Lecture 10: Reviews

Control Structures All C programs written in term of 3 control structures Sequence structures Programs executed sequentially by default Selection structures Used to choose among alternative courses of action Three types: if, if … else, and switch Repetition structures Used to repeat an action when some condition remains true. Three types: while, do … while and for

Sequence Structures Normally, statements are executed one after the other in the order written

Selection Structures - if if statement if (condition) { statement(s); } Only performs an action if the condition is true. Example: Flowchart if ( grade >= 60 ) { printf( “Passed\n” ); } if ( grade >= 60 ) { printf( “Passed\n” ); printf( “Congratulations!\n” ); } if ( grade >= 60 ) printf( “Passed\n” ); printf( “Congratulations!\n” );

Selection Structures - if…else If…else statement if (condition) { statement(s); } else { statement(s); } Specifies an action to be performed both when the condition is true and when it is false. Flowchart

Selection Structures - if…else Nested if…else statements Test for multiple cases by placing if…else selection statements inside if…else selection statement Once condition is met, rest of statements skipped if ( score >= 90 ) { printf( "The grade is 'A'.\n" ); } else if ( score >= 80 ) { printf( "The grade is 'B'.\n" ); } else if ( score >= 70 ) { printf( "The grade is 'C'.\n" ); } else if ( score >= 60 ) { printf( "The grade is 'D'.\n" ); } else { printf( "The grade is 'F'.\n" ); } >= 90:A 80 ~ 89:B 70 ~ 79:C 60 ~ 69:D < 60:F

Selection Structures - if…else if…else statements in sequence if ( score >= 90 ) { printf( "The grade is 'A'.\n" ); } else if ( score >= 80 ) { printf( "The grade is 'B'.\n" ); } if ( score >= 70 ) { printf( "The grade is 'C'.\n" ); } else if ( score >= 60 ) { printf( "The grade is 'D'.\n" ); } else { printf( "The grade is 'F'.\n" ); }

Selection Structures - switch switch statement Useful when a variable or expression is tested for all the values it can assume and different actions are taken. Format Series of case labels and an optional default case switch ( controlling expression ) { case value1: actions case value2: actions default: actions } The switch statement can be used only for testing a constant integral expression. switch (grade) { case >= 90: … case >= 80: … ….. } default case occurs if none of the case s are matched

Selection Structures - switch switch statement with break statements switch ( controlling expression ) { case value1: actions; break; case value2: actions; break; default: actions; break; } It causes program control to continue with the first statement after the switch statement.

Selection Structures - switch switch statement with break statements Example switch ( n ) { case 1: printf( “The number is 1\n”); break; case 2: printf( “The number is 2\n”); break; case 3: printf( “The number is 3\n”); break; default: printf( “The number is not 1, 2, or 3\n”); break; }

Selection Structures - switch switch statement with break statements Example switch ( n ) { case 1: printf( “The number is 1\n”); case 2: printf( “The number is 2\n”); break; case 3: printf( “The number is 3\n”); break; default: printf( “The number is not 1, 2, or 3\n”); break; }

Repetition Structures Two types of repetition structures Counter-controlled repetition Definite repetition: know how many times loop will execute Control variable used to count repetitions Sentinel-controlled repetition Indefinite repetition Used when number of repetitions not known Sentinel value indicates "end of data entry” Loop ends when user inputs the sentinel value

Repetition Structures Common error: Infinite loops are caused when the loop-continuation condition in a while, for or do...while statement never becomes false. To prevent: a) Make sure there is not a semicolon immediately after the header of a while statement. b) In a counter-controlled loop, make sure the control variable is incremented (or decremented) in the loop. c) In a sentinel-controlled loop, make sure the sentinel value is eventually input. j = 1; while (j <= 10); { printf(“%d\n”, j); j++; } j = 1; while (j <= 10) { printf(“%d\n”, j); } j = 1; while (j <= 10) { printf(“%d\n”, --j); j++; } j = 1; while (j <= 10) { printf(“%d\n”, j); j--; }

Repetition Structures - while while loop repeated until condition becomes false Counter-controlled repetition Sentinel-controlled repetition Format while ( condition ) { statement(s); } Flowchart condition while body

Repetition Structures - while Counter-controlled while loop Loop repeated until counter reaches a certain value Count the number of iterations j = 1; while (j <= 10) { printf(“Good Luck\n”); j++; } j = 0; while (j < 10) { printf(“Good Luck\n”); j++; } j = 0; while (j <= 10) { printf(“Good Luck\n”); j++; } j = 1; while (j <= 10) { printf(“Good Luck\n”); j += 2; } j = -1; while (j <= 10) { printf(“Good Luck\n”); j += 3; }

Repetition Structures - while Sentinel-controlled while loop Loop repeated until user inputs the sentinel value Sentinel value chosen so it cannot be confused with a regular input Examples Simple calculator ATM menu interface Prime number

Repetition Structures - do…while Similar to the while statement Condition for repetition only tested after the body of the loop is performed. All actions are performed at least once. Format do { statement(s); } while ( condition ); Flowcharting the do…while repetition statement

Repetition Structures - do…while Examples counter = 1; do { printf( “%d\n”, counter); counter ++; } while (counter <= 10); counter = 1; while (counter <= 10) { printf( “%d\n”, counter); counter ++; } counter = 12; do { printf( “%d\n”, counter); counter ++; } while (counter <= 10); counter = 12; while (counter <= 10) { printf( “%d\n”, counter); counter ++; }

Repetition Structures - for Can loop a known number of times Format when using for loops for ( initialization; loopContinuationTest; increment ) { statement(s); } initialization; while ( loopContinuationTest ) { statement(s); increment; } for (j = 1; j <= 10; j++) { printf(“%d\n”, j); } for (j = 10; j > 0; j--) { printf(“%d\n”, j); } for (j = 10; j > 0; j -= 2) { printf(“%d\n”, j); } incrementdecrement Increasing/decreasing other than 1

Nested Control Loops Nested while loops i = 1; while (i < 5) { j = 1; while (j < 5) { printf(“%d * %d = %d\n”, i, j, i*j); j++; } i++; } Nested for loops for (i = 1; i < 5; i++) { for (j = 1; j < 5; j++) { printf(“%d * %d = %d\n”, i, j, i*j); } i j

Nested Control Loops Nested while loops i = 1; while (i < 5) { j = 1; while ( j <= i ) { printf(“%d * %d = %d\n”, i, j, i*j); j++; } i++; } Nested for loops for (i = 1; i < 5; i++) { for (j = 1; j <= i; j++) { printf(“%d * %d = %d\n”, i, j, i*j); } i j j <= i

Nested Control Loops Nested while loops i = 1; while (i < 5) { j = 1; while ( j < 5 ) { printf(“%d * %d = %d\n”, i, j, i*j); j++; } i += 2; } Nested for loops for (i = 1; i < 5; i += 2) { for (j = 1; j < 5; j++) { printf(“%d * %d = %d\n”, i, j, i*j); } i j

Operators Arithmetic operators +, -, *, /, % Assignment operator = Arithmetic assignment operators +=, -=, *=, /=, %= Increment / decrement operators ++, -- Relational operators, >= Equality operators ==, != Logical operators &&, ||, !

== and = Swapping both operators does not ordinarily cause syntax errors. int payCode = 0; payCode = 4; if (payCode == 4) printf(“You get a bonus!\n”); int payCode = 0; payCode == 4; if (payCode == 4) printf(“You get a bonus!\n”); int payCode = 0; payCode = 3; if (payCode == 4) printf(“You get a bonus!\n”); int payCode = 0; payCode = 3; if (payCode = 4) printf(“You get a bonus!\n”);

Pre-increment and Post-increment Pre-increment/Pre-decrement Operator is used before the variable (++c or --c) Variable is changed before the expression it is in is evaluated Post-increment/Post-decrement Operator is used after the variable (c++ or c--) Expression executes before the variable is changed. c = 5; printf(“%d\n”, ++c); printf(“%d\n”, c); c = 5; printf(“%d\n”, c++); printf(“%d\n”, c); c = 5; ++c; printf(“%d\n”, c); c = 5; c++; printf(“%d\n”, c);

Logical Operators Logical AND - && Logical OR - || Logical NOT - ! x y x y

Logical Operators Examples int F = 0, T = 1; int x, y; x = !240; x = !0; x = (F && 1) || (T || F); y = (T || !F) && (!F && T); x = !(T && F || 1) if (grade >= 80 && grade <90) printf(“B\n”); while ((x > 0) || (y > 10)) { x--; y--; }

scanf and printf scanf() Used to obtain a value (values) from the user using standard input (usually keyboard). Examples int num, j; float x; scanf( “%d”, &num); scanf( “%d%d”, &num, &j); scanf( “%f”, &x); scanf( “%d%d%f”, &num, &j, &x); scanf( “%d”, num); scanf( “%d%d”, num);

scanf and printf printf() int num = 3, j = 2; float x = 3.0; printf( “Good Luck!\n”); /* prints a string */ printf( “num = %d\n”, num); /* prints the value of a variable */ printf( “%d + %d = %d\n”, num, j, num+j); /* prints the value of an expression */ printf( “%c\n”, ‘b’); /* prints the character ‘b’ */ j = ‘a’; printf( “%c\n”, j); /* prints the character ‘a’ */ printf( “x = %f\n”, x); /* prints a floating-point number */ printf( “x = %.2f\n”, x ); /* prints result with 2 digits after decimal point */