Array examples

Comments and questions to John Rowe.

Using scanf()

Here we use scanf() to read in sides[0] and sides[1] and use them in arithmetic expression, again just like a variable.

#include <stdio.h>
#include <math.h>
int main() {
  double sides[3];

  printf("Please enter the two sides of a right-angled triangle\n");
  scanf("%lf %lf", &sides[0], &sides[1]);

  sides[2] = sqrt(sides[0] * sides[0] + sides[1] * sides[1]);

  // ... Do something interesting 
  return 0;
}

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Reading an array using a function

Having said it would be useful to have a function called readarray(), here it is. To demonstrate that it is the address of the array that is passed to a function we print out the value of "numbers" and "input" to demonstrate they are the same.

//
// Demonstrate passing an array to a function
//
#include <stdio.h>
#define VALUES 1000000

void readarray(int input[], int n) {
  int i;

  printf("In readarray() the array is stored at address %p\n", input);
  for (i = 0; i < n; ++i ) {
    scanf("%d\n", &input[i]);
  }
}

void printarray(int output[], int n) {
  int i;

  for (i = 0; i < n; ++i ) {
    printf("%d\n", output[i]);
  }
}

int main() {
  int numbers[VALUES];

  printf("In main() the array is stored at address %p\n", numbers);
  readarray(numbers, VALUES);

  printf("The value of numbers[2] is: %d\n", numbers[2]);
  printarray(numbers, VALUES);
  return 0;
}

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The output is:

In main() the array is stored at address 0x7fff0f495e40
In readarray() the array is stored at address 0x7fff0f495e40
The value of numbers[7] is: 7

(Note that addresses are printed in hexadecimal.) Of course, the exact address printed out may be different each time. When I run it again a few seconds later I get:

In main() the array is stored at address 0x7fffe9c3d9f0
In readarray() the array is stored at address 0x7fffe9c3d9f0
The value of numbers[7] is: 7

Aside: a better version

The above version of readarray() doesn't check to see if it was actually able to read in the required number of integers. We can use what we learnt in the last lecture about the return value of fscanf() to improve it a little as follows:

//
// Read up to maxv integers, return the number read
// or -1 if the file could not be opened
int better_readarray(int input[], int maxv) {
  int i;
  FILE *infile;

  if ((infile = fopen("mydata.dat", "r")) == NULL ) {
    fprintf(stderr, "Failed to open mydata.dat for input\n");
    return -1;
  }

  printf("In readarray() the array is stored at address %p\n", input);
  for (i = 0; i < maxv; ++i ) {
    if (fscanf(infile, "%d\n", &input[i]) != 1)
      break;
  }

  return i;
}

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Dot product of vectors

As stressed in the lecture, in C a two-dimensional array is just an array of one-dimensional arrays. Consider the following dot-product function:

// Return the scalar (dot) product of two vectors
float dotprod(float v1[], float v2[], int n) {
  float res = 0;

  for (int i = 0; i < n; ++i)
    res += v1[i]*v2[i];

  return res;
}

#define VECLEN 2
#define NVECS  3
int main() {
  float dot[NVECS][NVECS], vector[NVECS][VECLEN] = 
    { 1.5, 3.9, 4.7, 7.2, 2.1, 5.13,  };
  int j, k;

  for (j = 0; j < NVECS; ++j) {
    for (k = j; k < NVECS; ++k)
      dot[j][k] = dotprod(vector[j], vector[k], VECLEN);
  }

  return 1;
}

Step through this code

 

Here we are using symmetry to save time and using the fact that we are allowed to write a = b = c which can occasionally be useful but should be used with care. For example y=2*(z=a+b) is also legal but is certainly not wise!

Notice also that the dotprod() function neither knows or cares if it has been passed two "genuine" one-dimensional arrays or two different parts of a two dimensional aray (or even if it has been passed the same array twice, which isn't a problem in this case but might be in other circumstances.)