Strings and IO

Strings

Integers and Characters Character Strings Input and Output

Strings

Types – So Far

To date all we have seen are numeric data types int, short, long, long long, ... float, double, ... char

The char type stores a numeric code which is translated to a character when appropriate And C treats them as numbers

Characters are Integers

It’s easy to print the ASCII code for a character char ch = ‘x’; printf(“code for %c = %d”, ch, ch);

The first placeholder print the letter that the code represents

The second placeholder prints the code C will also allow arithmetic to be

performed on char variables The underlying numeric codes are operated

Arithmetic and Char Let’s say that we want to print all of the letters from A to

Z We could write 26 printf statements▪ printf('A');▪ printf('B');▪ ...

Or we could do thischar ch = 'A';

while(ch < 'A' + 26){

printf("%c\n", ch);

ch++;

}

Character Strings

We have used character strings in printf calls e.g. printf(“Hello World”);

It would also be useful to store character strings in variables And to get user input in the form of

strings In C a character string is a sequence

of characters Stored in an array

Name and Age

Let’s write a program to find out the name and age of the user And then print them

Name and Age Programint main(){

char name[20];int age;

printf("What is your name? ");scanf("%s", name);printf("What is your age? ");scanf("%d", &age);

printf("Your name is %s, and your age is %d\n", name , age);

return 0;}

things to notesquare brackets

no &

What is your name? JennyWhat is your age? 11Your name is Jenny, and your age is 11

Character Arrays

The line char name[20]; declares an array of 20 characters A sequence in main memory with

enough space for twenty characters An array is an ordered sequence of data

elements of one type The brackets identify name as an

array rather than a single character And 20 indicates the size of the array

Arrays and Memory

... J e n n y \0

A sequence of 2o adjacent bytes in main memory

Why 20 bytes?

Because each character is stored in one byteWhat is the \0 in the 6th.

byte?A null character to indicate the end of the string

Why is this necessary?

Because memory locations can’t be empty, so it is important to distinguish between characters we want to store and garbage values

Strings and scanf

We used scanf like this: scanf(“%s”, name); The format specification %s is for a

string The variable name is not preceded by an

& Remember that when using scanf it

is necessary to provide the address of a variable Using the address of operator (i.e. &) Array variables, like name, in fact

contain addresses already, an array variable is a pointer▪ Which is why the & is not necessary

Characters and Strings

A string consists of an array containing the words in the string and the null character

Consequently, a string containing a single character is not the same as a char variable

The type of a character array is not the same as the type of a character That is char name[20] declares an array

not a char

athe character 'a' athe string "a" \

0

The Null Character

Usually we (as programmers) are not responsible for inserting the null character

This task is performed by a function that is responsible for creating a string Such as scanf, for string input

Note also that functions such as printf are able to detect the null character To print the appropriate part of a

character array

String Functions

There are a number of functions that are useful when working with strings

It can be important to know how long a character array is and how long a string is The maximum length And the length of the string currently

stored in the array The sizeof and strlen functions can

be used to determine this information

Finding Sizes – sizeof

The sizeof function can be used to find the size in bytes of any variable or typeint x = 212;double d = 2.713;char ch = 'a';char str[20];printf("sizeof(x) = %d\n", sizeof(x));printf("sizeof(d) = %d\n", sizeof(d));printf("sizeof(ch) = %d\n", sizeof(ch));printf("sizeof(str) = %d\n", sizeof(str));printf("sizeof(short) = %d\n", sizeof(short));

Finding the Size of a String The strlen function can be used to

find the size of a string stored in character array The number of characters before the null

characterchar name[20];printf("What is your name? ");scanf("%s", name);

printf("char[] size = %d\n", sizeof(name));printf("size of %s = %d\n", name, strlen(name));

#define(d) Strings

What gets printed?

#define BATMAN "Bruce Wayne"int main(){ printf("size of %s = %d\n", BATMAN, strlen(BATMAN)); printf("BATMAN size = %d\n\n", sizeof(BATMAN)); return 0;}

The difference is the null character

#define and Strings

The #define pre-processor directive can be used to create string constants As well as any other kind of constant▪ #define SFU "Simon Fraser University"

When using #define it is not necessary to specify the type of data being defined This is because the directive is essentially

replacing the constants with the data Before the compiler runs

#define Syntax

Define constants using #define like this #define <NAME> <value> #define PI 3.14159 #define TITLE “Lord of the Rings” #define DAYS 365

Constants are in upper-case by convention To distinguish them from variables

Size Constants

If you want to find the size of various types they are recorded in the limits.h file e.g. INT_MAX, LONG_MAX, LLONG_MAX This can be useful if you want to know

the byte size of a type on a particular OS There are similar constants for floats

in float.h

Other Constants

A more modern approach to creating constants is to use the const modifier const double PI = 3.14159;

Unlike #define this is not a pre-processor directive so must Declare the type, Use assignment, and End in a semi-colon

Defining constants with const is more flexible when dealing with projects with multiple files

Input and Output

Using printf

As we’ve seen the printf function can take the following arguments (i.e. input) A string – required Placeholders (conversion specifiers) in

the string▪ Introduced by the % symbol

Variable or literal arguments that should match the placeholders▪ If the variables don’t match the placeholders

then the output will be incorrect

Placeholders

Placeholders indicate the type to be printed e.g. %d for decimal integer

These can be further modified for Type size▪ e.g. %lld specifies a long long int

Field width, right justified by default▪ e.g. %4d specifies a minimum field width of four

Precision – used for %e, %E, and %f to indicate the number of digits to the right of the decimal place▪ e.g. %4.2f prints a float five characters wide with two digits

after the decimal place

Flags for printf Output can be further modified by the use of flags

- left justifies the output▪ e.g. %-20s prints a left justified string of width 20

+ indicates signed values with +, or –▪ e.g. "**%+8.2f" prints a float like this: ** +123.45

<space> indicates signed values should start either with a space, or the – sign, to allow columns of signed values to line up▪ e.g. "**% 10.3f" prints a float like this: ** 123.456

0 indicates that numbers should be padded with zeros▪ e.g. "**%8d" prints an int like this: **00000123

I’ve printed **s to show the spacing

More About Placeholders We used different placeholders to print a

character and a number Consider this common errordouble dbl = 123.1;printf("dbl = %d", dbl);

The placeholder indicates how the data stored in the argument should be printed Strictly speaking it also indicates how many

bytes of the argument should be accessed

Mismatched Conversions

If a variable doesn’t match the placeholder the output may be incorrect

The placeholder indicates two things The number of bytes of an argument to

look at What those bytes represent

Consider the %d placeholder It is an instruction to look at 4 bytes of

the appropriate argument And interpret them as a 2’s complement

int

Conversion ExampleHere is an example of a mismatched conversion

Incidentally printing the fourth line was fun ...

printf("printf(\"n1 = %%d, n2 = %%d, n3 = %%d\",n1, n2, n3);\n\n");

Note that the first placeholder prints the right most 4 bytes of n1 and the second prints the left most 4 bytesprintf, and all other function calls use an area of memory called the stack

...

n3(4byte

s)n2(4

bytes)

n1(8

bytes)...1st %d2nd %d3rd %d

part of the

stack

Printing Long Strings

A long C statement may take up more than one line Since the compiler ignores white space

A string may not take up more than one line The compiler gives an error if a string

takes up more than one line But printf will concatenate two strings

written on two linesprintf("blah blah blah");

error! printf("blah blah ""blah");

OK

Input With scanf

Similar to printf, scanf uses a control string with a list of arguments

The control string contains shows the destination data types for the input stream %s, %f, %s and so on Characters that do not match the

destination types will cause problems The arguments contains the

addresses of the variables

Whitespace

The scanf function uses whitespace to determine that the end of input is reached New lines, tabs or spaces

Other input functions allow whitespace to be included in input (for strings or characters)

The scanf function treats input as a stream of characters Processing or ignoring each character Ignored characters are left in the stream

ExampleHere is a code fragment to request a name and age

int age;char name[20];

printf("Enter your age, and last name: ");scanf("%d%s", &age, name);

printf("Your name is %s, and you are aged %d\n", name, age);

I could have written this in two lines:scanf("%d", &age);scanf("%s", name);

What should happen:

What might happen:

When a non-numeric character is reached (e) input stops for age

Then e3 was read as the name, and input stops once whitespace is encountered

Characters In Format String It is permissible to put ordinary

characters in the format string These characters must be matched

exactly by the input string▪ e.g. scanf(“%d,%d”, &x, &y);▪ scanf expects the user to type a number, a

comma, and another number▪ I would recommend not doing this!

scanf Return Value

The scanf function returns a value, allowing for functions to detect unexpected results The value is the number of items that

have been successfully readint x;int chars_read;printf("Please enter a number: ");chars_read = scanf("%d", &x);printf("Items successfully read = %d\n\n", chars_read);

More Later

We will cover more details on many of these subjects later Arrays Strings Pointers More string functions Other input and output functions