1 Structured Programming in C Welcome to CPSC 206

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Structured Programming in CStructured Programming in C

Welcome to CPSC 206Welcome to CPSC 206

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Lecture Information

http://people.cs.tamu.edu/ychen/Teaching/CPSC206

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Lecture Topics:

0. Introduction to Computer Science1. Overview of C Ch 1, 2

2. Flow of control and functions Ch 3, 43. Character processing & fundamental data types Ch 5, 64. File I/O Ch 135. Pointers, Arrays, and Strings Ch 8, 9, 106. Structures, and linked lists Ch 12

Features of C:Features of C:

7. Enumeration type and storage classes Ch 7, 88. Recursion Ch 11

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Review of Class on Sept. 16, Thursday

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Chapter 2:

Lexical Elements, Operators, and the C System

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Lexical Elements, Operators, and the C System

C is a language Alphabet syntax

What is C program?A C program is a sequence of characters

Lexical Elements, Operators, and the C SystemLexical Elements, Operators, and the C System

How a computer understands this sequence of characters?

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Introduction

How to check a C program is correct?The characters are collected by the

compiler into syntactic units called tokens

The compiler checks whether the tokens can be formed into legal strings according to the syntax of C language.


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Introduction

In C language, there are six kinds of tokens:

Keywords Identifiers Constants String constants Operators Punctuators


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Lexical Elements

Comment What is comment?

Arbitrary strings of symbols placed between the delimiters /* and */.

Single line comment: // text The compiler changes each comment into a

single blank character. Rules:

Multi-line comments cannot be placed in the middle of a keyword or identifier.

Multi-line comments may not be nested.

Lexical Elements, Operators, and the C SystemLexical Elements, Operators, and the C SystemLexical Lexical ElementsElements

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Lexical Elements

Keywords What is Keywords?

Keywords are explicitly reserved words that have a strict meaning as individual tokens in C.

Examples of KeywordsData Type: int, char, long, short

Keywords cannot be redefined or used in other contexts.


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Lexical Elements

Identifiers What is identifier?

The names of variables, functions, labels and other user-defined items are called identifier.

Special identifier Keywords, names of functions in C library,

main Rules:

1. composed of letters, digits, and underscore _ .2. The first character must be a letter or

underscore.3. case-sensitive4. would not be defined as the special identifiers:


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Lexical Element

Constants Integer constants: 0, 17

Decimal integer: 17Octal integer: 017Hexadecimal integer: 0x17

Floating constants:double, float, long double

Character constants: (enclosed between single quotes)

Lexical Lexical ElementsElements


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Lexical Elements

String Constants String constant is a sequence of characters

enclosed in a pair of double quote marks. String constants are differently form

character constants. Special characters: \”, \\ You mustn't split a string constant across

lines Two string constants that are separated

only by white space are concatenated by the compiler into a single string.



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Lexical Elements

Operators and Punctuator Precedence and Associativity determine

precisely how expressions are evaluated. Precedence of operators indicates when they when they

will be evaluatedwill be evaluated. Associativity

“left to right”: Operations are performed from left to right

o Examples: +,-,/,%“right to left”: Operations are performed from right

to lefto Examples: ++(prefix), --(prefix)



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Class on Sept 21

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Outline

An Example — Characters and Lexical ElementsLexical Elements

Comments Keywords Identifiers Constants String Constants Operators and Punctuators

An Example: Computing Powers of 2The C System


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Operators and Punctuators— Outline

Examples of Operators and PunctuatorsPrecedence and Associativity of

OperatorsIncrement and Decrement OperatorsAssignment Operators



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Operators and Punctuators — Increment ++ and Decrement Operators --

SemanticsPrecedence and AssociativityRules



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Increment ++i, i++ Each causes the stored value of i in memory

to be incremented by 1. Each of the expressions ++i and i++ has a

value.++i

o the stored value of i is incremented firsto the expression takes as its value the new

stored value of ii++

o the expression takes as its value the current stored value of i

o the stored value of i is incremented



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#include <stdio.h>int main(void){ int i, j, a, b; i=0; j=0; a = ++i; b = j++; printf("a=%d, b=%d\n",a,b); return 0;}

% gcc id.c% a.outa=1, b=0

id.c



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++ and + ++

Cause the value of a variable in memory to be changed

+Does not change the value of a

variable.



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Decrement Operator i-- and --i The value of i is decremented by 1. Each expression has a value.

--io the stored value of i is decremented by 1o the expression takes as its value the new

stored valued of ii--

o the expression takes as its value the current stored valued of i

o the stored value of i is decremented by 1



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Precedence and Associativity

Associativity ++ (postfix) -- (postfix) Left to right +(unary) –(unary) ++(prefix) --(prefix) Right

to left * / % Left to right + - Left to right



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Precedence and Associativity +(unary) –(unary) ++(prefix) --(prefix) Right to left

#include <stdio.h>int main(void){ int a=2; int result; result = - --a; printf("a=2, - --a = %d\n",result); return 0;}

% gcc id2.c% a.outa=2, - --a = -1

id2.c



Question1: What is the value of a after the operation --a?

Question2: What is the value of the expression --a?

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Examples#include <stdio.h>int main(void){ int a=1, b=2, c=3, d=4; int result1, result2; result1 = ++ a * b - c --; result2 = 7 - - b * ++ d; printf("++ a * b – c -- = %d\n",result1); printf(“ 7 - - b * ++ d = %d\n",result2); return 0;}

++ a * b – c --

++ (postfix) -- (postfix) Left to right+(unary) –(unary) ++(prefix) --(prefix) right to left* / % left to right+ - left to right


)()( )( ++ a * b – 3 2 * b – 3 4 – 3

1

)( )()(id2.c



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Examples#include <stdio.h>int main(void){ int a=1, b=2, c=3, d=4; int result1, result2; result1 = ++ a*b - c --; result2 = 7 - - b * ++ d; printf("++ a * b – c -- = %d\n",result1); printf(“ 7 - - b * ++ d = %d\n",result2); return 0;}

7 - - b * ++ d

++ (postfix) -- (postfix) Left to right+(unary) –(unary) ++(prefix) --(prefix) right to left* / % left to right+ - left to right


7 - (-2) * 5 7 - (-10) 17

)( )(

)(id2.c



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Examples#include <stdio.h>int main(void){ int a=1, b=2, c=3, d=4; int result1, result2; result1 = ++ a*b - c --; result2 = 7 - - b* ++ d; printf("++ a * b – c -- = %d\n",result1); printf(“ 7 - - b* ++ d = %d\n",result2); return 0;}

% gcc id1.c% a.out++ a * b - c -- = 1 7 - - b* ++ d = 17

id2.c



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Rules Applied to variables but not

to constants or ordinary expressions



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Operators and Punctuators— Increment ++ and Decrement Operators --

Examples:

#include <stdio.h>int main(void){ int a, result1, result2; a = 1; result1 = ++1; result2 = -- -a; return 0;}

% gcc id3.cid3.c: In function `main':id3.c:6: error: invalid lvalue in incrementid3.c:7: error: invalid lvalue in decrement

id3.c



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Operators and Punctuators— Increment ++ and Decrement Operators --

Summary ++

++i: the stored value of i is incremented; the expression takes as its value the new stored valued of i

i++: the expression takes as its value the current stored valued of i; the stored value of i is incremented by 1.

-- --i: the stored value of i is decremented by 1; the

expression takes as its value the new stored valued of i i--: the expression takes as its value the current stored

valued of i; the stored value of i is decremented by 1. Applied to variables but not to constants or

ordinary expression



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Operators and Punctuators— Outline

Examples of Operators and PunctuatorsPrecedence and Associativity of

OperatorsIncrement and Decrement OperatorsAssignment Operators



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Operators and Punctuators— Assignment Operators

Example: An assignment expression with = Format: variable = right_side

Two operands o variable o right_side: an expression



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Example: An assignment expression with = Format: variable = right_side Results:

The value of right_side is assigned to variable

Assignment expression variable = right_size has a value.

o The value of right_side is the value of the assignment expression.

Example: the value of c is 9

Assignment expression: a=2+(b=c+1)



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#include <stdio.h>int main(void){ int a, b, c; int a1, b1, c1; b = 2; c = 3; a = b + c; printf("b = %d, c = %d, a = %d \n",b, c, a);

a1=(b1 = 2) + (c1 = 3); printf("b1 = %d, c1 = %d, a1 = %d \n",b1, c1, a1);

return 0;}

% gcc ass1.c% a.outb = 2, c = 3, a = 5b1 = 2, c1 = 3, a1 = 5



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Assignment operators = op=:

+=, -=, *=, / =, %=, ……

Semantics: variable op= expression

equivalent to variable = variable op (expression) Example:

var*= expr var=var * expra *= 3 a = a * 3



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Assignment operators Precedence:

all the assignment operators have the same precedence

Lower than all the other operators which have been introduced (such as + - )

Associativity:right to left



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#include <stdio.h>int main(void){ int a, b, c; a = b = c = 0; printf("b = %d, c = %d, a = %d \n", b, c, a); return 0;} % gcc ass2.c

% a.outb = 0, c = 0, a = 0



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#include <stdio.h>int main(void){ int i=1, j=2, k=3, m=4; i += j + k; printf(" j = %d, k = %d, i += j+k = %d \n",j, k, i); printf(" m = %d, k = %d, ",m, k); j *= k = m + 5; printf("j *= k = m + 5 = %d \n",j); printf("k = %d \n",k); return 0;}

% gcc ass3.c% a.out j = 2, k = 3, i += j+k = 6 m = 4, k = 3, j *= k = m + 5 = 18 k = 9



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Summary Assignment operators

Precedence: they have the same precedence

o Lower than all the other operators which have been introduced (such as + - )

Associativity: right to left variable op= expr

variable = variable op (expr)The value of the expression is the value

of the expr



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Operators and Punctuators— Summary

Precedence and Associativity of OperatorsIncrement and Decrement Operators

i++, i++ i--, --i

Assignment Operators Variable op= expression

Variable = variable op (expression)The value of the expression is the value of

the expression



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Lexical Elements

Summary Comments Keywords Identifiers Constants String Constants Operators and Punctuators


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Lexical Elements

Comment What is comment?

Arbitrary strings of symbols placed between the delimiters /* and */.

Single line comment: // text The compiler changes each comment into a

single black character. Rules:

Multi-line comments cannot be placed in the middle of a keyword or identifier.

Multi-line comments may not be nested.


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Lexical Elements

Keywords What is Keywords?

Keywords are explicitly reserved words that have a strict meaning as individual tokens in C.

Examples of KeywordsData Type: int, char, long, short

Keywords cannot be redefined or used in other contexts.


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Lexical Elements

Identifiers What is identifier?

The names of variables, functions, labels and other user-defined items are called identifier.

Special identifier Keywords, names of functions in C library,

main Rules:

1. composed of letters, digits, and underscore _ .2. The first character must be a letter or

underscore.3. case-sensitive4. would not be defined as the special identifiers:


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Lexical Element

Constants Integer constants: 0, 17

Decimal integer: 17Octal integer: 017Hexadecimal integer: 0x17An integer may be too large to be stored in

a machine word. Floating constants: double, float, long double Character constants: (enclosed between single

quotes)



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Lexical Elements

String Constants String constant is a sequence of characters

enclosed in a pair of double quote marks. String constants are differently form

character constants. Special characters: \”, \\ You mustn't split a string constant across

lines Two string constants that are separated

only by white space are concatenated by the compiler into a single string.



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Lexical Elements

Operators and Punctuators Precedence and Associativity of Operators Increment and Decrement Operators

i++, i++i--, --i

Assignment OperatorsVariable op= expression

Variable = variable op (expression)o The value of the expression is the value of the

expression



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Outline





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An Example: Computing Powers of 2— Outline

Program pow_of_2 Dissection of the pow_of_2 Program

Lexical ElementsCommentsKeywordsIdentifiersConstantsString ConstantsOperators and Punctuators

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Purpose: prints on a line some powers of 2.

An Example: Computing Powers of 2— power_of_2.c

/* Some powers of 2 are printed. */

#include <stdio.h>

int main(void){ int e = 0, power_of_two = 1;

while (++e <= 10) printf("%5d", power_of_two *= 2); printf("\n"); return 0;}

pow_of_2.c

CommentsKeywordsIdentifiersConstantsString ConstantsOperators and Punctuators

How many times the body of the loop is executed?

power_of_two = power_of_two * 2

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An Example: Computing Powers of 2— power_of_2.c

/* Some powers of 2 are printed. */

#include <stdio.h>

int main(void){ int exponent = 0, power_of_two = 1;

while (++exponent <= 10) printf("%5d", power_of_two *= 2); printf("\n"); return 0;}

% gcc pow_of_2.c% a.out 2 4 8 16 32 64 128 256 512 1024%

Purpose: prints on a line some powers of 2.

pow_of_2.c

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Outline





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The C System

C is a small language The core language is small Non-essential functionality, such as math functions

or file handling, is provided by a standardized set of library routines.The standard library contains many useful

functions that add considerable power and flexibility to the c System.

What is standard library?How to use a function in standard library?

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The C System— The Standard Library

What is Standard library? (Contd.) A collection of header files and library files. Header file:

The names and characteristics of functions are included into computer files called header file.

Library file: The actual implementation of functions

are separated into a library file.The library contains compiled code that is

unreadable to humans.

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Examples of header files: <math.h>: For computing common mathematical

functions <stdio.h>: Provides the core input and output

capabilities of the C language. <stdlib.h>: For performing a variety of operations,

including conversion, pseudo-random numbers, memory allocation, process control, environment, signaling, searching, and sorting.

<string.h>: For manipulating several kinds of strings.

Check Appendix A for Details

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How to use a function in the standard library?

/*The traditional first program in honor of Dennis Ritchie who invented C at Bell Labs in 1972.*/

#include <stdio.h>int main(void){ printf(“Hello, world!\n”); return 0;}

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How to use a function in the standard library? The programmer needs to provide the

function prototype.Including appropriate header files.

Do we need to locate the function in the library file? No. The system knows where to find the code

that corresponds to functions from the standard library.

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Steps to use a function in the standard library. Find the header file which contains the

prototype of the function. Use #include preprocessing directive to

include the appropriate file.

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The C System— An example: prn_rand Program

Purpose of prn_rand program: Use rand() to generate some randomly

distributed integers.

Where is prototype of rand()? stdlib.h

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The C System— An example: prn_rand Program/*Printing random numbers. */#include <stdio.h>#include <stdlib.h>int main(void){ int i, n; printf("\n%s\n%s", "Some randomly distributed integers will be printed", "How many do you want to see? "); scanf("%d", &n); for(i=0;i<n;++i){ if (i%6==0) printf("\n"); printf("%9d", rand()); } printf("\n"); return 0;}

Hearder files are included

i=0;while (i++<n){……}

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The C System— An example: prn_rand Program/*Printing random numbers. */#include <stdio.h>#include <stdlib.h>int main(void){ int i, n; printf("\n%s\n%s", "Some randomly distributed integers will be printed", "How many do you want to see? "); scanf("%d", &n); for(i=0;i<n;++i){ if (i%6==0) printf("\n"); printf("%9d", rand()); } printf("\n"); return 0;}

% gcc prn_rand.c% a.out

Some randomly distributed integers will be printedHow many do you want to see? 17

16838 5758 10113 17515 31051 5627 23010 7419 16212 4086 2749 12767 9084 12060 32225 17543 25089%

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End of Chapter 2





Read Chapter 2Sections 2.1-

2.12

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End of Chapter 2

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1 Structured Programming in C Welcome to CPSC 206