1. Repetition and Loop Statements
Chapter 5
Repetition and Loop Statements
Instructor: Yuksel & Demirer

2. Repetition in Programs
In most software, the statements in the program may need to repeat for many times.
e.g., calculate the value of n!.
If n = 10000, it’s not elegant to write the code as 1*2*3*…*10000.
Loop is a control structure that repeats a group of steps in a program.
Loop body stands for the repeated statements.
There are three C loop control statements:
while, for, and do-while.
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3. Flow Diagram of Loop Choice Process
e.g., read the content in a file
e.g., calculate the value of n!
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4. Comparison of Loop Choices (1/2)
Kind
When to Use
C Structure
Counting loop
We know how many loop repetitions will be needed in advance.
while, for
Sentinel-controlled loop
Input of a list of data ended by a special value
Endfile-controlled loop
Input of a list of data from a data file
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5. Comparison of Loop Choices (2/2)
Kind
When to Use
C Structure
Input validation loop
Repeated interactive input of a value until a desired value is entered.
do-while
General conditional loop
Repeated processing of data until a desired condition is met.
while, for
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6. The while Statement in C
The syntax of while statement in C:
while (loop repetition condition)
statement
Loop repetition condition is the condition which controls the loop.
The statement is repeated as long as the loop repetition condition is true.
A loop is called an infinite loop if the loop repetition condition is always true.
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7. An Example of a while Loop
Loop repetition condition
Statement
Loop control variable is the variable whose value controls loop repetition.
In this example, count_emp is the loop control variable.
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8. Flowchart for a while Loop
Loop repetition condition
Statement
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9. Compound Assignment Operators (1/2)
The loop body usually consists of statements of the form: variable = variable op expression.
e.g., count_emp = count_emp + 1;
C provides compound assignment operators which enable a more concise notation for this kind of statements.
“variable op = expression” is the same to “variable = variable op expression.”
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10. Compound Assignment Operators (2/2)
Simple Assignment Operators
Compound Assignment Operators
count_emp = count_emp + 1;
count_emp += 1;
time = time -1;
time -= 1;
product = product * item;
product *= item;
total = total / number;
total /= number;
n = n % (x+1);
n %= x+1;
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11. The for Statement in C The syntax of for statement in C:
for (initialization expression;
loop repetition condition;
update expression)
statement
The initialization expression set the initial value of the loop control variable.
The loop repetition condition test the value of the loop control variable.
The update expression update the loop control variable.
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12. An Example of the for Loop
Initialization Expression
Loop repetition condition
Update Expression
count_emp is set to 0 initially.
count_emp should not exceed the value of number_emp.
count_emp is increased by one after each iteration.
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13. Increment and Decrement Operators
The statements of increment and decrement are commonly used in the for loop.
The increment (i.e., ++) or decrement (i.e., --) operators are the frequently used operators which take only one operand.
The increment/decrement operators increase or decrease the value of the single operand.
e.g., for (int i = 0; i < 100; i++){ … }
The variable i increases one after each iteration.
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14. Comparison of Prefix and Postfix Increments
The value of the expression (that uses the ++/-- operators) depends on the position of the operator.
The value of j is not
increased
The value of j is increased
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15. Sentinel-Controlled Loops
Sometimes we may not know how many times the loop will repeat.
One way to do this is to choose a sentinel value as an end marker.
The loop exits when the sentinel value is read.
If the user wish to exit the loop, he or she has to input the sentinel value.
It is similar to the “logout” function in many applications.
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16. An Example of Sentinel-Controlled while Loops
If the user wish to exit the loop,
he or she has to input -99.
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17. An Example of Endfile-Controlled Loops
fscanf is a function used to read file.
EOF stands for the special value of end-file returned by fscanf.
This loop repeats until reading the end of the file.
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18. Nested Loops Usually used to work with two dimensional arrays (later).
Nested loops consist of an outer loop with or more inner loops.
Each time the outer loop is repeated, the inner loops are reentered
Their loop control expressions are reevaluated
All required iterations are performed again.
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19. Nested Loops
Nested loops consist of an outer loop with one or more inner loops.
e.g.,
for (i=1;i<=100;i++){
for(j=1;j<=50;j++){ … }
The above loop will run for 100*50 iterations.
Outer loop
Inner loop
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20. What is the Output? lcv1 = 0; sum = 0; while (lcv1 < 100) {
sum = sum + 1;
lcv2++; }
lcv1++;
printf(“Sum is %d\n”, sum);
/* Output = */
Sum is 10000
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21. What is the Output? int a=0, b=0, sum=0; for(a=0; a<6; a+=2)
for(b=0; b>4; b--)
sum=sum+1;
printf(“%d”,sum);
/* Output = */
Sum is 0
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22. Do While statement
Both the for statement and the while statement evaluate the loop condition before the first execution of the loop body.
In most cases, this pre-test is desirable and prevents the loop from executing when there may be no data items to process
There are some situations, generally involving interactive input, when we know that a loop must execute at least one time.
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23. The do-while Statement in C
The syntax of do-while statement in C:
do statement while (loop repetition condition);
The statement is first executed.
If the loop repetition condition is true, the statement is repeated.
Otherwise, the loop is exited.
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24. An Example of the do-while Loop
/* Find even number input */
do{ printf(“Enter a value: ”);
scanf(“%d”, &num);
}while (num % 2 !=0)
This loop will repeat if the user inputs odd number.
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25. Do-While Example do { printf("Enter a letter from A through E> ");
scanf("%c", &letter_choice);
}while (letter_choice < ’A’ || letter_choice > ’E’);
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26. Flag Controlled Loops
Sometimes a loop repetition condition becomes so complex that placing the full expression in its usual spot is awkward.
In many cases, the condition may be simplified by using a flag.
while (flag) { …. }
A flag is a type int variable used to represent whether or not a certain event has occurred.
A flag has one of two values: 1 (true) and 0 (false).
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27. Endfile-controlled loops
Many library functions return helpful data as the value of the function.
For example, scanf returns the number of data items it actually obtained.
num = scanf(“%d%d%d”, &n1, &n2, &n3);
A data file is always terminated by an endfile character (EOF) that can be detected by the scanf and fscanf functions.
You can write a program that processes a list of data of any length from a file without requiring a special sentinel value at the end of the data file.
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28. #include <stdio.h> int main(void) { FILE *inp;
int sum = 0, score, input_status;
inp = fopen("scores.dat", "r");
input_status = fscanf(inp, "%d", &score);
while (input_status != EOF) {
printf("%d ", score);
sum += score;
input_status = fscanf(inp, "%d", &score); }
printf("\nSum of exam scores is %d\n", sum);
fclose(inp);
return 0;
/* Output */
Sum of exam scores is 165
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29. What is the Output? int i, sum=0,status=0; status = scanf(“%d”,&i);
while(status!=EOF) {
sum+=i; }
printf(“%d\n”, sum);
/* Input = */ 2 4 6
ctl-d
/* Output = */ 12
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30. In class work (1/2)
Write a program that prompts the user to input an integer n.
Draw a triangle with n levels by star symbols. For example,
n = 3,
* ** ***
After drawing the triangle, repeat the above process until the user input a negative integer.
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31. In class work #4 (2/2) An usage scenario:
Please input: 2 * ** Please input: 3 * ** *** Please input: -9 Thank you for using this program.
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