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Using for loops in C

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The need for loops

While solving certain programming problems, some blocks of code need to be executed repeatedly. The repetition of the same code until a particular condition is met justifies the need of for loops as a whole.

Syntax of a for loop

The for loop is a special kind of loop that only repeats the block of code it contains for a specific number of times. The number of repetitions is defined by a conditional statement within the loop. Let’s look at the syntax for declaring a loop to better understand how this works.

for(controlVariable; condition; changingControl){
//body of for loop
}

Now let’s look at each section of the for loop:

  • The controlVariable is the variable defined that will be used to initialize the loop counter i.e. the number of times the loop will run.

  • The condition is the statement on the basis of which the loop will be evaluated. Given that the statement returns false, the loop will no longer run.

  • The changingControl is the statement that will either increment, decrement or change the controlVariable for the next loop run.

  • The curly braces, {}, contain the code block that is to be executed.

How does a for loop work?

The illustration below shows the life cycle of the for loop and how it evaluates statements to run itself

Now that we know why we need a for loop and how it works, lets look at an example to re-inforce what we have learnt!

Applying theory to example

#include <stdio.h>
int main() {
// your code goes here
int sum =0;
for(int a=0;a<10;a++){
sum= sum+a;
printf("%d\n",sum);
}
}
Within the for loop declaration in ( ), each part is separated by a ;

In the code above, let’s see which part is what:

  • Here the controlVariable is the int a = 0 declaration which defines the initializer value for the for loop.
  • The condition in this case is the a < 10 statement; if a is less than 5 the loop will continue to run, otherwise it will terminate.
  • changingControl is the expression a++ indicating that before the next run of the loop, increase the value of a by 1.
  • The body of the loop to be executed each time is the block of code that adds a to sum and then prints the value of sum.
  • This entire process of checking the condition and subsequent actions is repeated once the code body executes.

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Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

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