Repetition in the programs •Example 1: Suppose we want to display the consecutive digits: 0,1,2,….9. For doing that adding one to the previous number should be repeated 10 times.• Example 2: Mars Observer spacecraft’s mission plan called for seven orbit adjustment maneuvers in order to move from initial orbit to the desired orbit for mapping Mars. The spacecraft would have needed to execute similar sets of instructions seven times, one set for each maneuver.

Repetition in the programs

The questions that determined whether loops are required in the algorithm:

1. Are there any repeated steps in the algorithm?

2. If repetition is required, how many times these steps should be repeated?

3. If the number of repetition is unknown, how long the repeating the steps should be kept?

The answer to question 1 indicates whether algorithm needs loop or not, and the answers to questions 2 and 3 determine which loop structure need to be used

Counting loop (counter-controlled loop)

The repetition is managed by a loop control variable whose value represents a count

Set loop control variable to an initial value (0)

while loop control variable < final value

……

increase loop control variable by increment value (1)

/* Loop to Process Seven Spacecraft Maneuvers */count_mvr = 0; /* no maneuvers done yet */while (count_mvr < 7) { /* test value of count_mvr */ printf("Number of orbits to next maneuver> "); scanf("%d", &orbits); printf("Orbit period, in hours and fraction of hours> "); scanf("%lf", &period); time to mvr = orbits * period; printf("Time to next maneuver is %6.1f hours\n", time to

mvr); count_mvr = count_mvr + 1; /* increment count_mvr */}printf("All maneuvers complete\n");

…….. increase count_mvr by 1

count_mvr <7 true

false

Exit loop

Flowchart of Flowchart of While While statementstatement

count_mvr = 0

Counting loop and while statement

Syntax of while statement: while (loop repetition condition)

statement

Three steps for a loop with a control variable are:1. Initialization of loop control variable (if missing loop

may repeat 0 or a wrong number of times)2. Testing loop control variable (loop will be executed

zero times if the condition is initially false)3. Updating loop control variable (if missing the loop will

execute forever, which is referred to as infinite loop)

Computing a Sum in the Loop printf("Enter number of maneuvers> "); scanf("%d", &num_mvrs); total_time = 0.0; count_mvr = 0; while (count_mvr < num_mvrs) { loop can be repeated for any number printf("Number of orbits to next maneuver> "); scanf("%d", &orbits); printf("Orbit period, in hours and fraction of hours> "); scanf("%lf", &period); time_to_mvr = orbits * period; printf("Time to next maneuver is %6.1f hours\n\n", time_to_mvr); count_mvr = count_mvr + 1; total_time = total_time + time_to_mvr; accumulator increases with

each loop iteration } printf("All maneuvers complete\n"); printf("Total time spent in maneuver phase is %6.1f hours\n", total_time); printing the value of accumulator

accumulator variable

Computing a Product in a Loop

The loop control variable holds the result of the computation that is performed in the loop

product = 1;

while (product < 10000) {

printf(“%d\n”, product); /* Display product so far */

printf(“Enter next item> “);

scanf(“%d”, &item);

product = product * item; /* Update product */

}

Compound Assignment Operators

Instead of variable = variable op expression;variable op= expression; can be used for +, -, *, /, and % operators. For example:ct = ct + 1; ct += 1;time = time – 1; time -= 1;product = product * data; product *=

data;n = n * (x + 1); n *= x+1;

The for Statement

• Syntax:for (initialization expression;

loop repetition condition; update expression)statement

Example:for (count_star = 0; count_star < 20; count_star+=1 )

printf(“*”);

The for Statement

• Combines the three loop control steps of initialization, testing and update in one place (more complex than while)

• Provides a consistent location for each of loop initialization, testing and update (more popular that while among the C programmers in industry)

• Can be used to count up or down by any interval

for (count_mvr = 0; /* initialization */ count_mvr < num_mvrs; /* loop condition*/ count_mvr += 1) { /* update */ printf("Number of orbits to next maneuver> "); scanf("%d", &orbits); printf("Orbit period, in hours and fraction of hours>"); scanf("%lf", &period); time_to_mvr = orbits * period; printf("Time to next maneuver is %6.1f hours\n\n", time_to_mvr); total_time += time_to_mvr; }

#include <stdio.h>intmain(void){ int time, start; printf("Enter starting time (an integer) in seconds> "); scanf("%d", &start); printf("\nBegin countdown\n"); for (time = start; time > 0; time -= 1) { printf("T - %d\n", time); } printf("Blast-off!\n"); return (0);}• Enter starting time (an integer) in seconds> 5Begin countdownT - 5T - 4T - 3T - 2T - 1Blast-off!

/* Conversion of Celsius to Fahrenheit temperatures */#include <stdio.h>/* Constant macros */#define CBEGIN 10#define CLIMIT -5#define CSTEP 5intmain(void){ /* Variable declarations */ int celsius; double fahrenheit; /* Display the table heading */ printf(" Celsius Fahrenheit\n");

/* Display the table */ for (celsius = CBEGIN; celsius >= CLIMIT; celsius -= CSTEP) { fahrenheit = 1.8 * celsius + 32.0; printf(" %3d %9.2f\n", celsius, fahrenheit); } return (0);} Celsius Fahrenheit 10 50.00 5 41.00 0 32.00 -5 23.00

Increment and Decrement operators

Suppose i =2 and j = ?• Prefix form: j= ++i; increments i and then use it => i=3,j=3• Postfix form: j= i++; uses i and then increment it => i=3, j=2Example : for n=4: printf ( %d,--n) => 3 printf ( %d,n--) => 4Self-Check 5.4.4: for i = 3 and j = 9 what are the results ofn = ++i * --j;m = i + j--;p = i + j;

Conditional loops with for statement

printf("Number of barrels currently in tank> "); scanf("%lf", &start_supply); for (current = start_supply; current >= min_supply; current -= remov_brls) { printf("%.2f barrels are available.\n\n", current); printf("Enter number of gallons removed> "); scanf("%lf", &remov_gals); remov_brls = remov_gals / GALS_PER_BRL; printf("After removal of %.2f gallons (%.2f barrels),\n", remov_gals, remov_brls); }

Sentinel-Controlled Loop

Correct Sentinel loop Incorrect Sentinel loop

1. sum=0 sum=0

2. get score while (score!=SENTINEL)

3. while (score! {

=SENTINEL){ get score

4 . sum+=score sum+=score

5. Get score } }

Sentinel-Controlled Loop/* Compute the sum of a list of exam scores. */#include <stdio.h>#define SENTINEL -99intmain(void){ int sum = 0, /* sum of scores input so far */ score; /* current score */ printf("Enter first score (or %d to quit)> ", SENTINEL); for (scanf("%d", &score); What is the result if score entered as 7o? score != SENTINEL; scanf("%d", &score)) { sum += score; printf("Enter next score (%d to quit)> ", SENTINEL); } printf("\nSum of exam scores is %d\n", sum); return (0);}

/* Batch Version of Sum of Exam Scores Program */#include <stdio.h> /* defines fopen, fclose, fscanf, fprintf, and EOF */intmain(void){ FILE *inp; /* input file pointer */ int sum = 0, /* sum of scores input so far */ score, /* current score */ input_status; /* status value returned by fscanf */ inp = fopen("scores.dat", "r"); printf("Scores\n"); for (input_status = fscanf(inp, "%d", &score); input_status != EOF; input_status = fscanf(inp, "%d", &score)) { printf("%5d\n", score); sum += score; } printf("\nSum of exam scores is %d\n", sum); fclose(inp); return (0);}

Avoiding Infinite Loops on Faulty Data • To avoid infinite loop when processing faulty data the loop in the

“Sum exam score” program can be modified as the follows:…… for (input_status = fscanf(inp, "%d", &score); input_status == 1; input_status = fscanf(inp, "%d", &score)) { printf("%5d\n", score); sum += score; }if (input_status == EOF) printf(“Sum of scores is %d\n”, sum);else printf(“Error in input, bad character”); ……• Note that in case of reaching EOF input_status is negative and in

case of faulty data input_status is zero otherwise it is always 1.

/* Finds the product of nonzero data. */#include <stdio.h>#define SENT -9999.0intmain(void){ double product = 1.0; /* product so far of nonzero data */ int status; /* status of input operation */ double data_item; /* current data value */ printf("Enter a data item (%.1f to quit)> ", SENT); for (status = scanf("%lf", &data_item); status == 1 && data_item != SENT; status = scanf("%lf", &data_item)) { if (data_item != 0) /* if statement is nested within the loop */ product *= data_item; printf("Enter next data item (%.1f to quit)> ", SENT); } if (status != 1) printf("Error in data list\n"); else /* The result will be shown for the correct input data */ printf("The product of nonzero data is %8.3f.\n", product); return (0);}

Nested Loops

#include <stdio.h>intmain(void){ int i, j; /* loop control variables */ printf(" I J\n"); /* prints column labels */ Program output for (i = 1; i < 4; ++i) { /* outer for loop */ printf("Outer %6d\n", i); for (j = 0; j < i; ++j) { /* inner loop */ printf(" Inner%9d\n", j); } /* end of inner loop */ } /* end of outer loop */ return (0);}

I JOuter 1 Inner 0Outer 2 Inner 0 Inner 1Outer 3 Inner 0 Inner 1 Inner 2

The do-while Statement

• The first iteration of the loop body occurs unconditionally (number of repetition is 1 or more instead of 0 or more in the while statement)

• All the statements in the loop are processed identically

• Can be used efficiently for checking the valid input data

The do-while Statement

• SYNTAX:

do

statement

while ( loop repetition condition)• Example

do /* Finding the first even number input */

status = scanf(“%d” &num);

while (status >0 && num %2 !=0);

Flowchart of do-while when using for

input validation

value not in desired range

false

prompt user to get a value forchecking valid input

true

/* Gives an error message on input of an invalid data type */printf("Enter minimum and maximum valid values> "); scanf("%d%d", &n_min, &n_max); do { printf("Enter an integer in the range from %d to %d inclusive> ", n_min, n_max); status = scanf("%d", &inval); if (status == 1) { error = 0; } else { error = 1; scanf("%c", &skip_ch); printf("\nInvalid character>>%c>> Skipping rest of line.\n", skip_ch); do { scanf("%c", &skip_ch); } while (skip_ch != '\n'); } /* Exit the loop only if the entry is valid */ } while (error || inval < n_min || inval > n_max);

do-while When Using for Input Validation

Flag_conrolled Validation Loop do { error = 0; /* No errors detected yet */ /* Get a fraction from the user */ printf("Enter a common fraction as two integers separated by "); printf("a slash\nand press <enter>\n> "); status = scanf("%d%c%d", &num, &slash, &den); /* Validate the fraction */ if (status < 3) { error = 1; printf("Input invalid-please read directions carefully\n"); } else if (slash != '/') { error = 1; printf("Input invalid-separate numerator and denominator"); printf(" by a slash (/)\n"); } else if (den <= 0) { error = 1; printf("Input invalid-denominator must be positive\n"); }do {/* Discard extra input characters */ scanf("%c", &discard); } while (discard != '\n'); } while (error); A flag (type int variable) is used as a condition of the while

Case Study: Computing Radiation Level

• Problem: ..Determine how long does it take before the radiation is down to a safe level of 0.466 millirem a day. Use a chart to display radiation level for every three days with the message unsafe and safe after every line. The program should stop just before the radiation level is one-tenth of the safe level.

• Analysis: data requirements:

Constants (SAFE_RAD, SAFETY_FACT),

Input (init_radiation), Output (day, radiation_lev)

Case study: Computing Radiation Level

Design :Algorithm 1. Initialize day to zero.2. Compute the stopping level3. Prompt user to enter the initial radiation level4. Compute day number and radiation level every 3 days

for the levels that exceed the stopping radiation level4.1 Initialize radiation_lev to init_radiation4.2 while radiation_lev exceeds min_radiation

4.3 Display the value of day, radiation_lev, and the string “Unsafe” or “Safe”

4.4 Add 3 to the value of day4.5 Compute radiation_lev for the next

period

Can we Change itto the for loop?

Case study: Computing Radiation Level • Implementation:............................./* Prompts user to enter initial radiation level */ printf("Enter the radiation level (in millirems)> "); scanf("%lf", &init_radiation); /* Displays table */ printf("\n Day Radiation Status\n (millirems)\n"); for (radiation_lev = init_radiation; radiation_lev > min_radiation; radiation_lev /= 2.0) { if (radiation_lev > SAFE_RAD) printf(" %3d %9.4f Unsafe\n", day, radiation_lev); else printf(" %3d %9.4f Safe\n", day, radiation_lev); day += 3; } return (0);…………………..

Case study: Computing Radiation Level

• Testing : Program result:Enter the radiation level (in millirems)> 150.0Day Radiation Status (millirems) 0 150.0000 Unsafe Initial value 3 75.0000 Unsafe 75 is 150 / 2, it shows calculation is ok 6 37.5000 Unsafe 9 18.7500 Unsafe The number of days increases by 3 12 9.3750 Unsafe 15 4.6875 Unsafe 18 2.3438 Unsafe 21 1.1719 Unsafe 24 0.5859 Unsafe 27 0.2930 Safe 30 0.1465 Safe Ending value, shows the loop exited correctly

How to Debug and Test Programs

• Using debugger programs: Quincy debugger• Debugging without a Debugger

1. Diagnostic call to printf

#define DEBUG 1

if (DEBUG)

printf(“***score is %d, sum is %d\n”,score,sum) ;

fflush(stdout); /* copying the contents of buffer to associated output device */

Debugging without a Debugger

2- Off-by-One loop errors:• Checking the loop repetition. For example:for (count =0; count <=n ;++count)

sum+=count /*executes n+1 times*/for (count =0; count <n ;++count)

sum+=count /*executes n times */• Checking the boundaries: initial value, end

value and tracing loop with a small loop control variable

Common Programming Errors

• Confusion between if , while and for• Forgetting to put braces for more than two

statements in the loop body• Making mistake when using tests for inequality

to control loop repetition:

while (balance !=0.0) instead of

while (balance>0.0)

The first while, repeats for negative balance

Common Programming Errors

• Using = instead of == in the loop repetition condition produces infinite loops:

do { …. } while (again=1);• do-while can be used when there is no possibility

of zero loop• Confusion in compound assignment operators: a *= b + c is a = a * (b+ c) it is not a = a * b + c• Confusion in increment and decrement operators

by ignoring operator’s side effect of changing the value of the operand