Exercise 6

Arrays

A block of many variables of the same type Array can be declared for any type

E.g. int A[10] is an array of 10 integers. Examples:

list of students’ marks series of numbers entered by user vectors matrices

Arrays in Memory

Sequence of variables of specified type The array variable itself holds the address in

memory of beginning of sequence Example: double S[10];

The k-th element of array A is specified by A[k-1] (0-based)

0 1 2 3 4 5 6 7 8 9

……

Example - reverse#include <stdio.h>

int main(void){ int i, A[10];

printf("please enter 10 numbers:\n"); for(i=0; i<10; i++) scanf("%d", &A[i]);

printf("numbers in reversed order:\n"); for(i=9; i>=0; i--) printf("%d\n", A[i]);

return 0;}

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

Reverse with #define

/* get 10 integers from the user and printing them in reversed order*/

#include <stdio.h>

#define NUM 10

int main(void){ int i; int A[NUM];

printf(“Please enter %d numbers:\n",NUM); for (i=0; i<NUM; i++) scanf("%d",&A[i]);

printf("numbers in reversed order:\n"); for (i=NUM-1; i>=0; i--) printf("%d\n",A[i]);

Like in the case of regular variables, we can initialize the array 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

Initialization

if you like, the array size can be inferred from the number of initializers 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};

Initialization

Example

Sort.c

Sort – step by step

#include <stdio.h>

#define SIZE 3

int main(void){

int A[SIZE] = {4,8,2}; int min_index;int i,j,tmp;

4 8 2A[0] A[1] A[2]

min_index----

---- ---- ----i j tmp

Sort – step by step

for(i=0; i<SIZE-1; i++){

min_index=i;for(j=i+1; j<SIZE; j++)

min_index=((A[min_index]>A[j] ? j) : min_index);

tmp=A[i];A[i]=A[min_index];A[min_index]=tmp;

4 8 2A[0] A[1] A[2]

min_index----

0 ---- ----i j tmp

Sort – step by step4 8 2A[0] A[1] A[2]

min_index0

0 ---- ----i j tmp

min_index0

0 1 ----i j tmp

min_index0

0 1 ----i j tmp

min_index0

0 2 ----i j tmp

min_index2

0 2 ----i j tmp

min_index2

0 3 ----i j tmp

min_index2

0 3 4i j tmp

min_index2

0 3 4i j tmp

min_index2

0 3 4i j tmp

min_index2

1 3 4i j tmp

min_index1

1 3 4i j tmp

min_index1

1 2 4i j tmp

min_index2

1 2 4i j tmp

min_index2

1 3 4i j tmp

min_index2

1 3 8i j tmp

min_index2

1 3 8i j tmp

min_index2

1 3 8i j tmp

min_index2

2 3 8i j tmp

Exercise

Write a program that gets an input line from the user (ends with ‘\n’) and displays the number of times each letter appears in it.

Example:For the input line: hello, world!The output should be:d – 1e – 1h – 1l – 3o – 2w – 1r – 1

Assume that the input is all in lower-case.

Solution

letter_count.c

Multi-dimensional arrays

Array of arrays:int A[2][3] = { {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 A[i][j]

Multi-dimensional arrays

The size of the array can be determined by the compiler:

double B[][2] = {{1,2}, {2,3}, {3,4}};

Cannot skip this!!

Example: matrix addition#include <stdio.h>

#define SIZE 3

int main() {

double A[][SIZE] = {{1,2,3}, {4,5,6}, {7,8,9}};double B[][SIZE] = {{1,1,1}, {2,2,2}, {3,3,3}};double C[SIZE][SIZE];int i, j;

for(i=0; i<SIZE; i++) for(j=0; j<SIZE; j++) C[i][j]=A[i][j] + B[i][j];

return 0;}

Exercise

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 = A*B)

Print all the matrices on the screen

Arrays as function arguments

Functions can accept arrays as arguments

Usually the array’s size also needs to be passed (why?)

Arrays as function arguments

For example:int CalcSum(int arr[], int size);

Within the function, arr is accessed in the usual way

Changes in the function affect the original array!!

Example

calc_sum.c

Exercise

Implement a function that accepts two integer arrays and returns 1 if they are equal, 0 otherwise

Write a program that accepts two arrays of integers from the user and checks for equality

Solution

compare_arrays.c

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