Transcript Slide 1
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 for each maneuver.
seven times , one set A.Abhari CPS125 1
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 A.Abhari CPS125 2
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)
A.Abhari CPS125 3
/* Loop to Process Seven Spacecraft Maneuvers */ count_mvr = 0; while (count_mvr < 7) /* no maneuvers done yet */ { /* 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"); A.Abhari CPS125 4
count_mvr = 0
Flowchart of While statement
count_mvr <7 true false Exit loop …….. increase count_mvr by 1 A.Abhari CPS125 5
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) A.Abhari CPS125 6
Computing a Sum in the Loop
printf("Enter number of maneuvers> "); scanf("%d", &num_mvrs ); total_time = 0.0; accumulator variable 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 A.Abhari CPS125 7
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 */ A.Abhari CPS125 8
Compound Assignment Operators
Instead of
variable = variable
op
expression; variable
op=
expression; can be used for
+, -, *, /, and % operators. For example: ct = ct + 1; time = time – 1; product = product * data; n = n * (x + 1); ct += 1; time -= 1; product *= data; n *= x+1; A.Abhari CPS125 9
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(“*”); A.Abhari CPS125 10
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 A.Abhari CPS125 11
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); time_to_mvr); total_time += time_to_mvr; */ 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", } A.Abhari CPS125 12
#include
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/* Conversion of Celsius to Fahrenheit temperatures */ #include
/* 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 10 5 Fahrenheit 50.00
41.00
0 -5 32.00
23.00
A.Abhari CPS125 15
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=2 Example : for n=4: printf ( %d,--n) => 3 printf ( %d,n--) => 4 Self-Check 5.4.4: for i = 3 and j = 9 what are the results of n = ++i * --j; m = i + j--; p = i + j; A.Abhari CPS125 16
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); } A.Abhari CPS125 17
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 } } A.Abhari CPS125 18
Sentinel-Controlled Loop
} /* Compute the sum of a list of exam scores. */ #include
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); A.Abhari CPS125 19
} /* Batch Version of Sum of Exam Scores Program */ #include
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.
A.Abhari CPS125 21
} { /* Finds the product of nonzero data. */ #include
int main(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); A.Abhari CPS125 22
Nested Loops
} #include
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 A.Abhari CPS125 24
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); A.Abhari CPS125 25
Flowchart of do-while when using for input validation prompt user to get a value for checking valid input false value not in desire d range A.Abhari CPS125 true 26
do-while When Using for Input Validation
/* 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 */ A.Abhari CPS125 } while (error || inval < n_min || inval > n_max); 27
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
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) A.Abhari CPS125 29
Case study: Computing Radiation Level
1.
2.
Design :Algorithm Initialize day to zero.
Compute the stopping level 3.
Prompt user to enter the initial radiation level 4.
Compute day number and radiation level every 3 days for the levels that exceed the stopping radiation level 4.1 Initialize radiation_lev to init_radiation 4.2 while radiation_lev exceeds min_radiation Can we Change it to the for loop?
4.3 Display the value of day, radiation_lev, and the string “Unsafe” or “Safe” 4.4 Add 3 to the value of day 4.5 Compute radiation_lev for the next period A.Abhari CPS125 30
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); …………………..
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Case study: Computing Radiation Level
•
Testing
: Program result: Enter the radiation level (in millirems)> 150.0
Day Radiation Status (millirems) 0 150.0000
3 75.0000 Unsafe Initial value 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 A.Abhari CPS125 correctly 32
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 */ A.Abhari CPS125 33
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 • 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 A.Abhari CPS125 35 • 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 A.Abhari CPS125 36Common Programming Errors
Common Programming Errors