I hope you:

1
CS110 Lecture 14Arrays For Strings
Jack Tumblinjet_at_cs.northwestern.edu

• I hope you
• Have turned in Proj-2due now
• Are preparing for Midterm Exam Wed Oct 29

2
(Recall) Arrays and Functions

• void cleanArray2(int siz, int a) /fcn
  prototype/...
• int main(void)
• int count5 5,4,3,2,1
• cleanArray(5, count) / use array name /
• printArray(5, count)
• return 0
• void cleanArray2int siz, int a) / fcn body
  /
• / clear 1st siz elements /
• int i
• for (i0 iltsiz i) ai0

3
(Recall) Arrays and Functions

• void cleanArray(int siz, int a)
  /prototype/...
• int main(void)
• int count5 5,4,3,2,1
• cleanArray(5, count)
• printArray(5, count)
• return 0
• Array name count gives arrays 0th byte location,
• function cleanArray() copies that address to
  define its own local array named a , so that
• a array starts at same memory location as count!

4
(Recall) Arrays and Memory
a
...4269427042714272427342744275427642774
2784279428042814282...
count
count0

• While cleanArray() runs, its a array uses the
  same memory locations use by the count array.
• Any changes to a arrayare changes to count.

count1
Address ? ? ?
count2
count3
5
Strings The 2nd Big Idea

• Text Computing gt Math Computing (for most
  of us)
• NOW were ready for it!
• we have interfaces/libraries,
• arrays / indexing / memory addresses
• It soon leads to a 3rd Big Idea pointers
• Chap.9 does NOT give you the whole story

6
Strings The 2nd Big Idea

• CS Jargon a string
• Is a single piece of text made of char values,
• Any desired length,
• Any desired characters (except one),
• Any desired order.
• String Examples
• asldk2 2_at_ 2 _VA12 78
• The night has d eyes\n
• ?THIS is not my beautiful life! gtTalkingHeads

7
We have already have
single quotes

• Character constants
• 'a' 'R' '9' '\n' '\t'
• Character variables
• char ans 'y'
• String constants
• "average value is f\n" "d of your base
  are belong to us"
• "hit any key to continue"
• ?String Variables? How?

double quotes
8
Strings in Memory
...4269427042714272427342744275427642774
2784279428042814282...
h e l l o w o r l d ! NU
LL

• in the C language,
• A string is a list of char values,
• stored sequentially in memory,
• always ended by a NULL
  char value.

Address ? ? ?
9
Strings in Memory

• ?!?! NULL ?!?!
• an unprintable special character (see Appendix)
• Why? NULL is a sentinel it means end of the
  string!
• How? in C, NULL character constant is also \0
• What? the stored numerical value for NULL is
  zero
• char ch2
• ch0 \0 /set both to NULL /
• ch1 NULL / (same as \0 character /
• printf(NULL is d,d,(int)ch0,(int)ch1)
  
• produces
• gt NULL is 0,0

(cast it from char to integer)
10
String Variables

• (Recall) A variable is
• a place-holder, a named chunk of memory to hold
  just one value that can change
• A string is a sequential list of char values,
  that ends at NULL (\0).
• Thus an array of type char is one
  way to make a string variable
• (but array size is fixed the string must fit
  inside)
• Lets try it

11
Put String in a char Array I
msg
...4269427042714272427342744275427642774
2784279428042814282...
h e l l o w o r l d ! \
0

• int main()
• char msg20
• msg0 h
• msg1 e
• msg2 l
• msg3 l
• msg4 o
• msg5 \0
• ... / WHY is this so tedious?!? /
• printf(s\n,msg)

uses char literals fill the char array with char
constants
Address ? ? ?
result gt hello world!
12
Put String in a char Array II
msg
...4269427042714272427342744275427642774
2784279428042814282...
h e l l o w o r l d ! \
0

• int main()
• char msg20h,e,l,l,o,
  , w,o,r,l,d,\0
• printf(s,msg)
• / not much better... /
• result
• gt hello world!

Address ? ? ?
uses char literals fill the char array with char
constants
13
Put String in a char Array III
msg
...4269427042714272427342744275427642774
2784279428042814282...
h e l l o w o r l d ! \
0

• int main()
• char msg20hello world!
• printf(s,msg)
• / AH! thats the way!... /
• result
• gt hello world!

Address ? ? ?
uses a string literal double-quotes make a
string constant
14
Strings in Arrays

• A char array is a fairly good string variable,
  and you can initialize it with a string
  constant
• char msg80 one line of text.

15
Strings in Arrays

• A char array is a fairly good string variable,
  and you can initialize it with a string
  constant
• char msg80 one line of text.
• Print a string variable using s and printf()
  printf(You typed s, msg)

16
Strings in Arrays

• A char array is a fairly good string variable,
  and you can initialize it with a string
  constant
• char msg80 one line of text.
• Print a string variable using s and printf()
  printf(You typed s, msg)
• Read a string variable using s and scanf()
  printf(Type answer, press return) scanf(
  s,msg)

17
Strings in Arrays
SURPRISE! scanf() uses NO ampersand () for
string variables (array name), unlike ordinary
variables WHY? --array name is an addressof
the 0th array element --for ordinary variable
var, writing var means give me the
address of var

• A char array is a fairly good string variable,
  and you can initialize it with a string
  constant
• char msg80 one line of text.
• Print a string variable using s and printf()
  printf(You typed s, msg)
• Read a string variable using s and scanf()
  printf(Type answer, press return) scanf(
  s,msg)

18
Basic String Variable Actions

• How can I . . .
• Assign copy a string from one array to another?
• Append copy one string onto the end of another?
• Find find a given character in a string? (find
  all e)
• Parse find the 1st,2nd,3rd... Nth word in a
  string?
• Match find a given key word in a string?
• Delete remove one character from a string?
• Erase remove one word from a string?
• You know how!---with loops, if/else, , etc.

19
Strings in Arrays

• Problem Many common tasks are tedious
• copy a string constant to a string variable
• append one string to the end of another
• remove the first part or last part of a string .
  . .
• Solution function library for tedious tasks
  includeltstring.hgt / see pg 666 in book /
• Side note cslibvc.lib replaces string.h (a
  big part of C) with his strlib.h (NOT a part
  of C) BAD IDEA!

20
Solution include ltstring.hgt

• Use the string.h library to do the tedium a
  vital part of C (see book, pg. 666 for summary)
• include ltstdio.hgt / for printf() /include
  ltstring.hgt / for strcpy() /int main()char
  msg20
• .
• strcpy(msg,"Hello world!")
• printf("my strings\n",msg)
• Result gt my stringHello world!
• gt

(Recall)String literal text in quotes
s means a string OK for scanf() too.
21
Solution include ltstring.hgt

• Book neglects it, but string.h has everything you
  need
• Find string length
• Copy a string
• Join 2 strings
• Compare 2 strings
• Find a char in a string
• Find a string in a string

(later)
22
String Length int strlen(src)

• include ltstdio.hgt / for printf() /
• include ltstring.hgt / for strlen() /int
  main()char msg20 "Hello world!"
  
• cnt strlen(msg)
• printf("length of \"s\" is
  d\n",msg,cnt)
• Result
• gt length of "Hello world!" is 12
• gt

23
String Copy strcpy(dest, src)

• include ltstdio.hgt / for printf() /include
  ltstring.hgt / for strcpy() /int
  main()char msg20
• strcpy(msg,"Hello world!")
• printf("my strings\n",msg)
• Result
• gt my string Hello world!
• gt

strcpy() is the string assignment operator
24
String Copy strncpy(dest,src,cnt)

• include ltstdio.hgt / for printf() /include
  ltstring.hgt / for strncpy()/int
  main()char msg20
• strcpy(msg,"Hello world!", 3)
• printf("my strings\n",msg)
• Result
• gt my string Hel
• gt

Copy only 1st 3 chars
25
String Merge strcat(str1,str2)

• include ltstdio.hgt / for printf() /
• include ltstring.hgt / for strcat() /int
  main()char msg181,msg281
• int diff
• strcpy(msg1,"Hello you!")
  strcpy(msg2,"Hello me!")
• diff strcat(msg1,msg2)
• printf("s\n", msg1)
• Result
• gt Hello you!Hello me!
• gt

msg1 is BOTH input and output to strcat() fcn!
26
String Merge strncat(str1,str2,cnt)

• include ltstdio.hgt / for printf() /
• include ltstring.hgt / for strncat()/int
  main()char msg181,msg281
• strcpy(msg1,"Hello you!")
  strcpy(msg2,"Hello me!")
• strncat(msg1,msg2,3)
• printf("s\n", msg1)
• Result
• gt Hello you!Hel
• gt

msg1 is BOTH input and output to strcat() fcn!
27
String Compare int strcmp(a,b)

• include ltstdio.hgt / for printf() /
• include ltstring.hgt / for strcmp() /int
  main()char msg181 "Hello to you!"char
  msg281 "Hello to me!"
• int diff diff
  strcmp(msg1,msg2)
• if(0diff) printf("same!\n")
• else printf("different!\n")
• Result
• gt different!
• gt

28
String Compare int strncmp(a,b,cnt)

• include ltstdio.hgt / for printf() /
• include ltstring.hgt / for strncmp()/int
  main()char msg181 "Hello to you!"char
  msg281 "Hello to me!"
• int diff
• diff strncmp(msg1,msg2,6)
• if(0diff) printf("same!\n")
• else printf("different!\n")
• Result
• gt same!
• gt

compare only the 1st 6 chars