Introduction to C Programming
Decision and Branching Concepts

Boolean Variables and Data Type ( or lack thereof in C )

• A true boolean data type could be used for storing logical values, and would only have two legal values - "true", and "false".
• C does not have boolean data types, and normally uses integers for boolean testing.
  • Zero is used to represent false, and One is used to represent true.
  • For interpretation, Zero is interpreted as false and anything non-zero is interpreted as true.
  • To make life easier, C Programmers typically define the terms "true" and "false" to have values 1 and 0 respectively.
    • In the old days, this was done using #define:
      • #define true 1
      • #define false 0
      • const int true = 1;
      • const int false = 0;
      • The type "bool" is the same as a newly defined type "_Bool"
        • _Bool is an unsigned integer, that can only be assigned the values 0 or 1
        • Attempting to store anything else in a _Bool stores a 1. ( Recall that C interprets any non-zero as true, i.e. 1 )
        • Variables can now be declared of type "bool".

        Relational Operators

        • Relational operators are binary operators that evaluate the truthhood or falsehood of a relationship between two arguments, and produce a value of true ( 1 ) or false ( 0 ) as a result.
        • The following table shows the 6 relational operators and their meaning, by example:

        True ( 1 ) if A is greater than B, false ( 0 ) otherwise

        True ( 1 ) if A is less than B, false ( 0 ) otherwise

        True ( 1 ) if A is greater than or equal to B, false ( 0 ) otherwise

        True ( 1 ) if A is less than or equal to B, false ( 0 ) otherwise

        True ( 1 ) if A is exactly equal to B, false ( 0 ) otherwise

        True ( 1 ) if A is not equal to B, false ( 0 ) otherwise

        • ( Note: There should not be a space between the two equals signs in = =. I am adding one in these HTML notes so the two signs do not run together, as ==. )
        • DANGER DANGER: Do not confuse the equality test operator, = =, with the assignment operator, =. It is a VERY common error to use the wrong number of equals signs, ( either too many or too few ), and the resulting code will usually compile and run but produce incorrect ( and often unpredictable ) results.
          • Example: "while( i = 5 )" is an infinite loop, because i is given the value 5 by the assignment, and since this is non-zero, it is always true. The programmer probably meant to use "while( i = = 5 )", which will test whether or not i is equal to 5, and will only continue to loop as long as this is true.
          • double x, y, tolerance = 1.0E-6;
          • BAD: if( x = = y )
          • GOOD: if( fabs( x - y ) < tolerance )
          • This is particularly important when testing the stopping condition of an iterative algorithm, because otherwise the roundoff error could cause the program to run forever. WRONG: while( estimatedError != 0.0 )

          Logical Operators

          • Logical operators are used to combine and evaluate boolean expressions, generally in combination with the relational operators listed above.
          • The following table shows the 3 logical operators and their meaning, by example:

          A AND B - True ( 1 ) if A is true AND B is also true, false ( 0 ) otherwise

          A OR B - True ( 1 ) if A is true OR B is true, false ( 0 ) otherwise

          NOT A - True ( 1 ) if A is false ( 0 ), false ( 0 ) if A is true ( non-zero ).

          • Note: There should not be a space between the | characters in | |, but one is being added here for clarity.
          • DANGER DANGER: If you only use a single & or | instead of the double && or | |, then you get valid code that is beyond the scope of this course, but which will not do what you want.
          • ! has a very high precedence, higher than the relational operators listed above. && and | | have lower precedence than the relational operators, with && having higher precedence than | |.
          • Example: To determine whether x is between 5.0 and 10.0:
            • INCORRECT: 5.0 < x < 10.0
            • CORRECT: 5.0 < x && x < 10.0
            • Question - What is the value of the incorrect expression ( regardless of the value of x ), and why?

            int year; bool leap_year; leap_year = ( ( ( ( year % 4 ) == 0 ) && ( ( year % 100 ) != 0 ) ) || ( ( year % 400 ) == 0 ) ); // Or equivalently: leap_year = !( year % 4 ) && year % 100 || !( year % 400 );

            • In a complex logical expression, the computer will stop evaluating the expression as soon as the result is known.
            • "A && B" will not evaluate B if A is already false.
            • "A | | B" will not evaluate B if A is already true.
            • Example: "if( x != 0.0 && 1.0 / x < 10.0 )" will never divide by zero.

            if-else

            • "if" blocks and "if-else" blocks are used to make decisions, and to optionally execute certain code.
            • The general syntax of the if-else structure is as follows:

            if( condition ) true_block else false_block

            • If the condition evaluates to true ( non-zero ), then the true_block of code is executed.
            • If the condition evaluates to false ( zero ), then the false-block of code is executed if it is present.
            • After one or the other block is executed, then execution continues with whatever code follows the if-else construct, without executing the other block.
            if( x
            When an if-else construct is used in the true block of another if-else, that is termed nested ifs.

            if( x < 0.0 ) printf( "Error - The x coordinate cannot be negative. ( x = %f ) \n", x ); exit( -1 );

            switch

            • In many cases, an integer valued expression must be compared against a number of possible choices, such as the following:

            const int left = 1, right = 2, up = 3, down = 4; // For improved readability int direction; . . . printf( "Please enter a direction, 1 for left, 2 for right, . " ); scanf( "%d", &direction ); if( direction = = left ) x--; else if ( direction = = right ) x++; else if ( direction = = up ) y++; else if ( direction = = down ) y--; else printf( "Error - Invalid direction entered.\n" );

            const int left = 1, right = 2, up = 3, down = 4; // For improved readability int direction; . . .

            printf( "Please enter a direction, 1 for left, 2 for right, . " ); scanf( "%d", &direction ); switch( direction ) < case left: x--; break; case right: x++; break; case up: y++; break; case down: y--; break; default: printf( "Error - Invalid direction entered.\n" ); break; >/* End of switch on direction */

            // Repeat the case sub-construct as many times as needed default: // Optional

            default code block; break; // Not needed. Added for defensive programming only.

            • If the break statement is omitted from the end of a case, then execution will continue into the next case, and so on until a break is eventually encountered, or until the end of the switch is reached.
            • Omitting the break statement is a common mistake.
            • Sometimes the break is omitted intentionally, in order to combine cases:

            bool done = false; // Assumes stdbool.h has been #included char choice; while ( ! done ) < // Code to print out a menu and ask for choice omitted scanf( "%c", &choice ); switch( choice ) < case 'E': case 'e': /* Code here to handle Entering data */ break; case 'C': case 'c': /* Code here to handle Calculations */ break; case 'H': case 'h': case '?': /* Code here to deliver Help */ break; case 'Q': case 'q': done = true; break; default: printf( "Error - Invalid choice: %c\n", choice ); printf( "Choose 'H' for help.\n"; break; >/* End of switch on menu choices */ > /* End of infinite while loop */

            The Conditional Operator

            Earlier we looked at unary and binary operators under C. In addition, the C language contains one ternary operator, known as the conditional operator, ?:

            • The conditional operator functions very much like an if-else construct, and the two constructs can often be used interchangeably.
            • However, due to its operator status, the results of the conditional operator can also be used as part of a larger expression and/or be assigned to a variable.
            • The syntax of the conditional operator is:

            condition ? true_clause : false_clause

            • The condition is first evaluated, ( along with any side effects. ) Then, either the true clause or the false clause will be executed, ( along with any side effects ), depending on whether the condition evaluates to true ( non-zero ) or false ( zero ) respectively. The value of the conditional operator as used in a larger expression is the value of whichever clause gets executed.
            • Examples:

            max = i > j ? i : j; abs = x > 0 ? x : -x; printf( "X is %s.\n", ( x < 0 ? "negative" : "non-negative" ) );
            // Parentheses needed in the last example because + and * higher than > and ?: pay = base + ( sales > quota ? 1.5 : 1.0 ) * bonus;

            if( error < tolerance ) step *= 2.0; else step = 1.0; if( x != 0.0 ) z /= x; else z = infinite; if( x >0.0 ) sign = 1.0; else if ( x = = 0.0 ) sign = 0.0; else sign = -1.0;

            Related Topics

            The following topics are not covered here, but may be found in many books on C/C++;

            • Bitwise logical operators, | and &, ^, and ~
            • Bit shift operators, >
            • Combinations of these with assignment, e.g. |=, &=, >=, and ^=.