // note that the precise value of px will depend on the machine // and may change each time the program is run, because its value // depends on what portion of memory is allocated to the program // by the operating system at the time that the program is run x++; // increment x
// // // //
printing them (above) produces something like: x=5 px=0xbffffce0 y=3 note that the value of x changes, because px contains the address of x
// what happens if we take away the parens? *px++; printf( "x=%d px=%p y=%d\n",x,px,y ); // printing them (above) produces something like: // x=5 px=0xbffffce4 y=3 // the value of px changes -- is that what you expected? // also note that it goes up by 4 -- because it is an integer pointer // and integers take up 4 bytes // since px has changed, what does it point to now? printf( "*px=%d\n",*px ); // the output is: // *px=3 // because px now points to ys address -- this is because y was
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printf( "x=%d px=%p y=%d\n",x,px,y ); // printing them (above) produces something like: // x=4 px=0xbffffce0 y=3 // note that the value of x changes, but not px or y (*px)++; // increment the value stored at the address pointed // to by px
printf( "x=%d px=%p y=%d\n",x,px,y );
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// declared right after x was declared. note that this is usually // the case, but not necessarily. use an array to ensure contiguity of // addresses. }
and the output step is... step step step 0: 1: 2: 3: here is what we start with: after incrementing x: after incrementing (*px): after incrementing *px: and *px=3 x=3 x=4 x=5 x=5 px=0xbffff874 px=0xbffff874 px=0xbffff874 px=0xbffff878 y=3 y=3 y=3 y=3
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and heres a picture of whats going on:
step 0: px this is the initial situation: x is initialized to the value 3 px is initialized to point to x y is initialized to the value pointed to by x
pointers and references
pointers (same as in C): int *p means pointer to int p = &i means p gets the address of object i references (not in C): they are basically aliases alternative names for the values stored at the indicated memory locations, e.g.:
x=
3
y=
3
step 1:
px
here is the situation after incrementing x
x=
4
y=
3
step 2:
px
here is the situation after incrementing (*px), i.e., the value that px points to, in other words, x y= 3
x=
5
int int double double
n; &nn = n; arr[10]; &last = arr[9];
the dierence between them:
step 3: px here is the situation after incrementing px i.e., the POINTER increments, in other words, it moves to point to the next contiguous item in memory, in this case, y
// refs.cpp #include <iostream> #include <cstdio> using namespace std;
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x=
5
y=
3
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cout << endl; int main() { int A = 1; int B = 2; int *p = &A; int &refA = A; *p = 3; cout cout cout cout << << << << // // // // // declare and define A declare and define next memory location after A p points to A alias (reference) for A *p points to A, so A is assigned 3 A++; cout cout cout cout p++; cout cout cout cout << << << << "after A++ "; " A=" << A << " p=" << p << " *p=" << *p << " refA=" << refA; " &A=" << &A << " &p=" << &p << " &refA=" << &refA; endl;
"initially: "; " A=" << A << " p=" << p << " *p=" << *p << " refA=" << refA; " &A=" << &A << " &p=" << &p << " &refA=" << &refA; endl;
<< << << <<
"after p++ "; " A=" << A << " p=" << p << " *p=" << *p << " refA=" << refA; " &A=" << &A << " &p=" << &p << " &refA=" << &refA; endl;
A = *p + 1; // A is assigned value of *p=3 plus 1 cout << "after A=*p+1: "; cout << " A=" << A << " p=" << p << " *p=" << *p << " refA=" << refA; cout << " &A=" << &A << " &p=" << &p << " &refA=" << &refA; cout << endl; A = refA + 1; // A is assigned value of refA, now 4, plus 1 cout << "after A=refA+1: "; cout << " A=" << A << " p=" << p << " *p=" << *p << " refA=" << refA; cout << " &A=" << &A << " &p=" << &p << " &refA=" << &refA;
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refA++; cout << cout << cout << cout << }
"after refA++ "; " A=" << A << " p=" << p << " *p=" << *p << " refA=" << refA; " &A=" << &A << " &p=" << &p << " &refA=" << &refA; endl;
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and the output is:
initially: after A=*p+1: after A=refA+1: after A...