1. Plan for the Day: I/O (beyond scanf and printf)
Input and Output Topic 7
Plan for the Day:
I/O (beyond scanf and printf)

2. Standard, String and File I/O

3. I/O in C
Input and output facilities provided by standard library <stdio.h> and not by the language
Text stream consists of series of lines ending with '\n'

4. Standard Input & Output
int putchar(int c)
prints char c to stdout
Returns c, or EOF on error
int getchar()
Returns next character from stdin
Returns EOF on error
EOF = end of file character

5. Standard I/O
#include<stdio.h> int main() { char c; printf("Enter a character: "); c = getchar(); printf("Character entered was: "); putchar(c); }
CExamples/IOExamples/getPutChar.c

6. Standard Input & Output
What does this do?
int main() {
char c;
while((c = getchar()) != EOF) {
if(c >= 'A' && c <= 'Z') c = c – 'A' + 'a';
putchar(c); }
return 0;
In Unix: to use a file instead of stdin, use < operator
Input redirection: ./a.out < file.txt
Treats file.txt as source of standard input
Ctrl-Z or Ctrl-D or Ctrl-C to produce EOF (depending on OS)
CExamples/IOExamples/testGetChar.c

7. String Input/Output Write formatted output to a string:
int sprintf(char str[], char format[], arg-list)
Format specification same as printf
Output written to str, size not checked
Returns # of characters written (excluding '\0') or negative value on error
Read formatted input from a string:
int sscanf(char str[], char format[], var-address-list)
format specification same as scanf
Input read from str
Returns # of variables filled or EOF on error
Remember that scanf, sscanf, fscanf leaves behind any \n on the input stream
Also: scanf automatically put null at end of input string
printf, puts assume output string ends with null
Make scanf safer by using the conversion specifier %ns, where n specifies max number of chars to be read
CExamples/IOExamples/sprintfEx.c, sprintfEx2.c

8. Example
Example:
int age;
char name[20];
char str[80] = "Sarah 23";
sscanf(str, "%s %d", name, &age);
printf("Name: %s, age: %d\n", name, age);
Example: Use sprintf to convert a double into an array of characters:
char answer[100];
double number = ;
sprintf(answer, "%f", number);
printf("%c\n", answer[0]);
Output:
Name: Sarah, age: 23
Code: CExamples/IOExamples/sscanfFun.c
If I print the return value of sscanf, I'll get 2
Exercise: Change this example so I make sure not to read too long a string into name
Read from str
Print to answer
Output: 9

9. Files Open a text or binary file:
FILE* fopen(char name[], char mode[])
modes: "r" (read only), "w" (write only), "a" (append).
Append "b" for binary files
Returns pointer to file if it exists, NULL otherwise (creates new file for mode "w")
Close stream:
int fclose(FILE* fp)
fclose() automatically called on all open files when program terminates
CExamples/IOExamples/fileEx.c
stdin and stdout are also type FILE*
stderr corresponds to standard error output (different from stdout) – different window on screen

10. Example
#include<stdio.h> int main() { FILE* fp = fopen("example.txt", "r"); if(fp == NULL) { printf("Can't open %s\n", "example.txt"); exit(1); // failure } // Read from the file... fclose(fp); return 0;

11. File Path In Windows, specify path with \\ or / instead of \
Use the call
fopen("c:\\project\\test1.dat", "r"); or
fopen("c:/project/test1.dat", "r");

12. File Input reads one character from stream
int getc(FILE* fp)
reads one character from stream
returns that character or EOF (error or end of file)
Note: getchar uses stdin to read a character
char *fgets(char *line, int maxlen, FILE* fp)
reads single line up to maxlen-1 characters from input, including (possibly) line break '\n'
returns a pointer to character array line
returns NULL if end of stream
fgets stops reading when it reaches \n, but includes \n in the string
Returns line if no error. NULL is returned if no chars read and end of file encountered.
CExamples/IOExamples/fileEx2.c. (getc example)

13. File Input
int fscanf(FILE* fp, char format[], var-address-list)
like scanf and sscanf except items read from input stream fp
Cexamples/IOExample/fscanfEx.c

14. File Output int putc(int c, FILE* fp)
writes character c to output stream
returns c (or EOF on error)
Note: putchar is equivalent to putc(c, stdout)
int fputs(char line[], FILE* fp)
writes line to output stream
returns 0 on success, EOF otherwise
int fprintf(FILE *fp, char format[], var-list)
Similar to printf, sprintf
CExamples/IOExamples/fprintfEx.c

15. Example
/* Read line from keyboard & write to file */ int main() { char fileName[80]; char str[80]; FILE *fp; printf("Enter file name: "); scanf("%79s", fileName); // remember: \n still in input stream getchar(); // grab newline character printf("Filename: %s\n", fileName); //fgets(fileName, 80, stdin); // Alternative: remember that \n // included in string though fp = fopen(fileName, "w"); if(fp == NULL) { printf("File %s failed to open\n", fileName); exit(EXIT_FAILURE); } printf("Enter a string: "); fgets(str, 80, stdin); fputs(str, fp); fclose(fp);
CExamples/IOExamples/fileEx.c
fgets: reads characters from stream and stores them in string str until 79 characters have been read or newline or EOF reached
Newline makes fgets stop reading but is also included in string str
Note: exit() is in stdlib.h. Terminates calling procedure immediately, returning status
EXIT_FAILURE is macro – indicates unsuccessful termination

16. Exercise
Write a program that reads in all the lines from a file, and writes each line to the console, inserting the line number at the beginning of the line.