1. Strings What is a string? It is an array of characters terminated with
an extra character '\0'
NULL character
e.g.
char name[5] = {'B','i','l','l','\0'};
equivalent to
char name[5] = "Bill";
double quotes
Constant string
C tacks on '\0' automatically
at the end of the string or
char name[] = "Bill";
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2. In the memory: name[0] is 'B' name[4] is '\0' tells the computer
where the string
ends
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3. Using strings do not forget Initialization like arrays of numbers
char name[5] = {'B','i','l','l','\0'}; or
char name[5];
name[0]='B'; name[1]='i'; name[2]='l';
name[3]='l'; name[4]='\0'; or
char name[5] = "Bill";
particular to
string arrays or
char name[] = "Bill";
Cannot do
char name[5]; /* OK */
name = "Bill"; /* NOT OK */
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4. Input/Output of strings
with scanf
char name[5];
scanf("%s",name);
placeholder
for strings
no & (why?)
no length check (if name is not big
enough, you will get run time problems)
skips initial white spaces
insert '\0' at the next white space
input:
CSC142 is a lot of fun
name contains
'C', 'S', 'C','1','4','2','\0'
with printf
printf("%s",name);
Specific for strings. There is no placeholder
to input or output arrays of numbers.
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5. Operations on strings As for arrays of numbers, there are no
operators in C that work directly on strings.
Be Careful!
char name[5] = "Bill"; /*OK*/
char name2[5]; /*OK*/
name2 = name; /*NOT OK*/
Instead
for(i=0; i<5; i++)
name2[i] = name[i];
Or more clever!
for(i=0; name[i]!='\0'; i++)
name2[i] = name[i];
name2[i] = '\0';
don’t need the length of the string
But, we don't need to write our own functions.
Check the standard string library
At the beginning of the program:
#include <string.h>
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6. Some string functions Copy string s2 into string s1 strcpy(s1,s2);
In C,
The C code for strcpy is similar to
void strcpy(char s1[], char s2[]) {
int i;
for(i=0; s2[i]!='\0', i++)
s1[i] = s2[i];
s1[i] = '\0'; }
Careful, C doesn't check if s2 fits in s1
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7. #include <string.h> ... char line1[5];
Don't do:
#include <string.h>
...
char line1[5];
char line2[]="The Universe"
strcpy(line1,line2);
Trouble ahead!
In the memory T h e U n i v r s
'\0'
space reserved
for line1
just erased some
(important?) data
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8. When using strcpy, check the length of the strings
How?
Use the function strlen
char s[]={'H','e','l','l','o','\0'};
strlen(s) is 5
5 because '\0' is not
counted
When using strcpy, write
char s2[] = "Once upon a time";
char s1[SIZE];
if (strlen(s2)<SIZE-1)
strcpy(s1,s2);
else
printf("String is too short!");
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9. Could also copy only the part of s2 that fits in s1
strncpy
The C code for strncpy is similar to
void strncpy(char dest[],
char source[], int destlen) {
int i;
for(i=0; i<destlen; i++)
if (i<=strlen(source))
dest[i] = source[i];
else
/* end of dest is padded
with nulls */
dest[i] = '\0' }
copy at most destlen characters from
source to dest
Note: '\0' might not be added to dest
When?
destlen <= strlen(source)
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10. String concatenation strcat strcat(s1,s2)
concatenates s2 to the end of s1
e.g.
char s1[14]="New York";/*8 characters*/
char s2[]=" city"; /*5 characters*/
strcat(s1,s2);
printf("%s",s1);
printf prints "New York city"
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11. String comparison How to compare strings?
use the (extended) alphabetical order
include characters
like [,
"a" < "d"
"ant" < "antelope"
"&123" <
But, cannot use <, >, == for strings in C
Use the function strcmp
strcmp(str1,str2) is
not C operators
<0 if str1 < str2
=0 if str1 = str2
>0 if str1 > str2
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12. Pitfalls of strings char str1[] = "Hello"; char str2[] = "Bonjour";
Can we write?
str1 = str2
/* ERROR */
if (str1 == str2)...
OK, but compare pointers (not the
string contents!)
if (str1 > str2)
OK, but compare pointers (not the
string contents!)
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13. Arrays of strings Same as multidimensional arrays
char day[7][10]={"Monday","Tuesday",
"Wednesday","Thursday","Friday",
"Saturday","Sunday"};
10 for Wednesday: 9 characters + 1 (for '\0')
Examples
printf("CSC142 lab is on %s",day[2]); or
char name[NUM_NAMES][MAX_NAME+1];
int age[NUM_NAMES];
int i;
for(i=0; i<NUM_NAMES; i++) {
scanf("%s%d",name[i],&age[i]);
printf("%s %d",name[i],age[i]); }
no &
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14. Check other functions in <string.h>
strncat, strncmp
strtod, strtol, strtoul
(to convert a string to a number)
See also the related function library for
characters:
<ctype.h>
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15. Summary string: Null terminated array of chars
No string assignment or comparison in C
Use built in functions in <string.h>
assignment: strcpy, strncpy
comparison: strcmp, strncmp
length: strlen
printf and scanf with %s
Pitfall: overrunning the allowed length of
a string
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