Agenda

1
Agenda

• Arrays
• Definition
• Memory
• Examples
• Passing arrays to functions
• Multi dimensional arrays

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Arrays

• Array sequential block of memory that holds
  variables of the same type
• Array can be declared for any type
• Example int A10 is an array of 10 integers.
• Examples
• list of students marks
• series of numbers entered by user
• vectors
• matrices

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Arrays in Memory

• Sequence of variables of specified type
• The array variable itself holds the address in
  memory of beginning of sequence
• Example
• double s10
• The k-th element of array A is specified by
  Ak-1 (0 based)

0
1
2
3
4
5
6
7
8
9


s
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Arrays in Memory

• Access arrays content
• Change arrays content

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Example Find Minimum

• int i, min, array10
• printf("please enter 10 numbers\n")
• for(i 0 i lt 10 i)
• scanf("d", arrayi)
• min array0
• for(i 1 i lt 10 i)
• if (arrayi lt min)
• min arrayi
• printf("the minimum is d\n", min)

6
Define

• Magic Numbers (like 10 in the last example) in
  the program convey little information to the
  reader
• Hard to change in a systematic way
• define defines a symbolic name
• During preprocessing phase, symbolic names are
  replaced by the replacement text

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minimum with define

• include ltstdio.hgt
• define ARRAY_SIZE 10
• int main(void)
• int i, min, arrayARRAY_SIZE
• printf("please enter d numbers\n",
  ARRAY_SIZE)
• for(i 0 i lt ARRAY_SIZE i)
• scanf("d", arrayi)
• min array0
• for(i 1 i lt ARRAY_SIZE i)
• if (arrayi lt min)
• min arrayi
• printf("the minimum is d\n", min)
  return 0

8
Initialization

• Can be initialized during declaration.
• the number of initializers cannot be more than
  the number of elements in the array
• but it can be less
• in which case, the remaining elements are
  initialized to 0
• if you like, the array size can be inferred from
  the number of initializers (not recommended)
• by leaving the square brackets empty
• so these are identical declarations
• int array1 8 2, 4, 6, 8, 10, 12, 14, 16
• int array2 2, 4, 6, 8, 10, 12, 14,
  16

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Exercise

• Write a program that gets 10 numbers from the
  user.
• It then accepts another number and checks to see
  if
• that number was one of the previous ones.

Example 1 Please enter 10 numbers 1 2 3 4 5 6 7
8 9 10 Please enter a number to search for 8 I
found it! Example 2 Please enter 10 numbers 1
2 3 4 5 6 7 8 9 10 Please enter a number to
search for 30 Sorry, its not there.
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Solution (simple_search.c)

• include ltstdio.hgt
• define ARRAY_SIZE 10
• int main(void)
• int arrayARRAY_SIZE, i, num
• printf("Please enter d numbers\n",
  ARRAY_SIZE)
• for (i 0 i lt ARRAY_SIZE i)
• scanf("d", arrayi)
• printf("Please enter a number to search
  for\n")
• scanf("d", num)
• for (i 0 i lt ARRAY_SIZE i)
• if (arrayi num)
• printf("I found it!\n")
• return 0

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Some Notes on Array

• Arrays versus basic variables
• Out of range
• Equality do not use the operator on arrays
  (until we learn otherwise). Copy instead
• Size is always a constant (until we learn
  otherwise)

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Agenda

• Arrays
• Definition
• Memory
• Examples
• Passing arrays to functions
• Multi dimensional arrays

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Arrays as function arguments

• Functions can accept arrays as arguments
• The arrays size also needs to be passed (why?)

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Arrays as function arguments

• For example int calc_sum(int arr, int size)
• Within the function, arr is accessed in the usual
  way
• Changes to the array in the function change the
  original array! (why?)

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Example (mult_all.c)

• void mult_all(int arr, int size, int
  multiplier)
• int i 0
• for (i 0 i lt size i)
• arri multiplier
• int main()
• int i,array1,2,3,4,5,6,7,8,9,10
• mult_all(array,10,5)
• printf(array is now\n)
• for (i0 ilt10 i)
• printf(d ,arrayi)
• printf(\n)
• return 0

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Example - Sort

• We would like to sort the elements in an array in
  an ascending order.

sort
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Example of Sort (Bubble Sort)
(done)
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Bubble Sort

• void sort(int a, int size)
• int i, j, temp
• for (i size - 1 i gt 0 --i) / counting
  down /
• for (j 0 j lt i j) / bubbling
  up /
• if (aj gt aj1) / if out of
  order... /
• / ... then swap /
• temp aj
• aj aj1
• aj1 temp

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Using sort

• include ltstdio.hgt
• define ARRAY_SIZE 5
• void sort(int a, int size)
• int main()
• int arrayARRAY_SIZE 7, 2, 8, 5, 4
• int i 0
• sort(array, ARRAY_SIZE)
• / print the sorted array /
• for (i 0 i lt ARRAY_SIZE i)
• printf("d ", arrayi)
• return 0

20
Exercise (_at_ home?)

• Implement a function that accepts two integer
  arrays and returns 1 if they are equal, 0
  otherwise.The arrays are of the same size
• Write a program that accepts two arrays of
  integers from the user and checks for equality

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Solution (compare_arrays.c)

• int compare_arrays(int arr1, int arr2, int
  size)
• int i 0
• / compare the elements one at a time /
• for (i 0 i lt size i)
• if (arr1i ! arr2i)
• return 0
• / if we got here, both arrays are identical
  /
• return 1

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Agenda

• Arrays
• Definition
• Memory
• Examples
• Passing arrays to functions
• Multi dimensional arrays

23
Multi-dimensional arrays

• Array of arrays
• int A23 1, 2, 3,
• 4, 5, 6
• Means an array of 2 integer arrays, each of
  length 3.
• Access j-th element of the i-array is
• Aij

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Multi-dimensional arrays

• The size of the array can be determined by the
  compiler (not recommended)
• int B2 1,2, 2,3, 3,4

Cannot skip this!!
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Example matrix addition
include ltstdio.hgt define SIZE 3 int main()
int ASIZESIZE 1,2,3, 4,5,6,
7,8,9 int BSIZESIZE 1,1,1, 2,2,2,
3,3,3 int CSIZESIZE int i 0, j
0 for (i 0 i lt SIZE i) for (j 0 j
lt SIZE j) Cij Aij
Bij return 0
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2D arrays as function arguments

• The second subscript must be specified and it
  must be constant

• void print_matrix(int mat33)
• int i, j
• for (i 0 i lt 3 i)
• for (j 0 j lt 3 j)
• printf("\td ", matij)
• printf("\n")

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Exercise _at_ home

• Write a program that defines 3 matrices A,B,C of
  size 3x3 with float elements initialize the
  first two matrices (A and B)
• Compute the matrix multiplication of A and B and
  store it in C (i.e. C AB)
• Matrix Multiplication

• Print all the matrices on the screen

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Solution
mat_mul.c
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Debugger (if time allows)
Use the debugger on one of the above examples,
see that you can follow execution step by step
and see the variables values at each stage
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More time (no chance)?
Talk about the loops-related questions from 2
weeks ago