For vs. While
In C, loops are the fundamental part of language that are used to repeat a block of code multiple times. The two most commonly used loops are the for loop and the while loop. Although they achieve the same result, their structure, use cases, and flexibility differ.
The below table highlights some primary differences between the for and while loop in C:
| Feature | For Loop | While Loop |
|---|---|---|
| Purpose | Used when the number of iterations is known beforehand. | Used when the number of iterations is unknown and depends on a condition. |
| Structure | Combines initialization, condition, and increment/decrement in a single line. | Requires separate statements for initialization, condition, and increment/decrement. |
| Syntax | for (initialization; condition; update) { // statements.... } | while (condition) { // statements... } |
| Condition Check | The condition is checked before each iteration (entry-controlled). | The condition is checked before each iteration (entry-controlled). |
| Use Case | Ideal for counting or iterating over arrays, loops with fixed limits. | Ideal for loops with dynamic conditions, like reading input until a condition is met. |
When to Use Each Loop?
- For Loop: When you know the number of iterations in advance, such as iterating over an array or printing a specific number of lines.
- While Loop: When the number of iterations depends on a dynamic condition, such as prompting the user for valid input or running a process until a certain condition is met.
for Loop in C
The for loop is best suited for scenarios where the number of iterations is known or fixed beforehand. It's a structured loop that combines initialization, condition checking, and iteration in a single line as a part of its syntax.
Syntax
for (initialization; condition; updation) {
// Body of the loop
}
Let's take a look at an example that prints the text "GfG" 5 times.
#include <stdio.h>
int main() {
for (int i = 1; i <= 5; i++) {
printf("GfG\n");
}
return 0;
}
Output
GfG GfG GfG GfG GfG
while Loop in C
The while loop, on the other hand, is more flexible and is often used when the number of iterations is not known upfront. It only requires the condition to be checked before each iteration and will continue running as long as the condition evaluates to true.
Syntax
while (condition) {
// Body
}
Let's take a look at an example that prints the text "GfG" 5 times.
#include <stdio.h>
int main() {
int i = 1;
while (i <= 5) {
printf("%d ", i);
i++;
}
return 0;
}
Output
Loop Loop Loop Loop Loop
When for and while Differ?
After looking at these examples, one may wonder whether there is any case when the two loops do not work in the same way? The answer is YES. Due to the structural differences, if there are some jump statements in the code, for and while loop may execute in different way.
Let's take an example where a continue statement is present in the loop body. For look at for loop:
#include <stdio.h>
int main()
{
for (int i = 0; i < 5; i++) {
if (i == 2) {
continue;
}
printf("%d\n", i);
}
}
Output
0 1 3 4
Now look at while loop:
#include <stdio.h>
int main() {
int i = 0;
while (i < 5) {
if (i == 2) {
continue;
}
printf("%d\n", i);
i++;
}
}
Output
Time Limit Exceeded
The problem in the above while Loop Code is that, when the condition (i == 2) becomes true the continue statement gets executed, which will take you again to the beginning of the loop without incrementing i. Due to this, the process will continue infinitely which leads to “Time Limit Exceeded” as the output.
Now, to solve this problem we have to do a small correction in the code as shown below:
#include <stdio.h>
int main()
{
// code
int i = 0;
while (i < 5) {
if (i == 2) {
// Increment i before continue statement which
// avoids infinite loop due to unreachable updation
i++;
continue;
}
printf("%d\n", i);
i++;
}
return 0;
}
Output
0 1 3 4
