PIC10A Lecture 16

Reading

p351-360, 372-375

Functions in `<cstring>`

strlen

strlen(cstring) find out the string length of the cstring
See example from last lecture

strcpy and strncpy

strcpy(target, src) copy the the src (a C-string) to the the target
You shouldn't use = to copy strings.
doesn't not check to make sure that the target is large enough to hold the value of the src
strncpy(target, src, int limit) similar to strcpy, but at most limit characters are copied. It is safer than strcpy.

"=" can't be used in string assignment

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

int main() {
    char a[6] = "Hello";
    char b[6];
    b = a; // compiling error
    cout << a << endl;
    cout << b << endl;

    return 0;
}

Use strcpy to copy strings.

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

int main() {
    char a[6] = "Hello";
    char b[6];
    strcpy(b,a); // copy the a to b
    cout << a << endl;
    cout << b << endl;

    return 0;
}

Another example of copying string.

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

int main() {
    char a[10];
    strcpy(a, "PIC10A");
    cout << a << endl;

    return 0;
}

strcpy does not check to make sure the array is large enough to hold all the characters.

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

int main() {
    char a[3];
    strcpy(a, "PIC10A"); // not enough space, "PIC10A" has 6 characters
    cout << a << endl;

    return 0;
}

Use strncpy to make sure that the string is big enough for all the characters.

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

int main() {
    char a[] = "Hello"; // 5 characters
    strncpy(a, "Absolute C++ is great!", 5); // don't copy more than 5 characters
    cout << a << endl; 

    return 0;
}

Output, only 5 characters are copied.

Absol

strcat, strncat

strcat(target, src) concatenates(appends) the src to the end of the target.
it does not check to see if the target is large enough to hold the the result.
strncat(target, src, int limit): similar to strcat, but at most limit characters are appended.

Example

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

int main() {
    char greeting[] = "Hello! "; 
    char name[] = "Charles";
    char sentence[30];

    strcpy(sentence, greeting);// copy greeting to sentence
    strcat(sentence, name); // append name to sentence
    cout << sentence << endl;
    return 0;   
}

Output

Hello! Charles

strcmp, strncmp

strcmp(string1, string2) compares 2 strings.
- equal to 0, if string1 is same as string2
- < 0, if string1 is smallar than string2, i.e. if string1 and string2 are put in a dictionary, string1 appears first.
- > 0, if string1 is greater than string2.
strncmp(string1, string2, int limit), similar to strcmp(string1, string2) but only compare at most limit characters.

Example

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

int main() {
    char a[80], b[80];
    cout << "Please input 2 words\n";
    cout << "First word: ";
    cin >> a;
    cout << "Second word: ";
    cin >> b;
    if(strcmp(a,b) == 0) {
        cout << "They are equal.\n";
    } else {
        cout << "They are not equal.\n";
    }
    return 0;
}

Output 1

Please input 2 words
First word: UCLA
Second word: UCLA
They are equal.

Output 2

Please input 2 words
First word: PIC10A
Second word: PIC10B
They are not equal.

C-String input

You can input a C-string by using cin >> charArray
however, the input of the string ends in space, tab or new line
To read a whole line, use
```
cin.getline(stringName, max_characters + 1)
```
It reads max_characters and copy the data to stringName

A simple input

#include<iostream>
using namespace std;

int main() {
    char name[80];
    cout << "Please enter your name: ";
    cin >> name;
    cout << "Hello! " << name << endl;;

    return 0;
}

Output1

Please enter your name: Amy
Hello! Amy

Output2 : the input only read up to the space. Only first name is showed.

Please enter your name: Charles Li
Hello! Charles

Use getline to input a whole line.

#include<iostream>
using namespace std;

int main() {
    char name[20];
    cout << "Please enter your name: ";
    cin.getline(name, 20); 
    cout << "Hello! " << name << endl;;

    return 0;
}

Output

Please enter your name: Charles Li
Hello! Charles Li

If you enter more than 19 characters, only the first 19 characters is stored.

Please enter your name: abcdefghijklmnopqrstuvwxyz
Hello! abcdefghijklmnopqrs

Character-manipulating functions

#include<cctype>
toupper(char ch) or (tolower(char ch)) returns the uppercase (or lowercase) version of the character ch. The function won't change the argument ch
islower(char ch), isupper(ch), isdigit(ch), isalnum(char), isspace(ch), ispunct(ch): returns ture or false, the meaning should be clear. islower(ch) returns true is ch is a lowercase letter. Refer to the book for the definitions of other functions.

Example

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

// return true if s is in number format
bool isNumber(char s[]);

int main() {
    char input[80];
    cout << "Your input: ";
    cin.getline(input, 80); 
    if(isNumber(input)) {
        cout << "The input is a number.\n";
    } else {
        cout << "The input is not a number.\n";
    }

    return 0;

}

bool isNumber(char s[]) {
    for(int i = 0; i < strlen(s); i++) {
        if(!isdigit(s[i])) { // if one character is not a digit
            return false;
        }
    }
    return true; // all of the characters are digits
}

Output1

Your input: 314159
The input is a number.

Output2

Your input: 3445ab33
The input is not a number.