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CS1061 C Programming Lecture 5: Building Blocks of Simple Programs

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Integer division truncates remainder. 7 / 5 evaluates to 1 ... Some arithmetic operators act before others (i.e., multiplication before ... – PowerPoint PPT presentation

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Title: CS1061 C Programming Lecture 5: Building Blocks of Simple Programs


1
CS1061 C ProgrammingLecture 5 Building Blocks
of Simple Programs
  • A. ORiordan, 2004

2
Arithmetic
  • Arithmetic calculations
  • Use (addition) - (subtraction)
    (multiplication) and / (division)
  • Integer division truncates remainder
  • 7 / 5 evaluates to 1
  • Modulus operator() returns the remainder
  • 7 5 evaluates to 2
  • Operator precedence
  • Some arithmetic operators act before others
    (i.e., multiplication before addition) - use
    parenthesis when needed.
  • Example Find the average of three variables a, b
    and c
  • Do not use a b c / 3
  • Use (a b c ) / 3

3
The if statement
  • The Selection structure is used to choose among
    alternative courses of action
  • Example If students grade is greater than or
    equal to 60 then Print Passed
  • Algorithm If condition true Print statement
    executed and program goes on to next statement.
    If false, print statement is ignored and the
    program goes onto the next statement.
  • Code
  • if (grade gt 60) printf("Passed\n")

4
The else clause
  • The if selection statement may have an else
    part.
  • Example If students grade is greater than or
    equal to 60 then Print Passed else Print
    Failed.
  • Here is the C code. Note use of relational
    operator (gt)
  • if (grade gt 60)
  • printf("Passed\n")
  • else
  • printf("Failed\n")
  • Statement could have been written (using ?
    notation)
  • grade gt 60 ? printf(Passed\n)
    printf(Failed\n)

5
The else clause (continued)
  • You can have a set of statements within a pair of
    braces, example
  • if (grade gt 60)
  • printf("Passed.\n")
  • else
  • printf("Failed.\n")
  • printf("You must take this course
    again.\n")
  • The second printf is part of the else clause.
    Note also the use of the relational operator gt
    (less than or equal to)

6
Booleans
  • The if statement above contains a Boolean
    expression. This is an expression with two values
    that eveluates to either true or false.
  • if (x lt 4) printf(x is less than 4)
  • You can also declare Boolean variables and give
    them an initial value.
  • bool myBoolean false
  • if (myBoolean)
  • printf(This statement is never displayed)
  • Booleans named after George Boole, First
    Professor of Mathematics, U.C.C.

7
Relational Operators
8
C Characters
  • Enclosing a character in single quotes, e.g. H,
    represents a printable character constant.
    Internally, characters are represented
    numerically, usually by their ASCII codes. For
    example, the ASCII code for A is 65.
  • Example Here we declare a character variable
    answer with the initial value y (for yes) and
    change it in the couse of the program to n (for
    no).
  • char answer y / set as yes /
  • ...
  • answer n / changed to no /
  • Non-printable characters are represented using
    the escape sequence backslash (\). The same
    convention is used to represent the characters \
    and which would otherwise cause confusion, i.e.
    \\ and \.

9
Special Characters
  • C has special characters that need to be
    escaped
  • new-line \n
  • horizontal tabulation \t
  • vertical tabulation \v
  • backspace \b
  • carriage return \r
  • bell \a
  • backslash \\
  • question mark \?
  • single quote \
  • double quote \
  • Here is an example within a printf statement
  • printf(\tThis is \layed\ out nicely\n)

10
Reading in data
  • Before we can write useful programs we need to be
    able to read in data from the keyboard.
  • A scanf() performs a similar function to printf
    except its for (formatted) input. Note the use of
    the address-of operator () in the argument list.
  • Heres an example where we read in an integer and
    a character into variables (and then display
    them).
  •  
  • int anInt
  • char aChar
  •  
  • printf(Enter one character and one integer)
  • scanf(c d, aChar, anInt)
  • printf(The value of the integer is d and the
    value of the character is c\n, anInt, aChar)

11
Reading in data (continued)
  • The scanf function uses standard input (usually
    keyboard).
  • Here is a simple example
  • int int_var
  • scanf(d,int_var)
  • This scanf statement has two arguments
  • d - indicates data should be a decimal integer
  • int_var - location in memory to store variable
  • The user responds to this scanf statement by
    typing in a number, and then pressing the enter
    (return) key.
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