1. Input and Output in C

2. Standard Input and Output
Output: printf
Input: scanf
Remember the program computing the distance between two points!
/* Declare and initialize variables. */
double x1=1, y1=5, x2=4, y2=7,
side_1, side_2, distance;
How can we compute distance for different points?
It would be better to get new points from user, right? For this we will use scanf
To use these functions, we need to use
#include <stdio.h>

3. Standard Output printf Function prints information to the screen
requires two arguments
control string
Contains text, conversion specifiers or both
Identifier to be printed
Example
double angle = 45.5;
printf(“Angle = %.2f degrees \n”, angle);
Output:
Angle = degrees
Conversion Specifier
Control String
Identifier

4. Conversion Specifiers for Output Statements
Frequently Used

5. Standard Input scanf Function inputs values from the keyboard
required arguments
control string
memory locations that correspond to the specifiers in the control string
Example:
double distance;
char unit_length;
scanf("%lf %c", &distance, &unit_length);
It is very important to use a specifier that is appropriate for the data type of the variable

6. Conversion Specifiers for Input Statements
Frequently Used

7. Arguments to main() int main(int argc, char *argv[]) { int i;
printf("argc = %d\n",argc);
for (i=0; i<argc; i++)
printf("argv[%d] = %s\n",i,argv[i]);
return 0; }

8. Arguments to main() fox01> ./myprog this is apple argc = 4
argv[0] = ./myprog
argv[1] = this
argv[2] = is
argv[3] = apple

9. Fahrenheit to Celcius #include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
double fahrenheit, celcius;
fahrenheit = atof(argv[1]);
celcius = (fahrenheit-32)*5.0/9;
printf("Celcius = %f\n",celcius); }

10. Arguments to main()
fox01> ./fahtocelcius 36
Celcius =

11. Fahrenheit to Celcius #include <stdio.h>
#include <stdlib.h>
int main() {
double fahrenheit, celcius;
printf("Enter fahrenheit");
scanf("%lf",&fahrenheit);
celcius = (fahrenheit-32)*5.0/9;
printf("Celcius = %f\n",celcius); }

12. Arguments to main() fox01> ./fahtocelcius2 < f.txt
Enter fahrenheitCelcius =
f.txt file has the following 34

13. Output Redirection Printf() sends output to stdout by default
Redirect output with > in command lime
fox01> ./fahtocelcius2 > ftoc.out
Enter Fahrenheit 34
fox01>
ftoc.out file has the following
Celcius =

14. Strings 14

15. Strings A string is an array of characters Use printf to print strings
char data[10] = “Hello”;
Use printf to print strings
printf(“%s”,data);
Can be accessed char by char
data[0] is first character
End of String Symbol H e l l o \0
data 15

16. Strings Each character has an integer representation a b c d e … … … …z
………………………112 A B C D E … … … … Z
……………………… 90 1 2 3 4 5 6 7 8 9 \0 \n 10 16

17. Strings Characters can be interpreted as integers char c = ‘A’;
printf(“%c\n”,c); prints A
printf(“%d\n”,c); prints 65
printf(“%c\n”,65); prints A 17

18. String Literals
String literal values are represented by sequences of characters between double quotes (“)
Examples
“” - empty string
“hello”
“a” versus ‘a’
‘a’ is a single character value (stored in 1 byte) as the ASCII value for a
“a” is an array with two characters, the first is a, the second is the character value \0

19. Referring to String Literals
String literal is an array, can refer to a single character from the literal as a character
Example:
printf(“%c”,”hello”[1]);
outputs the character ‘e’
During compilation, C creates space for each string literal (# of characters in the literal + 1)
referring to the literal refers to that space (as if it is an array)

20. Duplicate String Literals
Each string literal in a C program is stored at a different location
So even if the string literals contain the same string, they are not equal (in the == sense)
Example:
char string1[6] = “hello”;
char string2[6] = “hello”;
but string1 does not equal string2 (they are stored at different locations)

21. String Variables
Allocate an array of a size large enough to hold the string (plus 1 extra value for the delimiter)
Examples (with initialization):
char str1[6] = “Hello”;
char str2[] = “Hello”;
char *str3 = “Hello”;
char str4[6] = {‘H’,’e’,’l’,’l’,’o’,’\0’};
Note, each variable is considered a constant in that the space it is connected to cannot be changed
str1 = str2; /* not allowable, but we can copy the contents
of str2 to str1 (more later) */

22. Changing String Variables
Can change parts of a string variable
char str1[6] = “hello”;
str1[0] = ‘y’;
/* str1 is now “yello” */
str1[4] = ‘\0’;
/* str1 is now “yell” */
Important to retain delimiter (replacing str1[5] in the original string with something other than ‘\0’ makes a string that does not end)
Have to stay within limits of array

23. String Input Use %s field specification in scanf to read string
ignores leading white space
reads characters until next white space encountered
C stores null (\0) char after last non-white space char
Reads into array (no & before name, array is a pointer)
Example:
char Name[11];
scanf(“%s”,Name);
Problem: no limit on number of characters read (need one for delimiter), if too many characters for array, problems may occur

24. String Input
Can use the width value in the field specification to limit the number of characters read:
char Name[11];
scanf(“%10s”,Name);
Remember, you need one space for the \0
width should be one less than size of array
Strings shorter than the field specification are read normally, but C always stops after reading 10 characters

25. String Input (cont) Edit set input %[ListofChars] Examples:
ListofChars specifies set of characters (called scan set)
Characters read as long as character falls in scan set
Stops when first non scan set character encountered
Note, does not ignore leading white space
Any character may be specified except ]
Putting ^ at the start to negate the set (any character BUT list is allowed)
Examples:
scanf(“%[ ]”,Number);
scanf(“%[^\n]”,Line); /* read until newline char */

26. String Output Use %s field specification in printf:
characters in string printed until \0 encountered
char Name[10] = “Rich”;
printf(“|%s|”,Name); /* outputs |Rich| */
Can use width value to print string in space:
printf(“|%10s|”,Name); /* outputs | Rich| */
Use - flag to left justify:
printf(“|%-10s|”,Name); /* outputs |Rich | */

27. Input/Output Example #include <stdio.h> int main() {
char LastName[11];
char FirstName[11];
printf("Enter your name (last , first): ");
scanf("%10s%*[^,],%10s",LastName,FirstName);
printf("Nice to meet you %s %s\n",
FirstName,LastName); }

28. Reading a Whole Line Commands: char *gets(char *str)
reads the next line (up to the next newline) from keyboard and stores it in the array of chars pointed to by str
returns str if string read or NULL if problem/end-of-file
not limited in how many chars read (may read too many for array)
newline included in string read
char *fgets(char *str, int size, FILE *fp)
reads next line from file connected to fp, stores string in str
fp must be an input connection
reads at most size characters (plus one for \0)
to read from keyboard: fgets(mystring,100,stdin)

29. Printing a String Commands: int puts(char *str)
prints the string pointed to by str to the screen
prints until delimiter reached (string better have a \0)
returns EOF if the puts fails
outputs newline if \n encountered (for strings read with gets or fgets)
int fputs(char *str, FILE *fp)
prints the string pointed to by str to the file connected to fp
fp must be an output connection
returns EOF if the fputs fails
outputs newline if \n encountered

30. fgets/fputs Example #include <stdio.h> int main() {
char fname[81];
char buffer[101];
FILE *instream;
printf("Show file: ");
scanf("%80s",fname);
if ((instream = fopen(fname,"r")) == NULL) {
printf("Unable to open file %s\n",fname);
exit(-1); }

31. fgets/fputs Example (cont)
printf("\n%s:\n",fname);
while (fgets(buffer,sizeof(buffer)-1,instream)!= NULL)
fputs(buffer,stdout);
fclose(instream); }

32. Printing to a String
The sprintf function allows us to print to a string argument using printf formatting rules
First argument of sprintf is string to print to, remaining arguments are as in printf
Example:
char buffer[100];
sprintf(buffer,”%s, %s”,LastName,FirstName);
if (strlen(buffer) > 15)
printf(“Long name %s %s\n”,FirstName,LastName);

33. sprintf Example printf("File Prefix: "); scanf("%80s",basefname);
#include <stdio.h>
#include <string.h>
void main() {
FILE *instream;
FILE *outstream;
char basefname[81];
char readfname[101];
char savefname[81];
char buffer[101];
int fnum;
printf("File Prefix: ");
scanf("%80s",basefname);
printf("Save to File: ");
scanf("%80s",savefname);
if ((outstream =
fopen(savefname,"w")) ==
NULL) {
printf("Unable to open %s\n",
savefname);
exit(-1); }

34. sprintf Example for (fnum = 0; fnum < 5; fnum++) {
/* file name with basefname as prefix, fnum as suffix */
sprintf(readfname,"%s.%d",basefname,fnum);
if ((instream = fopen(readfname,"r")) == NULL) {
printf("Unable to open input file %s\n",readfname);
exit(-1); }
while (fgets(buffer,sizeof(buffer)-1,instream) != NULL)
fputs(buffer,outstream);
fclose(instream);
fclose(outstream);

35. Reading from a String
The sscanf function allows us to read from a string argument using scanf rules
First argument of sscanf is string to read from, remaining arguments are as in scanf
Example:
char buffer[100] = “A ”;
sscanf(buffer,”%c%d%f”,&ch,&inum,&fnum);
/* puts ‘A’ in ch, 10 in inum and 50.0 in fnum */

36. Character Functions
toupper(ch) If ch is a lowercase letter, this function returns the corresponding uppercase letter; otherwise, it returns ch
isdigit(ch) Returns a nonzero value if ch is a decimal digit; otherwise, it returns a zero.
islower(ch) Returns a nonzero value if ch is a lowercase letter; otherwise, it returns a zero.
isupper(ch) Returns a nonzero value if ch is an uppercase letter; otherwise, it returns a zero.
isalpha(ch) Returns a nonzero value if ch is an uppercase letter or a lowercase letter; otherwise, it returns a zero.
isalnum(ch) Returns a nonzero value if ch is an alphabetic character or a numeric digit; otherwise, it returns a zero.
isspace(ch) Returns a non-zero value if ch is a white-space character
(‘ ‘,’\t’, ‘\n’…)

37. Data Files

38. Data Files Output Input Read input of a program from a file
Write output of a program to a file
Files are stored on disk
Consider the following example
Output
Input
3 3 3
1 3 3
3 1 3
3 3 1
1 2 3
Equilateral
Isosceles
Scalene

39. Read information from file
Data Files
Each data file must have a file pointer
file pointer must be defined
FILE *sensor1;
FILE *balloon;
file pointer must be associated with a specific file using the fopen function
sensor1 = fopen(“sensor1.dat”, “r”);
balloon = fopen(“balloon.dat”, “w”);
Read information from file
Write information
to a file

40. I/O Statements Input file - use fscanf instead of scanf
fscanf(sensor1, “%1f %lf”, &t, &motion);
Output file - use fprint instead of printf
fprintf(balloon, “%f %f %f\n”, time, height, velocity);

41. End of file controlled loop
#include <stdio.h>
int main() {
FILE *scorefile;
int score;
scorefile = fopen("scores.txt","r");
while (feof(scorefile) <= 0)
fscanf(scorefile,"%d",&score);
printf("%d\n",score); }
fclose(scorefile);
return(0);
Available on webpage as lecture2e11.c
File 56 78 93 24 85 63
Available on webpage as scores.txt

42. Exercise
Given a file of integers. Write a program that finds the minimum number in a file.
// algorithm to find minimum in a file
open file
set minimum to a large value
while (there are items to read)
read next number x from file
if (x < min)
min = x
display the minimum
close file
File 56 78 93 24 85 63
Solution available on webpage as lectrue2e13.c

43. Exercise
Given a file of integers. Write a program that searches for whether a number appears in the file or not.
// algorithm to check for y in a file
open file
set found to false
while (there are items to read and found is false)
read next number x from file
if (x equals y)
set found to true
Display found message to user
Display not found message to user
close file
File 56 78 93 24 85 63
Solution available on webpage as lecture2e14.c

44. Exercise Read from one file and write to another FILE *scorefile;
FILE *outfile;
int score;
scorefile = fopen("scores.txt","r");
outfile = fopen("newscores.txt","w");
while (feof(scorefile) <= 0) {
fscanf(scorefile,"%d",&score);
fprintf(outfile,"%d\n",score); }
Solution available on webpage as lecture2e15.c

45. Exercise
Write a program that reads a number n from user and prints all the numbers 1 to n to file “numbers.txt”. 1 2 3 4 5 6 7 1 2 3 4 5
n= 5
n= 7

46. More on inputs Function to read a string
char* fgets(strVar, maxLength, file)
Read from specified file (stdin, by default) until either maxLengh-1 chars or until a line feed char is encounted
stringVar: input buffer, need enough space; normally char array:
char strvar[100];

47. More on inputs Function to read from a string
int sscanf(inputbuffer, formatstring,…)
Same as scanf, except to get input data from inputbuffer
Normally, use fgets to read one line into an inputbuffer from a file and use sscanf to read the data.

48. Example sscanf(szinputbuffer,formatS,szV1,&iV2,&dV3)
Seq szInputBuffer Format Cnt szV iV dV3
%s %d %lf '123456'
%5s %d %lf '12345'
%[1-9] %d %lf '123456'
%[1-9],%d,%10lf '123456' n/a n/a
,123, %[1-9],%d,%10lf '123456'
,123, %[^,],%d,%10lf '123456'
,123, %6[^,] ,%d,%10lf '123456'
, 123, %[^,],%d,%10lf ' '
, 123, %[^, ],%d,%10lf '123456' n/a n/a
, 123, %[^, ] ,%d,%10lf '123456'
, 123, %[^, ] ,%d,%10lf '123456'
, 123, %[^, ] ,%d,%10lf '123456'
,123, %[^,] %d %10lf '123456' n/a n/a

49. Errors to stderr
Errors should be output to stderr using fprintf rather to stdout using printf( )
Do this
fprintf( stderr, “this is the error message\n” );
instead of this
printf( “this is the error message\n” );
For example
ofp = fopen("test.out", “w") ;
if (ofp == NULL) {
fprintf (stderr, "Error opening test.out\n");
exit (-1); }

50. Header Files Split program into multiple files
File is included during compilation
Only the c file is compiled

51. Header Files File: lecture3header.h double x; double y; char color;
double width;
double height;

52. Header Files REST OF YOUR PROGRAM File: lecture3example.c
#include <stdio.h>
#include <stdlib.h>
#include "lecture3header.h"
REST OF YOUR PROGRAM

53. Data Conversion
Sometimes it is convenient to convert data from one type to another
For example, in a particular situation we may want to treat an integer as a floating point value
These conversions do not change the type of a variable or the value that's stored in it – they only convert a value as part of a computation

54. Data Conversion
Conversions must be handled carefully to avoid losing information
Widening conversions are safest because they tend to go from a small data type to a larger one (such as a short to an int)
Narrowing conversions can lose information because they tend to go from a large data type to a smaller one (such as an int to a short)
Data conversions can occur in three ways:
assignment conversion
promotion
casting

55. Assignment Conversion
Assignment conversion occurs when a value of one type is assigned to a variable of another
For example, the following assignment converts the value stored in the dollars variable to a double value
double money;
int dollars = 123;
money = dollars; // money == 123.0
Only widening conversions can happen via assignment
The type and value of dollars will not be changed

56. double result = sum / count;
Data Conversion
Promotion happens automatically when operators in expressions convert their operands
For example, if sum is a double and count is an int, the value of count is promoted to a floating point value to perform the following calculation:
double result = sum / count;
The value and type of count will not be changed

57. double result = (double) total / count;
Casting
Casting is a powerful and dangerous conversion technique
Both widening and narrowing conversions can be done by explicitly casting a value
To cast, the desired type is put in parentheses in front of the value being converted
For example, if total and count are integers, but we want a floating point result when dividing them, we cast total or count to a double for purposes of the calculation:
double result = (double) total / count;
Then, the other variable will be promoted, but the value and type of total and count will not be changed