Module 17 · Interactive lesson

for Loop

The counting loop

The for loop is the loop you will use most. It puts the three parts every loop needs — start, condition and step — on one line, so a counting loop is easy to read. In the lab below you watch the three parts light up in the order C runs them, add and multiply with a loop, and try four variations.

  • ①start — runs once
  • ②condition — before every round
  • ③step — after every round
for tracer Ready
main.c 

                

                
            


            

                

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                        Statements run 0
                        Lines printed 0
                    

                

            


            

                

                    
                    
                    
                    
                


                

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            Keyboard: ← → step · Space play / pause ·
            R reset · Esc exit classroom mode
        

    







    

        

            

                How it works
                
The ideas behind the lab

                
Exactly what you watched happen, in plain steps.

            

        


                    

                

                    
Three parts in one line

                    
A for loop has three parts in its parentheses, separated by semicolons: ① the start (runs once), ② the condition (checked before every round), ③ the step (runs after every round). The order is ① ② body ③ ② body ③ … until ② is false.

                

                
for (int i = 1;   i <= 5;   i++) {
//    ① start      ② test    ③ step
    printf("%d\n", i);      // body
}

// order:  ①  ② body ③  ② body ③  …  ② (false) → done

            

        
        
                    

                

                    The same loop written with for and with while
                    
                        

                                                            Part
                                                            for
                                                            while
                                                    

                    
                    
                                                    

                                                                    ① start
                                                                    for (int i = 1; …; …)
                                                                    int i = 1;   (before the loop)
                                                            

                                                    

                                                                    ② condition
                                                                    for (…; i <= 5; …)
                                                                    while (i <= 5)
                                                            

                                                    

                                                                    ③ step
                                                                    for (…; …; i++)
                                                                    i++;   (last line of the body)
                                                            

                                                    

                                                                    Best for
                                                                    a known number of rounds (counting)
                                                                    an unknown number of rounds (until something happens)
                                                            

                                            
                

            

        
        

                            

                    
How for works

                    
    
                                                    
  1. ① The start part runs ONCE, before anything else.
    2. 
                                                    
  2. ② The condition is checked before every round — false means the loop is over.
    3. 
                                                    
  3. The body runs.
    4. 
                                                    
  4. ③ The step part runs after the body, then back to ②.
    5. 
                                                    
  5. If ② is false at the start, the body runs 0 times.
    6. 
                                            

                

                            

                    
Counting patterns

                    
    
                                                    
  1. Up:      for (i = 1; i <= n; i++)
    2. 
                                                    
  2. Down:    for (i = n; i >= 1; i--)
    3. 
                                                    
  3. In twos: for (i = 0; i <= 10; i += 2)
    4. 
                                                    
  4. n times: for (i = 0; i < n; i++)   — the most common form
    5. 
                                                    
  5. Sum: start at 0 and add; product: start at 1 and multiply.
    6. 
                                            

                

                            

                    
Details

                    
    
                                                    
  1. A variable declared in the header (int i = 0) exists only inside the loop.
    2. 
                                                    
  2. Each part may contain several expressions with commas: i = 0, j = 10.
    3. 
                                                    
  3. Every part may be left out; for (;;) loops for ever.
    4. 
                                                    
  4. Do not change the loop variable inside the body as well — it confuses the count.
    5. 
                                                    
  5. Separate the parts with semicolons, not commas.
    6. 
                                            

                

                    

    







    

        

            

                Walkthrough
                
A multiplication table and powers of 2

                
Every line, explained.

            

        


        

                            

                    1
                    #include <stdio.h>
                                    

                            

                    2
                    
                                    

                            

                    3
                    int main(void)
                                    

                            

                    4
                    {
                                    

                            

                    5
                        int n = 7;
                                    

                            

                    6
                        for (int i = 1; i <= 10; i++)
                                            i takes 1 … 10: ten rounds.
                                    

                            

                    7
                            printf("%d x %d = %d\n", n, i, n * i);
                                            Prints 7 x 1 = 7 … 7 x 10 = 70.
                                    

                            

                    8
                        int power = 1;
                                            A running product starts at 1.
                                    

                            

                    9
                        for (int k = 0; k < 8; k++) {
                                            k takes 0 … 7: eight rounds (k < 8 is the usual "n times" form).
                                    

                            

                    10
                            printf("2^%d = %d\n", k, power);
                                            Prints 2^0 = 1 … 2^7 = 128.
                                    

                            

                    11
                            power *= 2;
                                            Double it for the next round.
                                    

                            

                    12
                        }
                                    

                            

                    13
                        return 0;
                                    

                            

                    14
                    }
                                    

                    

    







    

        

            

                Common mistakes
                
Spot the bug

                
Mistakes almost every beginner makes — and how to fix them.

            

        


        

                            

                    
A semicolon after for ( … )

                    

                        

                            ✗ Wrong
                            
for (i = 0; i < 3; i++);
    printf("%d\n", i);

                        

                        

                            ✓ Right
                            
for (i = 0; i < 3; i++)
    printf("%d\n", i);

                        

                    

                    
The ; is the body. The loop does nothing three times, then printf runs once and prints 3.

                

                            

                    
Commas instead of semicolons

                    

                        

                            ✗ Wrong
                            
for (i = 0, i < 5, i++)

                        

                        

                            ✓ Right
                            
for (i = 0; i < 5; i++)

                        

                    

                    
The three parts are separated by semicolons. With commas the compiler says: expected ';' before ')'.

                

                            

                    
One round too many

                    

                        

                            ✗ Wrong
                            
// print "Hi" 10 times
for (i = 0; i <= 10; i++)

                        

                        

                            ✓ Right
                            
// print "Hi" 10 times
for (i = 0; i < 10; i++)

                        

                    

                    
0 to 10 inclusive is 11 values. Start at 0 with <, or start at 1 with <=.

                

                            

                    
Using the loop variable after the loop

                    

                        

                            ✗ Wrong
                            
for (int i = 0; i < 3; i++) { … }
printf("%d", i);

                        

                        

                            ✓ Right
                            
int i;
for (i = 0; i < 3; i++) { … }
printf("%d", i);

                        

                    

                    
A variable declared in the for header only exists inside the loop: 'i' undeclared.

                

                            

                    
Changing i in the body too

                    

                        

                            ✗ Wrong
                            
for (i = 0; i < 10; i++) {
    printf("%d ", i);
    i++;
}

                        

                        

                            ✓ Right
                            
for (i = 0; i < 10; i += 2)
    printf("%d ", i);

                        

                    

                    
i is increased twice per round, which is easy to miss. Put the whole step in part ③.

                

                    

    







    

        

            

                Practice
                
Check your understanding

                
Answer each question — you will see the explanation straight away.

            

            0 / 5 correct
        


        

                            

                    Question 1
                    
for (int i = 0; i < 4; i++) printf("%d", i); — output?

                    

                                                    
                                                    
                                                    
                                                    
                                            

                    
                

                            

                    Question 2
                    
How many times does the start part ① of a for run?

                    

                                                    
                                                    
                                                    
                                                    
                                            

                    
                

                            

                    Question 3
                    
for (int i = 10; i > 0; i -= 3) printf("%d ", i); — output?

                    

                                                    
                                                    
                                                    
                                                    
                                            

                    
                

                            

                    Question 4
                    
Which loop runs exactly 5 times?

                    

                                                    
                                                    
                                                    
                                                    
                                            

                    
                

                            

                    Question 5
                    
int s = 0; for (int i = 1; i <= 4; i++) s += i; — s is?

                    

                                                    
                                                    
                                                    
                                                    
                                            

                    
                

                    

    







    

        

            

                Assessment
                
for Loop quiz

                

                                            Test yourself: 10 questions, 70% to pass. Create a free account to take the quiz,
                        save your progress and work towards the certificate.
                                    

            

                            

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                    while Loop
                
            
                            
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                    do-while Loop