Computer Science 210 Computer Organization Strings and Text Files in C

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

The String “Type” and Variables char *greeting = "Hi Ken!\n"; printf("%s", greeting); greeting 'H' 'i' ' ' 'K' 'e' 'n' '!' '\n' nul 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*

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

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

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

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

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' nul The ctype library includes several common character functions (test for letter or digit, convert to uppercase, etc.)

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

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

Other Points about Strings Can be the element type of an array Can be the type of a field in a struct

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

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 length, char *argVec[]){ int i; for (i = 0; i < length; i++) printf("Argument %d: %s\n", i, argVec[i]); }

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)

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)

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

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*

Example: Print to File or Terminal int main(int length, char *argVec[]){ if (length == 1) return; FILE *infile = fopen(argVec[1], "r"); if (infile == NULL){ printf("The input file does not exist in this directory\n"); } FILE *outfile; if (length == 3) outfile = fopen(argVec[2], "w"); // Using an output file else outfile = stdout; // Using the terminal processFile(infile, outfile); fclose(infile); if (outfile != stdout) fclose(outfile);

Pointers and Dynamic Storage For Monday Pointers and Dynamic Storage