PIC10A Lecture 15

String

C-String

We can use an array of chars to represent a string.
Example, my name Charles can be represented by:

char name[] = {'C', 'h', 'a', 'r', 'l', 'e', 's'};

Not quite!

The computer doesn't know where is the end of the string.
We need to indicate the end of a string by putting a null character '\0' at the end of a string.

char name[] = {'C', 'h', 'a', 'r', 'l', 'e', 's', '\0'};

A string stored in this way is called C-string.
It is very inconvenient to initialize a string in the way shown above.
C++ allows us to initialize a C-String like this :

char name[] = "Charles";

This means

name[0]	name[1]	name[2]	name[3]	name[4]	name[5]	name[6]	name[7]
C	h	a	r	l	e	s	\0

use double quotation
The program assigns the size of the string for you
a null character '\0' is automatically added for you
So the size of the char array is actually the length of the string + 1 (for the extra null character)

You can also specific the size of an array in a C-String declaration. Example :

char name[10] = "Hi Mom!";

name[0]	name[1]	name[2]	name[3]	name[4]	name[5]	name[6]	name[7]	name[8]	name[9]
H	i		M	o	m	!	\0	?	?

The extra characters(the characters after '\0') are unknown.
Always make sure the size of the array is the max possible string size + 1. The extra 1 is for the null character.
Example, if you want to declare a string to represent a name, and if you know that a name should have less or equal to 50 characters. Which one of the following is the correct choice?
- char name[50]
- char name[51]

What's wrong about the following program?

#include<iostream>
using namespace std;

int main() {
    char a[3] = "abc";
	return 0;
}

Copy a string is not as easy as you think. There will be a compiling error in the following program

#include<iostream>
using namespace std;

int main() {
    char name[8];
    name = "charles"; // you can't assign string
                      // instead, use strcpy function
    return 0;
}

cout a CString

A cstring is actually an array of characters with a '\0' at the end
you can cout a cstring, every single element before '\0' will be printed out
this is something you can't do with other types of array.

Example

#include<iostream>
using namespace std;

int main() {
    char greeting[] = "Hello!";
    char name[] = "Charles";

    cout << greeting; // Hello!
    cout << " "; // a space
    cout << name; // Charles
    cout << endl;

    return 0;
}

Output

Hello! Charles

Compare to the following program.

#include<iostream>
using namespace std;

int main() {
    int a[] = {1,2,3};
    cout << a << endl;
    return 0;
}

Something meaningless is printed(actually the address of a is printed).

0012FF74

Also compare to the following program

#include<iostream>
using namespace std;

int main() {
    char name[] = {'A', 'm', 'y', '\0'}; // a C-string
    char name2[] = {'A', 'm', 'y'}; // not a C-string

    cout << name; // normal output
    cout << endl;
    cout << name2; // strange output
    cout << endl;

    return 0;
}

Output

Amy
Amy#Amy

Recall:
A C-string is an array of chars with a null character '\0' at the end.

C-string in function

C-string is an array of chars.
you can use an array of chars in function(refer to previous lectures)
use '\0' to indicate the end of an array
no need to pass the size to the function

Question: Write a function int length(char s[]). It returns the number of charaters of a C-string.
Answer

#include<iostream>
using namespace std;

int length(char s[]);

int main() {
    char greeting[] = "Hi PIC10A!!!";
    cout << "The length of the string is: "
         << length(greeting)
         << endl;

    return 0;
}


int length(char s[]) {
    int index = 0;
    while(s[index] != '\0') { // when the charater is not null
        index++;
    }

    return index;
}

Output

The length of the string is: 12

Keypoint
Use the following while-loop to go through every element in the string.

    while(s[index] != '\0') { 
        // do something to s[index]
        // your codes
		
        index++;
    }

One more question: Write a function to change all 'a' in a string to 'z'.

#include<iostream>
using namespace std;

void aToZ(char s[]);

int main() {
    char string[] = "Pic10a students, how are you?";
    cout << "Before the function call, string is \n"
         << string << endl;
    aToZ(string);
    cout << "After the function call, string is \n"
         << string << endl;

    return 0;
}


void aToZ(char s[]) {
    int index = 0;
    while(s[index] != '\0') { // when the charater is not null
        // do something to s[index]
        if(s[index] == 'a')
            s[index] = 'z';

        index++;
    }
}

Output

Before the function call, string is
Pic10a students, how are you?
After the function call, string is
Pic10z students, how zre you?

Useful functions

There are already a lot of useful functions written
#include<cstring>
refer to the book p357, p865
functions:
- strcpy, strncpy: string copy.
- strcat, strncat: append one string to the other.
- strlen: length of the string
- strcmp, strncmp : compare string.

Example, the program about length function can be rewritten as

#include<iostream>
#include<cstring>
using namespace std;


int main() {
    char greeting[] = "Hi PIC10A!!!";
    cout << "The length of the string is: "
         << strlen(greeting)
         << endl;

    return 0;
}