Computer Science 210Computer Organization

Strings in C

Representing Stringsprintf("Hi Ken!\n");

'\n''!''n''e''K'' ''i''H' 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 Variableschar *greeting = "Hi Ken!\n";

printf("%s", greeting);

'\n''!''n''e''K'' ''i''H' 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*

greeting

The String “Type” and Variableschar *greeting = "Hi Ken!\n";

printf("%s", greeting);

'\n''!''n''e''K'' ''i''H' nul

greeting

GREETING .STRINGz "Hi Ken!\n"

LEA R0, GREETINGPUTS

View a String as an Array of charchar *greeting = "Hi Ken!\n";int i = 0;for (; greeting[i] != 0; i++) putchar(greeting[i]);

'\n''!''n''e''K'' ''i''H' nul

A string looks like an array in memory

So, we can use an index to access or modify a character in any cell

greeting

The string Library and strlen#include <string.h>

char *greeting = "Hi Ken!\n";int i = 0;for (; i < strlen(greeting); i++) putchar(greeting[i]);

'\n''!''n''e''K'' ''i''H' nul

The string library includes several common string functions (length, concatenation, comparison, etc.)

strlen is O(n), because it searches for the nul character

greeting

The string LibraryFunction 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]);

'\n''!''N''E''K'' ''I''H' nul

greeting

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 andany 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]);}