1. Introduction to Problem Solving and Programming
Lecture 7 – Strings

2. Strings
A string is any sequence of characters enclosed in double quotes
“Hello World".
There is no separate data type for strings as char, integer, float or double.
Instead, a string is represented in C as an array of type char.

3. Strings
We can declare and initialize a string variable using any of the following:
char str1[20]={'H', 'e','l','l','o',' ','W','o','r','l','d','\0'}; //as other arrays
char str2[20] = “Hello World"; /*with size specified explicitly */
char str3[]= “Hello World"; /*with size specified implicitly */
The string is terminated by the \0 character:

4. Strings For example, the declaration: char str[20] = “Hello World";
is actually represented in the memory as shown below: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 H e l o W r d \0

5. Input / Output of strings
We can use %s to read and print a string
printf (“Name is %s \n”, Name);
scanf (“%s”, str);
Pointer to character array

6. Input / Output of strings
We can use %s to read and print a string
printf (“Name is %s \n”, Name);
scanf (“%s”, str);
Where is &

7. Input / Output with gets and puts
A problem with scanf when reading a string is that it stops scanning the moment it encounters a white space.
Thus, it cannot scan a string such as: “Ali Zaki Street” in one variable.
An alternative to scanf is the function gets that takes a string variable as argument.
char school[SIZE];
gets(school);

8. Input / Output with gets and puts
A problem with scanf when reading a string is that it stops scanning the moment it encounters a white space.
Thus, it cannot scan a string such as: “Ali Zaki Street” in one variable.
An alternative to scanf is the function gets that takes a string variable as argument.
char school[SIZE];
gets(school);
No &

9. Input / Output with gets and puts
The gets function continues to scan for characters until it encounters the new line character – until the user types the Enter key.
The function puts can be used to print a string.
puts(school);

10. String.h
To use some operations or functions on strings, we need to include file string.h
#include<string.h>
So, we can use functions
strcpy(s1, s2)
strlen(s)
strcmp(s1, s2)
strcat (s1, s2)

11. String Functions strcpy(s1, s2)
we can not use operator “ = “ with string
strcpy is copying s2 into s1 (s1  s2)

12. Strcpy Array style
void strcpy(char dest[], char src[]) { int i = 0; while (src[i] != ‘\0’) { dest[i] = src[i]; i++; } dest[i] = ‘\0’

13. Strcpy Pointer style
void strcpy(char* dest, char* src) { while (*src != ‘\0’) { *dest = *src; dest++; src++; }

14. String Functions strlen(S) Return the length of string S
strcpy(S, “industrial robot”);
strlen(S) will return 16
Note the difference between this and: char s[]=“industrial robot” ;

15. Strlen Array style int strlen(char str[]) { int i = 0;
while (str[i] != ‘\0’) {
i++; }
return i;

16. Strlen Pointer style
int strlen(char* str) { int i = 0; while (*str != ‘\0’) { i++; str++; } return i;

17. String Functions strcmp(s1, s2) It is for comparison of s1 and s2
It return 0 if s1=s2
Return –ve if s1 is less than s2
Return +ve if s1 is greater than s2

18. String Functions It appends/concatenates s2 to the end of s1
strcat(s1, s2)
It appends/concatenates s2 to the end of s1
Example: s1= “Lion” , s2 = “King”
strcat(s1,s2)  s1=“LionKing”

19. Example (1) #include <stdio.h> #include <string.h>
int main()
{ int k;
char src [30];
char dest [10]=" Tutorial";
strcpy(src, "This is a");
strcat(src, dest);
k=strlen(src);
printf("Final string : %s, with length = %d\n", src,k);
return(0); }

20. Example (2) /* Compare two strings and determine which is greater*/
gets(s);
gets(d);
k=strcmp(s,d);
if (k<0)
printf("\n First is less than second \n");
else if (k==0)
printf("\n First equals second \n");
else if (k>0)
printf("\n First is greater than second \n");

21. Example (3) Write a function to reverse a string.
Write a program to check if a given string is a “palindrome”. Discuss alternative approaches, one of which you can use the reverse function.

22. Array of Strings
To represent arrays of strings we need 2- dimensional arrays of characters.
The first-dimension represents the number of strings in the array and the second-dimension represents the strings.
The following are statements to declare an array to store up to 30 names, each of maximum length, 25 characters.
#define NUM_NAMES 30
#define NAME_LEN 25
...
char names[NUM_NAMES][NAME_LEN];

23. Array of Strings
We can also initialize an array of strings at declaration in the following manner:
char month[12][10] = {"January", "February", "March", "April",
"May", "June", "July", "August",
"September", "October", "November",
"December"};

24. Example // Print array of strings #include <stdio.h> int main()
{ int i;
char Names[3][20]={"Akram Ali“ , “Mido Zaki",
, ”Said Abdo”};
for (i=0; i<3; i++)
printf("%s\n",Names[i]);
return(0);}

25. Array of Pointers to strings
For example: an array of strings
char *suit[ 4 ] = { "Hearts",
"Diamonds",
"Clubs",
"Spades" };
Strings are pointers to the first character
Each element of suit is a pointer to a char
The strings are not actually stored in the array suit, only pointers to the strings are stored
suit array has a fixed size, but strings can be of any size

26. Arrays of Pointers to strings
char * – each element of suit is a pointer to a char
The strings are not actually stored in the array suit, only pointers to the strings are stored
suit array has a fixed size, but strings can be of any size
suit[3]
suit[2]
suit[1]
suit[0]
’H’
’e’
’a’
’r’
’t’
’s’
’\0’
’D’
’i’
’m’
’o’
’n’
’d’
’C’
’l’
’u’
’b’
’S’
’p’

27. Example (Array of strings)
char *suit[ 4 ] = { "Hearts",
"Diamonds",
"Clubs",
"Spades" };
int main()
{ int x ;
for (x = 0; x < 4 ; x++)
printf("\n %s %d ", suit[x],strlen(suit[x]));
return 0; }

28. Questions?