Introduction to Computing – B142L

Control structures

Academic year 2011/12



Objectives:

1. Conditional syntax in the C language
2. Iteration syntax in the C language

Theory of control structures in computing

Paper written by Bohm and Jacopini in 1966 proposed that all programs can be written using 3 types of control structures: sequential conditional repetition

Source: Bohm, C., Jacopini, G. (1966). Flow diagrams, turing machines and languages with only two formation rules. Communications of the ACM, 9, 366-371.

Control structures in the C language

Decision with if statement

Pseudo code: display have a nice day if raining      display reminder to bring umbrella Be careful to avoid division by zero in test conditions.

Flowchart:

#include <stdio.h> int main( void ) { int raining = 1; printf( "Have a nice day\n" ); if ( raining ) /* This is the test condition */ printf( "Bring an umbrella" ); }

Decision with if statement

Pseudo code: obtain input number if multiple of 5      display message that multiple found display remainder of division by 5 Be careful to avoid division by zero in test conditions.

Flowchart:

#include <stdio.h> int main( void ) { int number; scanf( "%d", &number ); if ( ( number % 5 ) == 0 ) { printf( "Number is multiple of 5\n" ); } printf( "The remainder is %d", number % 5 ); }

Decision with if / else statement

Pseudo code: obtain 2 cards if sum of cards equals to 21      display blackjack message else      display sum of cards Be careful to avoid division by zero in test conditions.

Flowchart:

#include <stdio.h> int main( void ) { int card1, card2, sum; scanf( "%d %d", &card1, &card2 ); sum = card1 + card2; if ( sum == 21 ) { printf( "Blackjack!" ); } else { printf( "Sum is %d", sum ); } }

Decision with if / else statement and logical negation

Pseudo code: obtain 2 cards if sum of cards not equal to 21      display sum of cards else      display blackjack message Be careful to avoid division by zero in test conditions.

Flowchart:

#include <stdio.h> int main( void ) { int card1, card2, sum; scanf( "%d %d", &card1, &card2 ); sum = card1 + card2; if ( sum != 21 ) { printf( "Sum is %d", sum ); } else { printf( "Blackjack!" ); } }

Nested decisions

Pseudo code: obtain 2 cards calculate sum of cards if sum of cards less than 12      if first card is an ace            add 10 to sum display the sum The decision logic affects the order of the test conditions. Be careful to avoid division by zero in test conditions.

Flowchart:

#include <stdio.h> int main( void ) { int card1, card2, sum; scanf( "%d %d", &card1, &card2 ); sum = card1 + card2; if ( sum < 12 ) { if ( card1 == 1 ) sum = sum + 10; } printf( "Sum of cards is %d", sum ); }

It may be possible to optimize the logic of nested decisions.

#include <stdio.h> int main( void ) { int card1, card2, sum; scanf( "%d %d", &card1, &card2 ); sum = card1 + card2; if ( ( sum < 12 ) && ( card1 == 1 ) ) { sum = sum + 10; } printf( "Sum of cards is %d", sum ); }

Chained decisions

Pseudo code: obtain 2 cards calculate sum of cards if sum equal to 21      display blackjack message else if 2 cards are actually a pair      suggest that the player split the pair else if sum is more than 17      suggest that player stand the hand display the sum The decision logic affects the order of the test conditions. Chained decision structures have only one path of execution. Be careful to avoid division by zero in test conditions.

Flowchart:

#include <stdio.h> int main( void ) { int card1, card2, sum; scanf( "%d %d", &card1, &card2 ); sum = card1 + card2; if ( sum == 21 ) printf( "Blackjack!\n" ); else if ( card1 == card2 ) printf( "Split your cards!\n" ); else if ( sum > 17 ) printf( "Stand!\n" ); printf( "Sum of cards is %d", sum ); }

Selection decisions with switch statement

Pseudo code: obtain 2 cards calculate sum of cards evaluate value of sum      21            display blackjack message      20      19      18            suggest that player stand the hand      default           display the sum switch statements must use integer variables. The default selection is always executed. It must be the last selection of the switch statement. Selection decision structures may have more than one path of execution. The decision logic affects the order of the comparison conditions. Comparisons are made in sequence from top to bottom.

Flowchart:

#include <stdio.h> int main( void ) { int card1, card2, sum; scanf( "%d %d", &card1, &card2 ); sum = card1 + card2; switch ( sum ) { case 21 : printf( "Blackjack!\n" ); case 20 : case 19 : case 18 : printf( "Stand!\n" ); default : printf( "Sum of cards is %d", sum ); } }

Selection decisions with switch statement and break keyword

Pseudo code: obtain 2 cards calculate sum of cards evaluate value of sum      21            display blackjack message            break      20      19      18            suggest that player stand the hand            break      default           display the sum            break The break keyword immediately exits the current switch statement. The default selection is only executed if the switch statement has not ended.

Flowchart:

#include <stdio.h> int main( void ) { int card1, card2, sum; scanf( "%d %d", &card1, &card2 ); sum = card1 + card2; switch ( sum ) { case 21 : printf( "Blackjack!\n" ); break; case 20 : case 19 : case 18 : printf( "Stand!\n" ); break; default : printf( "Sum of cards is %d", sum ); break; } }

Decisions may have one or more paths of execution

Chained decision structures have only one path of execution.

Selection decision structures may have more than one path of execution.

Software development cycle

Design and test the program logic with pseudo code and flow charts before implementing it with source code.


Use extra printf() functions to help follow the program logic.


Remember to check for matching braces when replacing a simple statement with a compound statement.

Iteration using for loops

Pseudo code: setup variables start loop by initializing counter to zero repeat loop if counter is less than 3      obtain input card value      add input to sum increase counter by 1 and go to test condition display sum of all cards The break keyword immediately exits the current for loop. It is possible that the statements inside a for loop is never executed because the test condition was never successful. Be careful to avoid division by zero in test conditions.

Flowchart:

#include <stdio.h> int main( void ) { int card = 0; int sum = 0; int counter; for ( counter = 0; counter < 3; counter++ ) { scanf( "%d", &card ); sum = sum + card; } printf( "Sum of cards is %d", sum ); }

Iteration using while loops

Pseudo code: setup variables start loop by initializing counter to zero repeat loop if counter is less than 3      obtain input card value      add input to sum      increase counter by 1      (remember this step to avoid infinite loops) display sum of all cards The break keyword immediately exits the current while loop. It is possible that the statements inside a while loop is never executed because the test condition was never successful. Be careful to avoid division by zero in test conditions.

Flowchart:

#include <stdio.h> int main( void ) { int card = 0; int sum = 0; int counter = 0; while ( counter < 3 ) { scanf( "%d", &card ); sum = sum + card; counter++; } printf( "Sum of cards is %d", sum ); }

Iteration using do / while loops

Pseudo code: setup variables initialize counter to zero start loop      obtain input card value      add input to sum      increase counter by 1      (remember this step to avoid infinite loops) repeat loop if counter is less than 3 display sum of all cards The statements inside a do / while loop always execute at least once because the body of the loop executes before it reaches the test condition. The break keyword immediately exits the current do / while loop. Be careful to avoid division by zero in test conditions.

Flowchart:

#include <stdio.h> int main( void ) { int card = 0; int sum = 0; int counter = 0; do { scanf( "%d", &card ); sum = sum + card; counter++; } while ( counter < 3 ); printf( "Sum of cards is %d", sum ); }

Dry run table during the software development cycle

A dry run is a manual step by step trace of the program logic and algorithm. It uses a table where the first column is the step number and one column is added for each data variable used in the program. Each statement in the program increases the dry run table by one step. Using a dry run table is useful for testing and finding errors.

Pseudo code: setup variables initialize counter to zero start loop      obtain input card value      add input to sum      increase counter by 1 repeat loop if counter is less than 3 display sum of all cards
Source code: #include <stdio.h>   int main( void ) {   int card = 0;   int sum = 0;   int counter = 0;     do   {     scanf( "%d", &card );     sum = sum + card;     counter++;   } while ( counter < 3 );     printf( "Sum of cards is %d", sum ); }

Counter-controlled loops compared to sentinel-controlled loops

Counter-controlled loop is designed to repeat for a defined number of iterations. Pseudo code: setup variables start loop by initializing counter to zero repeat loop if counter is less than 3      obtain input card value      add input to sum      increase counter by 1 display sum of all cards

Sentinel-controlled loop is designed to repeat for a unknown number of iterations. Pseudo code: setup variables repeat loop if card value is less than 15      add input to sum      obtain input card value display sum of all cards Sentinel-controlled loops are widely used for event-based programming. For example: repeat loop if player is sleeping      increase health repeat loop if accelerator is pressed down      increase speed repeat loop if player is walking      move forward by 1

Counter-controlled loops compared to sentinel-controlled loops

Be careful of infinite loops when writing test conditions to use with sentinel-controlled loops.

Counter-controlled loop is designed to repeat for a defined number of iterations. Pseudo code: setup variables start loop by initializing counter to zero repeat loop if counter is less than 3      obtain input card value      add input to sum      increase counter by 1 display sum of all cards Source code: #include <stdio.h>   int main( void ) {    int card = 0;    int sum = 0;    int counter = 0;      while ( counter < 3 )    {       scanf( "%d", &card );       sum = sum + card;       counter++;    }      printf( "Sum of cards is %d", sum ); }

Sentinel-controlled loop is designed to repeat for a unknown number of iterations. Pseudo code: setup variables repeat loop if card value is less than 15      add input to sum      obtain input card value display sum of all cards Source code: #include <stdio.h>   int main( void ) {    int card = 0;    int sum = 0;      while ( card < 15 )    {       sum = sum + card;       scanf( "%d", &card );    }      printf( "Sum of cards is %d", sum ); }

Review questions

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Control structures

What is the output of the program below?

A logic error occurs.
"Three" is displayed to output.
A runtime error occurs.
"Four" is displayed to output.

Review questions

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Control structures

The program below would display C to output for which range of input values?

step <= 100
100 <= step <= 200
0 < step < 100
step <= 0

Review questions

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Control structures

How many times will the body of the loop repeat in the program below?

49
Once
None
50

Review questions

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Control structures

What is the best description of the variable num1 in the program below?

Sum of all positive input values
Sum of all negative input values
Count of all negative input values
Count of all positive input values