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Chapter 8: Arrays and pointers
BİL 133 – Algoritma ve Programlama
Maltepe University
Computer Engineering Department
Copyright ©2004 Pearson Addison-Wesley. All rights reserved.
Basics int * ptr1, * ptr2;
int a[10];
ptr1 = &a[2];
ptr2 = a; // equivalent to ptr2 = &a[0];
• An array variable is actually a pointer to
the first element of the array.
• ptr2 points to the first element of the
array and get others by offset.
• Referring a[i] is same as referring *(a+i).
a
a+1
a+2
.
.
?
?
?
?
?
?
?
?
0
1
2
3
4
5
6
7
Memory Addresses
2
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Array Elements as Function arguments
• If we want to print the ith element of the array x[i], then
we can do the following:
printf(”%d\n”, x[i]);
scanf(”%d”, &x[i]);
• We can also pass array elements as arguments to
functions that we write. For example:
swap1(x[i], x[i+1]);
swap2(&x[i], &x[i+1]);
3
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Having arrays as function arguments
• Besides passing individual array elements to functions, we can write functions that take entire arrays as arguments.
• There are several ways of passing arrays to functions but in each case we only pass the address of the array.
• This is very similar to what we did during “passing variables by reference…”
• As we are not passing a copy of the array – any changes to the array made within the function will also effect the original array.
• When an array name with no subscript appears in the argument list of a function call, what is actually stored in the function’s corresponding parameter is the address of the array. Example,
int a[10]; foo(a); foo(&a[0]); // same as above
4
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Array Arguments
• We can also pass an entire array as the input
argument for a function.
• However, the function does not copy the array
but just manipulates the original array.
– An assignment to one of the array elements by a
statement in the function changes the contents of the
original array.
5
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Passing array as function parameter
• Array names are pointers.
• The name of an array contain the address of its initial element, a = &a[0];
• Passing an array as function argument copies the value of the pointer to the formal array parameter and points to the same location.
• Where is the array actually located?
– At the data area of the function that declared it.
– For an array declared as formal parameter, space is allocated in the function data area only for the address of the initial array element of the array passed as actual argument.
6
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Using arrays in formal parameter list void foo(int x[10])
• Store the address of the corresponding array argument (a in
previous slide) to variable x, compiler ignores 10 since x is a
pointer.
void foo(int x[])
• This is just a different format to make code easy to read, for
compiler this format or the others have the same meaning.
void foo(int *x)
• Function foo can take any integer array as argument.
• You should also give the size as a parameter to make the
function more portable.
• void foo(int *x, int size)
7
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Arrays as Input Arguments
• ANSI C provides a qualifier that we can include in the declaration of the array formal parameter in order to notify the C complier that the array is only an input to the function and that the function does not intend to modify the array.
• This qualifier allows the compiler to mark as an error any attempt to change an array element within the function.
void foo(const int x[], int size) • The const tells C not to allow any element of the array x
to change in the function foo.
8
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C code Example
int main(void){
int a[5]={1,2,3,4,5};
int i;
clear1(a,5);
clear2(a,5);
for(i=0; i<5; i=i+1)
printf("%d ", a[i]);
return 0;
}
void clear1(int x[], int size){
int i;
for(i=0; i<size; i=i+1)
x[i] = 0;
}
void clear2(int *x, int size){
int *p;
for(p=x; p<(x+size); p=p+1)
// for(p=&x[0]; p<&x[size]; p=p+1)
*p = 0;
}
9
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An Example of Array Arguments
• We have the flexibility to pass an array of any
size to the function.
– Because C does not allocate any memory space
for the array, and thus the compiler does not need
to know the size. int list[] is
the declaration for
passing an int
array
10
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Data Areas After Calling fill_array (x, 5, 1);
The value of each element in the
original array are changed to 1.
11
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Alternative Formats of Array
Arguments • fill_array(x, 5, 1) is the same as
fill_array(&x[0], 5, 1).
– This call may lead to misunderstanding of the reader.
• In the declaration for function fill_array,
we can declare the array argument as either
– int list[] or int *list.
• If you wish to prevent the modification of the
array argument, you can write
– fill_array(const int list[], …)
12
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Using Array Elements as Function
Arguments (1)
• Suppose that we have a function prototype as
shown below.
– void do_it(double arg_1, double
*arg2_p, double *arg3_p);
• Let x be an array of type double elements.
• We can call the function do_it as follows.
– do_it(x[0], &x[1], &x[2]);
• The modification on arg2_p and arg3_p will
change the values of x[1] and x[2]. 13
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Using Array Elements as Function
Arguments (2)
arg_1 has a
copy of the
value from x[0]
arg_3 is a pointer
pointed to the
memory of x[2]. 14
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Returning an Array from a function(1)
• Returning arrays should work too - right?
int a[5];
a = foo();
…..
int* foo() {
int c[5] = {1,2,3,4,5};
return c;
}
• Wrong! In C, it is not legal for a function’s return
type to be an array.
15
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Returning an Array from a function(2)
• Type problem between int[] and int*
– a is a constant pointer type, you can’t change it.
• Array c is stored in function foo’s stack frame.
– Return just copies the pointer, not the array.
– Memory where the array c resides may be overwritten once
function foo finishes its execution.
• To do it properly
– Pass an array as a function argument from your calling
function and modify it inside the called function.
16
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Returning an Array Result
• The add_arrays function computes the sum
of corresponding elements in arrays ar1 and
ar2, and then stores results into ar3.
Input array arguments
Output array
arguments
17
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Memory for calling
add_arrays(x, y, x_plus_y)
Indirectly read the values of array
x
Indirectly read the values of array y
Indirectly write the values of x+y
into x_plus_y 18
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Searching and Sorting an Array
• Sometimes we may need to
– search a particular value in an array, or
– sort an array to rearrange the array elements in
numerical order.
• Commonly used searching methods:
– Sequential search, binary search, etc.
• Commonly used sorting methods:
– Selection sort, bubble sort, quick sort, etc.
19
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Sequential Search in an Array
target is the value we want to
locate in the array
Traverse the array to look for
the value identical to target.
20
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Selection Sort in an Array
• Selection sort is an
intuitive sorting
algorithm.
– Find the index of the
smallest element in
the array.
– Swap the smallest
element with the first
element.
– Repeat the above
steps for the 2nd, 3rd,
…, smallest elements.
Not included
21
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A Function for Selection Sort
Suppose we have a function that
can find the minimal element.
Swap the minimal
element with the
previous larger
element. 22
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Multidimensional Arrays
• A multidimensional array is an array with two or more dimensions.
• We will use two-dimensional arrays to represent tables of data, matrices, and other two-dimensional objects.
• Thus int x[3][3] would define a three by three matrix that holds integers.
• Initialization is bit different
int x[3][3] = {{1,2,3}, {4,5,6}, {7,8,9}};
• Both indices starts at 0.
• Think of it as x[rows][cols]. x[0][0]=1 x[0][1]=2 x[0][2]=3
x[1][0]=4 x[1][1]=5 x[1][2]=6
x[2][0]=7 x[2][1]=8 x[2][2]=9
Row Col inside a row
23
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Multidimensional Arrays
• Multidimensional arrays stand for the arrays with two or more dimensions.
• Syntax for normal usage: element-type aname[size1][size2]…[sizen]; Syntax for parameter in function prototype: element-type aname[][size2]…[sizen]
• e.g., char tictac[3][3];
– A 2-dimensional array with three rows and three columns.
• e.g., tictac[][3];
– The declaration for the function prototype. 24
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The Memory Allocated for tictac[3][3]
• The memory allocated for array
tictac[3][3]is shown below.
• The number of total allocated element is
3*3=9 elements.
25
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Initialization of Multidimensional
Arrays • The multidimensional array can also be
initialized in declarations.
• The initialized values are grouped in rows.
– e.g., char tictac[3][3]={{‘a’, ’b’, ’c’}, {‘d’, ’e’, ’f’}, {‘ ’,‘ ’,‘
’}};
• We can also declare arrays with more dimensions.
– e.g., declare a 3-dimensional array. int
enroll[course][campus][cls_rank]; 26
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2D array in address space(1)
• How to store a multi-
dimensional array into a one-
dimensional memory space.
• Row major ordering assigns
successive elements, moving
across the rows and then down
the columns, to successive
memory locations.
Base_Address is the address of the first element of the array (A[0][0] in this case).
char *base = &A[0][0];
Referring A[i][j] is same as referring *(base+4*i+j); //4 is column here
27
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2D array in address space(2)
int a[2][3]; // 2 rows and 3 cols
• Alternative interpretation: Three 1D array of 4 integers.
• The base address of the array is &a[0][0].
• The array name a by itself is equivalent to &a[0]. This time it is
a pointer to an array of 3 elements.
• Different ways to access (i,j) th element:
a[i][j]
*(a[i] + j)
*((*(a+i)) + j)
*(&a[0][0] + 3*i + j)
a[1][2]
a[1][1]
a[1][0]
a[0][2]
a[0][1]
a[0][0] a
a+1
28
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Multi-dimensional Arrays.
Declaration int a[N]; int b[M][N]; int c[L][M][N];
Array element a[i] b[i][j] c[i][j][k]
Pointer to element a b[i] c[i][j]
Pointer to 1D array of N element
b c[i]
Pointer to 2D array of M*N element
c
29
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Passing Multidimensional array
arr[7][9][2] = {0}; … sum(arr,7); … int sum(int a[][9][2], int asize){ int i, j, k, sum = 0 for( i = 0; i<asize; ++i) { for( j = 0; j<9; ++j) { for( k = 0; k<2; ++k) { sum += a[i][j][k]; return sum; }
For a multidimensional array
we must declare all but the first
dimension. Only 7 can be
omitted.
30
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The Memory Allocated for enroll[100][5][4]
enroll[100][5][4]
consists of 100*5*4
=2000 elements.
31
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Figure 8.9 Function to Add Two Arrays
32
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Figure 8.10 Function Data Areas for
add_arrays(x, y, x_plus_y, 5);
33
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Partially Filled Arrays
• Frequently, a program will need to process many lists of similar data
• These list may not all be the same length
• In order to reuse an array for processing more than one data set, the programmer often declares an array large enough to hold the largest data set anticipated.
• This array can be used for processing shorter lists as well, provided that the program keeps track of how many array elements are actually in use.
• One way to do this is to have the original array size declared as the highest needed, and then store a sentinel value after the last value inputted.
34
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Sorting an Array – Selection Sort
• Similar to sorting cards.
35
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Figure 8.16 Trace of Selection Sort Not included
36
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Common Programming Errors
• The most common error in using arrays is a subscript-range error.
• An out-of-range reference occurs when the subscript value is outside the range specified by the array declaration. – In some situations, no run-time error message will be
produced – the program will simply produce incorrect results.
– Other times, you may get a runtime error like “segmentation fault” or “bus error”
• Remember how to pass arrays to functions.
• Remember that the first index of the array is 0
38
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Study (1)
• Write a function that computes the average grades and
sorts the grades of two classes.
• Your program should input two lists of grades for each
class.
• Your program should output:
(1) average grades for each class;
(2) a list of sorted grades of two classes.
• You can use the sorting function in p.401 in the text
book.
• You can either assume the number of students in each
class is fixed or prompt the user to input the number. 39
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Study (2)
• e.g.,
Please input grades of 1st class:
55 45 65 85 75
Please input grades of 2nd class:
80 70 50 60 40
Output:
The average grades of 1st class is: 65
The average grades of 2nd class is: 60
The sorted grades of two classes are:
40 45 50 55 60 65 70 75 80 85 40