- Arrays store sequences of data of a single type!
- Declaration:
`char A[MAX_N]`

declares an array with elements`A[0], ... A[MAX_N-1]` - Arrays in C start always at 0!

- Arrays are static!

the size has to be known in advance and can't be changed during runtime. - Pointers allow dynamic memory management!

(e.g. arrays of flexible size - more next time)

#include<stdio.h> int main() { int A[10]; int i; /* * store seven numbers in array A * Questions: What are these numbers? */ A[0] = 1; A[1] = 2; A[2] = 4; A[3] = 6; A[4] = 10; A[5] = 12; A[6] = 16; /* print numbers in A */ printf("forward\n"); for (i = 0; i < 7; ++i) printf("A[%d] = %d\n",i,A[i]); /* print numbers in A backwards */ printf("\nbackward\n"); for (i = 6; i >= 0; --i) printf("A[%d] = %d\n",i,A[i]); }

#include<stdio.h> #define MAX_N 20 int main() { /* * Consider a sequence of numbers a_0, a_1, ..., a_(n-1), * and a number x. * * We want to count how many numbers of A are less than x. * This quantity is usually called rank_A(x) * */ float A[MAX_N]; float x; int rank, i; int n; printf("how many numbers:\t"); scanf("%d",&n); /* read numbers */ for (i = 0; i < n; ++i) { printf("number:\t"); scanf("%f",&A[i]); } /* read x */ printf("x:\t"); scanf("%f",&x); /* count elements smaller than x */ rank = 0; for (i = 0; i < n; ++i) rank += (A[i] < x); /* output */ printf("rank(%f) = %d\n",x,rank); }

#include<stdio.h> #define MAX_N 20 int main() { /* Consider a set of numbers a_0, a_1, ..., a_(n-1) * without repeats. * * In this example, we're sorting the set using ranks. * This sorting algorithm is bad for two reasons. Why? * Think in terms of required space and time... * * What do you need to change to allow repeated numbers? */ int A[MAX_N]; int Asorted[MAX_N]; int i, j; int rank[MAX_N]; int n; printf("how many numbers:\t"); scanf("%d",&n); /* read numbers */ for (i = 0; i < n; ++i) { printf("number:\t"); scanf("%d",&A[i]); } /* count elements smaller than x */ for (j = 0; j < n; ++j) { rank[j] = 0; for (i = 0; i < n; ++i) rank[j] += (A[i] < A[j]); } /* sort elements */ for (i = 0; i < n; ++i) Asorted[rank[i]] = A[i]; /* output */ for (i = 0; i < n; ++i) printf("%d \t",Asorted[i]); }

#include<stdio.h> #define MAX_N 20 #define MAX_M 100 int main() { /* Consider a set of numbers a_0, a_1, ..., a_(n-1) * from 0 to MAX_M without repeats. * * Here's a different sorting algorithm. * Can you figure out how it works? What's so special * about this one? Compare it to the previous one. * Do you know other algorithms? * * Can you fix it to work with repeats? * In which situation is this algorithm bad? * How would you fix it? */ int A[MAX_N]; int B[MAX_M]; int C[MAX_N]; int i, j, n; /* initialization */ for (i = 0; i < MAX_M; ++i) B[i] = 0; /* input */ printf("how many numbers:\t"); scanf("%d",&n); for (i = 0; i < n; ++i) { printf("number:\t"); scanf("%d",&A[i]); } /* sorting */ for (i = 0; i < n; ++i) ++B[A[i]]; for (i = 0; i < MAX_M-1; ++i) B[i+1] += B[i]; for (i = 0; i < n; ++i) C[B[A[i]]-1] = A[i]; /* output */ for (i = 0; i < n; ++i) printf("%d \t",C[i]); }

#include<stdio.h> #define MAX_N 200 int main() { /* * Compute prime number in interval from 1 to MAX_N. * * Questions: * How many prime numbers are in that interval? How * does this number grow for large MAX_N? Draw the * graph if you know how to use any graphics functions. */ int P[MAX_N]; int i, k; /* mark all numbers */ for (i = 0; i < MAX_N; ++i) P[i] = 1; for (i = 2; i < MAX_N; ++i) if (P[i]) { printf("%d\n", i); for (k = i+i; k < MAX_N; k+= i) P[k] = 0; } }