Understanding Arrays in C: Declaration, Initialization, and Sorting Techniques

Exercise 6 Arrays

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 … S

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]);

Initialization • 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};

Example Sort.c

A[0] A[1] A[2] Sort – step by step 4 8 2 min_index ---- i j tmp #include <stdio.h> #define SIZE 3 int main(void) { int A[SIZE] = {4,8,2}; int min_index; int i,j,tmp; ---- ---- ----

A[0] A[1] A[2] Sort – step by step 4 8 2 min_index ---- i j tmp 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; } 0 ---- ----

A[0] A[1] A[2] Sort – step by step 4 8 2 min_index 0 i j tmp 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; } 0 ---- ----

A[0] A[1] A[2] Sort – step by step 4 8 2 min_index 0 i j tmp 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; } 0 1 ----

A[0] A[1] A[2] Sort – step by step 4 8 2 min_index 2 i j tmp 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; } 0 3 4

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 – 1 e – 1 h – 1 l – 3 o – 2 w – 1 r – 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

Understanding Arrays in C: Declaration, Initialization, and Sorting Techniques

Understanding Arrays in C: Declaration, Initialization, and Sorting Techniques

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