Transcript C Programming

C Programming
Lecture 10
Instructor: Wen, Chih-Yu
Department of Electrical Engineering
National Chung Hsing University
Functions and 2-D Arrays
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Topics
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Passing 2-D arrays to functions
Declare the size of a 2-D array
Reading an entire 2-D array form a file
Calling a function with a 2-D array
Using a 2-D array in a function
Functions and 2-D Arrays
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In a function prototype, a 2-D
array can have the left set of
brackets empty, but the other set
of brackets must be filled.
/* Program for Lesson 6_6 */
#include <stdio.h>
#define MAX_NUM_ROWS 8
#define MAX_NUM_COLS 10
void function1(int m, int n, int b[ ][MAX_NUM_COLS]);
void main (void)
{
int i,j, num_rows, num_cols;
int a[MAX_NUM_ROWS][MAX_NUM_COLS];
FILE *infile;
infile = fopen ("L6_6.DAT","r");
Input file L6_6.DAT
34
1234
2468
3579
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/***************************************************************
SECTION 1- READING A 2-D ARRAY FROM A FILE
***************************************************************/
fscanf (infile,"%d %d", &num_rows, &num_cols) ;
for (i=0; i<num_rows; i++)
{
for (j=0; j<num_cols; j++)
{
fscanf(infile,"%d ", &a[i][j]);
}
}
Functions and 2-D Arrays
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How to envision the array a[ ][ ]?
1234******
2468******
3579******
**********
**********
**********
**********
**********
Functions and 2-D Arrays
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How to have a program to determine the actual numbers
of rows and columns filled for two-dimensional arrays?
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Sentinel values: values that distinct from and cannot be
confused with other data values (similar to how we handle EOF)
From the input file
How to read a 2-D array from a data file?
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for (i=0; i<num_rows; i++)
Loop over the number of rows
{
Loop over the number of columns
for (j=0; j<num_cols; j++)
{
fscanf(infile,"%d ", &a[i][j]);
}
}
Functions and 2-D Arrays
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How to use a multidimensional array in the parameter list of a
function call?
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/***********************************************************
** SECTION 2 - CALLING A FUNCTION WITH A 2-D ARRAY
***********************************************************/
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function1(num_rows, num_cols, a);
Calling function1. it is advisable to
pass the number of rows and columns
along with the array address to a function.
/***********************************************************
** SECTION 3 - PRINTING A 2-D ARRAY
***********************************************************/
for (i=0; i<num_rows; i++)
{
for (j=0; j<num_cols; j++)
{
printf("%d ", a[i][j]);
}
printf("\n");
}
}
Functions and 2-D Arrays
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Passing the ability to access all array elements to a
function
function1(num_rows, num_cols, a);
Address of first element
passed to pointer variable
indicated with brackets
void function1(int m, int n, int b[ ][MAX_NUM_COLS]);
Functions and 2-D Arrays
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/***************************************************************
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SECTION 4 - FUNCTION THAT PERFORMS OPERATIONS ON A 2-D ARRAY
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***************************************************************/
void function1 (int m, int n, int b[ ][MAX_NUM_COLS])
{
int i,j;
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for (i=0; i<m; i++)
{
for (j=0; j<n; j++)
{
b[i][j] += 100;
}
}
}
Functions and 2-D Arrays
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Why to include the maximum number of columns in
the second set of brackets of the array parameter in
the prototype?
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For an array element, a[x][y] of an array declared with the
size a[MAX_NUM_ROWS][MAX_NUM_COLS], C uses
a formula to locate the element’s position in memory:
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sequence location = (x * MAX_NUM_COLS) + y + 1
a[1][2] (x = 1, y = 2)
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sequence location = (1 * 10) + 2 + 1 = 13, which is the 13th
element.
Functions and 2-D Arrays
For a 4-D array, what to include in the function
prototype?
#define I 10
#define J 5
#define K 8
#define L 3
int a[I][J][K][L];
 Function prototype
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void function1(… int a[ ][J][K][L]…);
Functions and 2-D Arrays
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Function prototype and function declarator
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void function1(int m, int n, int b[ ][MAX_NUM_COLS]);
void function1(int, int, int [ ][MAX_NUM_COLS]);
C ignores the variable names in the function prototype.
Single Character Data
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Topics
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The set of characters
Single character input and output
Characters treated as integers
Input buffer
Flushing the buffer
Single Character Data
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How to declare character variables?
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How to write an assignment statement with these
character variables?
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Declared with the key word char.
char var1, var2, var3, …;
Enclose the constant in single quotes, ‘ ’.
For instance, c1 = ‘g’;
How does C handle ANSI C character constants?
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C actually uses the integer value of the character
functions and operations.
Appendix A (ANSI C characters and their codes/values)
Single Character Data
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How to print characters?
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Use %c or %d conversion specification
 printf(“%c %c”, c1, c2);
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printf(“%d %d”, c4, c5);
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print the character variables c1 and c2
Cause the integer values to be printed
How the putchar function work?
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Prints the character that is argument to the standard output device
(screen).
putchar (character)
putchar(c2);
 Causes the value of the character c2 to be printed to the screen
putchar(‘y’);
 Causes the character y to be printed to the screen
Single Character Data
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What does putchar(32) do?
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C uses the integer value of the character in its functions.
putchar(32) causes the character represented by the
integer value 32 to be printed to the screen (the
character ‘space’).
printf(“%c %c”, c1, c2);
putchar(c1);
putchar(32);
putchar(c2);
Single Character Data
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How to read characters from the keyboard using
the scanf function?
Use %c conversion specification.
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scanf(“%c%c”, &c4, &c5);
what does the function getchar do?
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Returns a character that has been input from the
standard input device (keyboard) to the program.
Works with the input buffer to get the information
typed at the keyboard
getchar();
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Nothing should be put in the parentheses.
c6 = getchar();
Single Character Data
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What is input buffer and how does it work with getchar?
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A buffer is a portion of memory reserved for temporarily holding
the information that is being transferred.
A position indicator keeps track of the point at which no further
information has been read.
The getchar function works with the buffer position indicator to
retrieve the next character in the buffer and advance the position
indicator.
xp return
x
p
\n
Position indicator
Single Character Data
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getchar();
c6 = getchar();
c7 = getchar();
x
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p
\n
Position indicator
vs return
x
p
\n v s \n
Position indicator
Single Character Data
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What are the difficulties in dealing with the
getchar function?
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Need Enter to work
An extra \n always will be in the buffer
As further getchar calls are executed, the extra
character may cause execution to be different from
what it should be.
How to solve this problem?
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The function fflush
Single Character Data
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What does the function fflush do?
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The function fflush can be used to flush or empty the
buffer after obtaining the characters of interest.
fflush(stdin);
Flushed the input buffer
c8 = getchar();
before and after the two
c9 = getchar();
calls to getchar.
fflush(stdin);
Flushing the buffer after the two calls allows the next
calls to getchar to avoid reading extraneous characters.
Single Character Data
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What is stdin?
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stdin is a file pointer, which is declared in stdio.h.
In stdio.h, stdin is defined to point to the standard input
stream. Therefore,
fflush(stdin);
flushes what is pointed by stdin, which is the standard
input stream – the input buffer from the keyboard.
 Cannot choose to use stdin as an identifier for a variable
Single Character Data
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How to avoid the problems of getchar?
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The function getche
This function does not work with the input buffer and,
therefore, produced unbuffered input.
It deals with the OS and does not require the Enter key
to be pressed for the character code to be transferred.
The form is getche(); where nothing should be put in
the parentheses.
Lose some portabilities of your program
(not ANSI C compatible)
Single Character Data
Why not forget getchar and just use scanf?
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scanf
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When the scanf function is activated, execution of the
program stops.
Execution begins again when the user presses the Enter key,
causing scanf to read the information in the input buffer.
Interprets the information in the input buffer based on the
string literal used in the function call.
1.
2.
3.
Conversion or format specifiers (begun with a %)
White-space characters (treats a space, Tab, and Enter as white
space)
Non-white-space characters
The scanf Function Call
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scanf (“%c%c”, &c4, &c5);
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The string literal here has two conversion specifiers with no whitespace and no non-white-space characters.
It is difficult to deal with scanf characters because of the way that
scanf function deals with white space in both the literal and the
input buffer.
A space can be interpreted as a character.
scanf(“%c[ ]%c”, &c1, &c2);
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Will not accept [ ] [ ] [ ] as input ([ ] is a single space)
Until a non-white-space character is pressed, scanf will not move
forward
Strings and Pointers
Declaring, Initializing, and Printing Strings (Lesson 7.1)
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Topics
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Character arrays
Initializing single characters
Initializing strings
Printing strings
Memory arrangement
Arrays and Strings
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What is a string?
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A string is an array of characters including the
terminating null (\0) character.
bb[0] = ‘c’;
bb[1] = ‘a’;
bb[2] = ‘t’;
bb[3] = ‘\0’;
The last memory cell contains the escape sequence ,‘\0’,
which is treated as a single character.
‘\0’ is called the null character.
Arrays and Strings
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“This is a string constant.”
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C recognizes this statement to be a string and adds \0
after the period when stored in memory
The character array must be large enough to include
the \0 character.
A string is stored in a character array, whereas a
single character is stored in a character variable.