1. 16.216 ECE Application Programming
Instructor: Dr. Michael Geiger
Fall 2013
Lecture 22
String examples
File I/O

2. ECE Application Programming: Lecture 22
Lecture outline
Announcements/reminders
Program 6 due today
Program 7 to be posted; due 11/13
Advising: 10/28 to 11/8
Review
String functions
Today’s lecture
String examples
5/28/2018
ECE Application Programming: Lecture 22

3. Review: String functions
In <string.h> library:
Copying strings:
char *strcpy(char *dest,
const char *source);
char *strncpy(char *dest,
const char *source,
size_t num);
Return dest
Does not append ‘\0’ unless length of source < num
Comparing strings:
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t num);
Character-by-character comparison of character values
Returns 0 if s1 == s2, 1 if s1 > s2, -1 if s1 < s2
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ECE Application Programming: Lecture 22

4. Review: String functions (cont.)
Find # of characters in a string
size_t strlen(const char *s1);
Returns # characters before ‘\0’
Not necessarily size of array
“Add” strings together—string concatenation
char *strcat(char *dest,
const char *source);
char *strncat(char *dest,
const char *source,
size_t num);
Returns dest
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ECE Application Programming: Lecture 22

5. Example: Using string functions
Works with main program in PE
Assume input strings have max of 49 chars (+ ‘\0’)
Write a function to do each of the following:
int readStrings(char *s);
Repeatedly read strings from standard input until the input string matches s. Return the number of strings read.
void copyNull(char *s1, char *s2, int n);
Copy the first n characters of s2 into s1, and make sure that the new version of s1 terminates with a null character.
int fillString(char *s, int n);
Repeatedly read strings from standard input and concatenate them to s until there is no room in the string. Return the final length of the string.
For example, if s is a 6-character array already holding “abcd”:
User enters “e”—string is full; return 5
User enters “ef”—there’s not enough room; return 4
Assume s initially contains null terminated string (or is empty)
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ECE Application Programming: Lecture 22

6. ECE Application Programming: Lecture 22
Example solution
int readStrings(char *s) { char str[50]; // Assume max 50 chars int count = 0; do { scanf(“%s”, str); // NOTE: do not // need &str count++; } while (strcmp(str, s) != 0); return count; }
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ECE Application Programming: Lecture 22

7. Example solution (cont.)
void copyNull(char *s1, char *s2, int n) {
strncpy(s1, s2, n);
s1[n] = ‘\0’; }
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ECE Application Programming: Lecture 22

8. Example solution (cont.)
int fillString(char *s, int n) {
char input[50]; // Assume max
// 50 chars
int charsLeft; // Space remaining
// in s
do {
scanf(“%s”, input);
// Calculate # chars left in array if input
// string is added. Need to leave room for ‘\0’
charsLeft = n – (strlen(s) – 1) – strlen(input);
if (charsLeft > 0) // Enough space to add this string
strcat(s, input); // and continue
else { // Out of room
if (charsLeft == 0) // Can add input, but then out of room
strcat(s, input);
return strlen(s); }
} while (1);
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ECE Application Programming: Lecture 22

9. ECE Application Programming: Lecture 22
File information
Name z:\Visual Studio 2010\Projects\fileio\fileio\myinput.txt
Read/Write
Type (binary or ASCII text)
Access (security; single/multiple user)
Position in file
All above info is stored in a FILE type variable, pointed to by a file handle
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ECE Application Programming: Lecture 22

10. File i/o function calls
FILE *fopen(fname, faccess)
fname: name of file (e.g., "f1.txt")
faccess: up to three characters, in double quotes
First char: r/w/a (read/write/append)
Write starts at beginning of file, append starts at end
Either write or append creates new file if none exists
Second (optional) char: + (update mode)
Allows both reading and writing to same file
Third (optional) char: b/t (binary/text)
If text files, characters may be adapted to ASCII/Unicode
Binary files are just raw bytes
Returns FILE address if successful; NULL otherwise
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ECE Application Programming: Lecture 22

11. File i/o function calls
fclose(FILE *file_handle)
Closes a file
Argument is address returned by fopen()
Recommended for input files
Required for output files
O/S often doesn’t write last bit of file to disk until file is closed
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ECE Application Programming: Lecture 22

12. Example of basic file function usage
void main() { FILE *fp; // Open text file for reading fp = fopen("in.txt", "r"); if (fp == NULL) { printf("Error: could not open in.txt"); return; } ... // CODE TO EXECUTE IF FILE OPENS fclose(fp); // Close file when done
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ECE Application Programming: Lecture 22

13. File i/o function calls: formatted I/O
fprintf(file_handle, format_specifier, 0+ variables)
file_handle: address returned by fopen()
Other arguments are same as printf()
Example: fprintf(fp, "x = %d", x);
fscanf(file_handle, format_specifier, 0 or more variables)
Other arguments are same as scanf()
Example: fscanf(fp, "%d%d", &a, &b);
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ECE Application Programming: Lecture 22

14. ECE Application Programming: Lecture 22
Example: File I/O
Write a program to:
Read three integer values from the file myinput.txt
Determine sum and average
Write the original three values as well as the sum and average to the file myoutput.txt
Note that:
The program should exit if an error occurs in opening a file
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ECE Application Programming: Lecture 22

15. ECE Application Programming: Lecture 22
The program (part 1)
#include <stdio.h> void main() { FILE *infile; // Input file pointer FILE *outfile; // Output file pointer int x, y, z, sum; // Input values and sum double avg; // Average of x, y, and z // Open input file, exit if error infile=fopen("myinput.txt","r"); if (infile==NULL) printf("Error opening myinput.txt\n"); return; } // Can actually open file as part of conditional statement if ((outfile=fopen("myoutput.txt","w"))==NULL) printf("Error opening myoutput.txt\n");
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ECE Application Programming: Lecture 22

16. ECE Application Programming: Lecture 22
The program (part 2)
// Read the three values
fscanf(infile, "%d %d %d", &x, &y, &z);
// Compute sum and average
sum = x + y + z;
avg = sum / 3.0;
// print out values
fprintf(outfile, "Values: %d, %d, %d\n", x, y, z);
fprintf(outfile, "Sum: %d\n",sum);
fprintf(outfile, "Avg: %lf\n",avg);
// close the files
fclose(infile);
fclose(outfile); }
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ECE Application Programming: Lecture 22

17. ECE Application Programming: Lecture 22
Final notes
Next time
More string examples
File I/O
Reminders:
Program 6 due 10/28
Advising: 10/28 to 11/8
5/28/2018
ECE Application Programming: Lecture 22