1. Computer Science 210 Computer Organization
Strings and Text Files in C

2. Representing Strings
printf("Hi Ken!\n");
'H'
'i'
' '
'K'
'e'
'n'
'!'
'\n'
null
A string is a set of ASCII values that inhabit a sequence of bytes
A string should always end in a null character (ASCII 0)
Many string functions use null as a sentinel

3. The String “Type” and Variables
char *greeting = "Hi Ken!\n";
printf("%s", greeting);
greeting
'H'
'i'
' '
'K'
'e'
'n'
'!'
'\n'
null
The pointer to a char named greeting holds the address of the first byte
The standard string processing functions view strings as of type char*

4. The String “Type” and Variables
char *greeting = "Hi Ken!\n";
printf("%s", greeting);
greeting
'H'
'i'
' '
'K'
'e'
'n'
'!'
'\n'
null
GREETING .STRINGz "Hi Ken!\n"
LEA R0, GREETING
PUTS

5. View a String as an Array of char
char *greeting = "Hi Ken!\n";
for (int i = 0;; greeting[i] != 0; i++)
putchar(greeting[i]);
greeting
'H'
'i'
' '
'K'
'e'
'n'
'!'
'\n'
null
A string looks like an array in memory
So, we can use an index to access or modify a character in any cell

6. The string Library and strlen
#include <string.h>
char *greeting = "Hi Ken!\n";
for (int i = 0; i < strlen(greeting); i++)
putchar(greeting[i]);
greeting
'H'
'i'
' '
'K'
'e'
'n'
'!'
'\n'
null
The string library includes several common string functions (length, concatenation, comparison, etc.)
strlen is O(n), because it searches for the null character

7. The string Library
Function
What it does
int strlen(char *str)
Returns the number of characters in the string, not including the nul character.
strcmp(char *str1, char *str2)
Returns 0 if str1 equals str2, a negative integer if str1 < str2, or a positive integer if str1 > str2.
strcat(char *str1, char *str2)
Adds the characters in str2 to the end of str1, overwriting its nul character.
strcpy (char *str1, char *str2)
Copies the characters of str2 into the storage of str1. There must be enough storage!
You can pass either a char* or a char array to these functions

8. The ctype Library
#include <string.h>
#include <ctype.h>
char *greeting = "Hi Ken!\n";
int i = 0;
for (; i < strlen(greeting); i++)
greeting[i] = toupper(greeting[i]);
greeting
'H'
'I'
' '
'K'
'E'
'N'
'!'
'\n'
null
The ctype library includes several common character functions (test for letter or digit, convert to uppercase, etc.)

9. String Input with scanf
#include <stdio.h>
int main(){
char name[20];
printf("Enter your first name: ");
scanf("%s", name);
printf("%s\n", name); }
scanf stops at the first whitespace character and adds a nul character (works for one-word inputs)
Must pass scanf the storage, usually an array of char

10. String Input with gets
#include <stdio.h>
int main(){
char name[20];
printf("Enter your first name: ");
gets(name);
printf("%s\n", name); }
gets stops at the first newline character, which is not included, and adds a nul character (works for multi-word inputs)
Must pass gets the storage, usually an array of char

11. Example: The Argument Vector
The argument vector is an array of strings that are gathered up when a C program is run at the command prompt
The first string is the name of the command
The remaining strings are the arguments, if any
The argument vector and its length are optional formal parameters of the main function

12. Print the Command Line Arguments/*
Prints contents of the argument vector (the name of the program and
any command-line arguments). */
#include <stdio.h>
int main(int argc, char *argv[]){
int i;
for (i = 0; i < argc; i++)
printf("Argument %d: %s\n", i, argv]); }

13. Opening Files for I/O Syntax: fopen(fileName, modeString)
#include <stdio.h>
int main(){
FILE *infile = fopen("test.asm", "r");
FILE *outfile = fopen("test.bin", "w");
if (infile == NULL)
printf("The input file does not exist.\n");
else{
processFile(infile, outfile);
fclose(infile);
fclose(outfile); }
Syntax: fopen(fileName, modeString)

14. Text File I/O Functions
What it does
char* fgets(char* array,
int arraySize,
FILE *infile)
Returns NULL if end of file. Otherwise, loads the next line, including the newline and nul characters, into array. Note: there must be storage for array!
int fputs(char* array,
FILE *outfile)
Returns EOF if the string cannot be written to outfile. Otherwise, writes the string to outfile. A newline is not automatically written.
fprintf(FILE *outfile,
char* formatString,
arg1, . . ., argn)
Works just like printf with the standard output file (the terminal)

15. Example: Create a Numbered Listing
void processFile(FILE *infile, FILE *outfile){
int max = 80;
int fieldWidth = 4;
char line[max];
int lineNumber = 1;
while (fgets(line, max, infile)){
fprintf(outfile, "%*d%s", fieldWidth, lineNumber, "> ");
fputs(line, outfile);
lineNumber++; }
"%*d" allows a variable to be used as a field width for a datum
The variable for the field width precedes the datum being formatted

16. The Standard I/O Files
stdin names the standard input file, which is the keyboard
stdout names the standard output file, which is the terminal screen
Both variables are of type FILE*

17. Example: Print to File or Terminal
int main(int argc, char *argv[]){
if (argc == 1) // no argument provided (?)
return;
FILE *infile = fopen(argv[1], "r");
if (infile == NULL){
printf("The input file does not exist in this directory\n"); }
FILE *outfile;
if (argc == 3)
outfile = fopen(argv[1], "w"); // Using an output file
else
outfile = stdout; // Using the terminal
processFile(infile, outfile);
fclose(infile);
if (outfile != stdout)
fclose(outfile);

18. Pointers and Dynamic Storage
For Monday
Pointers and Dynamic Storage