C for, while and Loop Control
C for, while and Loop Control
A school needs room labels from one to three. Repeating the same print statement by hand makes longer sequences hard to maintain.
A loop repeats a body while its continuation rule holds. for places initialization, a condition and an update together; while checks before the body; do-while checks after at least one iteration.
Explain
Trace the initialization, test, body and update of a bounded loop.
Trace
Print rooms one to five but skip room three using continue.
Apply
Identify the stopping condition and avoid stepping beyond the last item.
Loops: follow the data
Start
A school needs room labels from one to three. Repeating the same print statement by hand makes longer sequences hard to maintain.
Operation
A loop repeats a body while its continuation rule holds. for places initialization, a condition and an update together; while checks before the body; do-while checks after at least one iteration.
Result
Room 1 Room 2 Room 3
Understand the operation
In for (int room = 1; room <= 3; room++), initialization runs once, the condition is checked before each body, and the update follows each completed body. The last failed condition stops the loop.
break leaves the nearest enclosing loop or switch. continue skips the remaining body and proceeds to the next iteration’s appropriate update or test. Ensure a while loop still changes the values that allow termination.
Read the complete C program
This complete C17 program uses a small school example. Read declarations before the statements that use their values. Each printf format describes how the corresponding value is written to standard output.
C17 · complete program
#include <stdio.h>
int main(void) {
for (int room = 1; room <= 3; room++) {
printf("Room %d\n", room);
}
return 0;
}
Expected output
Room 1
Room 2
Room 3
Follow the values through the code
C17 · code used in this explanation
#include <stdio.h>
int main(void) {
for (int room = 1; room <= 3; room++) {
printf("Room %d\n", room);
}
return 0;
}
room takes values 1, 2 and 3, each satisfying room <= 3.
After printing Room 3, the update makes room 4. The false condition stops the loop without printing a fourth line.
Follow the worked calculation
Read the starting values
room takes values 1, 2 and 3, each satisfying room <= 3.
Perform the operation
After printing Room 3, the update makes room 4. The false condition stops the loop without printing a fourth line.
Check the observable result
Room 1 Room 2 Room 3
Room 1 Room 2 Room 3
Try a small variation
C17 · code used in this explanation
#include <stdio.h>
int main(void) {
for (int room = 1; room <= 3; room++) {
printf("Room %d\n", room);
}
return 0;
}
Print rooms one to five but skip room three using continue.
Worked answer
continue skips printf for room three but still reaches the for loop’s update. The sequence advances to four and eventually terminates after five.
C17 · complete program
#include <stdio.h>
int main(void) {
for (int room = 1; room <= 5; room++) {
if (room == 3) continue;
printf("Room %d\n", room);
}
return 0;
}
Expected output
Room 1
Room 2
Room 4
Room 5
Recognize the important boundaries
Using <= count for an array indexed from zero accesses one element too many. Use an index strictly less than the element count.
An accidental semicolon after the loop header creates an empty body. A loop also needs a termination argument; increasing a signed counter forever is not safe.
Check your understanding
Choose the answer, then compare the reasoning for every option.
Use the idea independently
A loop repeats a body while its continuation rule holds. for places initialization, a condition and an update together; while checks before the body; do-while checks after at least one iteration.
Continue with indexed sequences.
Language reference: WG14 C11 committee draft. These examples use the C17-compatible subset of the explained language features.
Reaching the recap and completing the quizzes are separate milestones.