Lecture 5:  2-D Arrays & Functions

 

Objectives of this lecture

q       Learn how to declare, initialize, and process 2-D arrays

q       Be able to pass 2-D array as a parameter to a function

q       Be able to apply 2-D array in problem solving

 

Declaration

q       C allow the declaration of array of any dimension

q       2-D array can be visualized as a matrix or table of data elements consisting of rows and columns

q       Declaration is similar to 1-D array except that the size of both rows and columns must be specified, each in its own bracket.

q       Example:   int grades[100][6]

q       The first size indicates the number of rows, the second the number of columns 

q       The indexing starts from 0.  0…99 & 0…5 in the  above example

 

Initialization

q       2-D array can be initialized at declaration, using either of the following methods:

1.     int  X[3][2] = { {17,22}, {-33,65}, {14,31}}

2.     int  X[3][2] = {17, 22, -33, 65, 14, 31}

q       Each of these methods inserts the data into the array row-wise.

q       In either of these methods, if the data is not enough, the remaining elements are set to zero.

q       Data can also be read into a 2-D array using nested loops.  This can be row-wise or column-wise as shown below:

 

q       Row-wise

for (row=0; row<rsize, row++)

for (col=0; col<csize; col++)

       scanf(“%d”, &a[row][col]);

 

q       Column-wise

for (col=0; col<csize, col++)

for (row=0; row<rsize; row++)

       scanf(“%d”, &a[row][col]);

 

Example 1:

Problem:  The data for three students in three exams of ICS201 is shown in the table below:

 

Write a C program that prints the above data with additional column containing the average of each student and additional row containing the average of each exam.  The program should also print the overall average.

 

Reasoning:  We use a 2-D array to store the data. We then use nested loops row-wise and column-wise to compute the required averages

Input: no input

Ouput: the averages of each student and each exam.

Process:  average = sum of grades / no of exams (or students)

 

Solution:

Step 1: LOOP no of students

a.  initialize sum to zero

b.  LOOP number of exams

sum=sum+grade[j]

c.  student[i] average = sum/no. of exams

step 2: LOOP number of exams

a.  initialize sum to zero

b.  LOOP number of students

Sum = sum+grade[i]

c.  exam [j] average =sum/no. of students

Step 3:  initialize sum to zero

Step 4: LOOP number of students

              LOOP number of exams

                     Sum=sum+grade[i][j]

Step 5: average=sum/(no. students * no. of exams)

Step 6: LOOP number of students

a.  LOOP number of exams

OUTPUT grade[i][j]

b.  OUTPUT student[i] average

Step 7: LOOP number of exams

              OUTPUT exam average

Step 8: OUTPUT overall average

Step 9:  STOP

 

/* computes the average grades of students in 3 exams.

   also computes the average grade in each exam. */

 

#include <stdio.h>

#define NEXAMS 3

#define NSTUDS 3

 

main()

{  int row, col;

   float grade[NSTUDS][NEXAMS]={{50.0,75.0,60.0},

                                                      {90.0,45.0,70.0},

                                                      {80.0,55.0,67.0}};

   float avg_stu[NSTUDS], avg_exm[NEXAMS],sum,avg;

 

   for (row=0; row<NSTUDS; row++)

   {   sum=0;

       for (col=0; col<NEXAMS; col++)

            sum+=grade[row][col];

       avg_stu[row]=sum/NEXAMS;

   }

 

   for (col=0; col<NEXAMS; col++)

   {   sum=0;

       for (row=0; row<NSTUDS; row++)

             sum+=grade[row][col];

       avg_exm[col]=sum/NSTUDS;

   }

 

   sum=0

   for (row=0; row<NSTUDS; row++)

       for (col=0; col<NEXAMS; col++)

             sum+=grade[row][col];

   avg=sum/(NEXAMS*NSTUDS);

 

   for (row=0; row<NSTUDS; row++)

   {   for (col=0; col<NEXAMS; col++)

             printf("%7.2f",grade[row][col]);

       printf("%7.2f\n",avg_stu[row]);

   }

   for (col=0; col<NEXAMS; col++)

       printf("%7.2f",avg_exm[col]);

 

   printf("\n\nThe overall average = %5.2f\n",avg);

 

   return 0;

 

}

 

 Passing multi-dimensional array as a parameter

q       This is similar to 1-D array  - the argument is the array name followed by brackets for each of the dimensions

q       However, the sizes of all the dimensions except the last, must be specified

q       As Actual parameter, we just use the array name.

 

Example 2:

Problem:  Modify example 1 above so that it reads the number of students (maximum 30) and to include the following functions:

Ø      a function get_data to read the data

Ø      a function avr_row that returns the average of a given row

Ø      a function avg_col that returns the average of a given column

Ø      a function average that returns the overall averge.

 

/* computes the average grades of students in 3 exams.

   also computes the average grade in each exam. */

 

#include <stdio.h>

#define NEXAMS 3

#define MAXSTD 30

 

void get_data(float grade[][NEXAMS], int n);

float avg_row(float grade[][NEXAMS], int i, int colsize);

float avg_col(float grade[][NEXAMS], int j, int rowsize);

float average(float grade[][NEXAMS], int rowsize, int colsize);

 

main()

{  int row, col, n;

   float grade[MAXSTD][NEXAMS], avg_stu[MAXSTD], avg_exm[NEXAMS];

 

   printf("Enter number of students >");

   scanf("%d",&n);

 

   get_data(grade,n);

 

   for (row=0; row<n; row++)

       avg_stu[row]=avg_row(grade,row,3);

 

   for (col=0; col<NEXAMS; col++)

        avg_exm[col]=avg_col(grade,col,n);

  

   for (row=0; row<n; row++)

   {   for (col=0; col<NEXAMS; col++)

       printf("%7.2f",grade[row][col]);

       printf("%7.2f\n",avg_stu[row]);

   }

   for (col=0; col<NEXAMS; col++)

       printf("%7.2f",avg_exm[col]);

 

   printf("\n\nThe overall average = %7.2f\n",average(grade,n,NEXAMS));

 

   return 0;

 

}

 

/* reads the grades of n students in 3 exams */

void get_data(float grade[][NEXAMS], int n)

{   int row,col;

    float sum, avg;

    printf("Please enter grades for %d students row-wise\n",n);

    printf("In the form exam1 exam2  exam3\n");

    for (row=0; row<n; row++)

scanf("%f%f%f",&grade[row][0],&grade[row][1],&grade[row][2]);

} 

 

/* returns the average of a given row of a 2D array */

float avg_row(float grade[][NEXAMS], int i, int colsize)

{    int  col;

     float sum=0.0, avg;

     for (col=0; col<colsize; col++)

 sum+=grade[i][col];

     avg=sum/colsize;

     return avg;

}

 

/* returns the average of a given col of a 2D array */

float avg_col(float grade[][NEXAMS], int j, int rowsize)

{     int row;

      float sum=0.0, avg;

      for (row=0; row<rowsize; row++)

  sum+=grade[row][j];

      avg=sum/rowsize;

      return avg;

}

 

/* Computes the average of elements in a 2-d array */

float average(float grade[][NEXAMS], int rowsize, int colsize)

{     int row, col;

      float sum=0, avg;

      for (row=0; row<rowsize; row++)

  for (col=0; col<colsize; col++)

      sum+=grade[row][col];

      avg=sum/(rowsize*colsize);

      return avg;

}