Double For Loop syntax in C

Question

I am new to C and have to write some code that emulates a given function. However, I am having a difficult time understanding what the second for loop in this code is doing, explicitly. The syntax doesn't seem to follow standard for loop syntax of:

for ( init; condition; increment ) {
statement(s);
}

Here is the code I am examining and it's the second for loop that I am not following and I don't see any online version of this

for (i = 0; i <= (n1-n2); i++){   
    count=0;
    for(j = i,k = 0; k < n2; j++,k++){
        if (*(s1+j)!=*(s2+k)){
            break;
        }
        else
            count++;
        if(count==n2)    
            total++;                         
    }
}

I'm assuming it's two for loops in one, since there are two increments. And I think the inequality is similar to a 'while loop', but I'm not certain. The confusing piece is that there doesn't seem to be a condition for the j loop. I'm not sure if it's a syntax shortcut or if there is a special loop I can not find a resource for online.

k < n2 is the condition for the inner loop - there is no requirement that it has to use the variable j — UnholySheep
– UnholySheep, Commented Apr 22, 2017 at 16:18

Sourav Ghosh · Accepted Answer · 2017-04-22 16:24:32Z

6

I'm assuming it's two for loops in one, since there are two increments.

Well, not really. The second for loop also follows the prescribed syntax, it's just a little fancy about handling the loop related variables.

For a loop statement like

 for(j = i,k = 0; k < n2; j++,k++){

can be interpreted as

 for( (j = i,k = 0)  ; (k < n2) ; (j++,k++) ){
       ^^^^^^^^^^^      ^^^^^^     ^^^^^^^  
       init            condition    post loop body operation.

Now, the initialization and post-body expression blocks makes use of comma operator.

Related, quoting C11, chapter §6.5.17

The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value.

To elaborate, the same syntax can be broken down to

 j = i;                               //initialization, only once
 for( k = 0  ; k < n2 ; k++ ){        // good ol' syntax
  //......loop body
  j++;                                //last expression in the body.
 }                                    //end of loop

edited Apr 22, 2017 at 16:24

answered Apr 22, 2017 at 16:18

Sourav Ghosh

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8 Comments

Joseph Over a year ago

Ok that is fantastic. So the j is 'along for the ride' sort of speak. And the k loop acts as a 'while' loop, it counts from 0 to 1 less than n2. is that an appropriate analogy?

Sourav Ghosh Over a year ago

@Joseph yes, almost, adding to that, j is also increased alongside k after every loop body execution.

Joseph Over a year ago

So I ran the code and it appears the k loop resets on the k < n2 condition. The j loop continues counting until the 'break' condition is met in the if statement. Is that what should happen?

Sourav Ghosh Over a year ago

@Joseph what do you mean by the k loop and j loop?

Joseph Over a year ago

I need to get the right terminology - but i'm fairly certain I understand the syntax now. thank you!

|

haltode · Accepted Answer · 2017-04-22 16:19:37Z

3

You can do multiple initialization and increment in a single for loop exactly as in your example (separated with a comma).

for(first = 0, second = 0 ; your_condition ; ++first, ++second)
    ...

This is useful when you need multiple variables to be updated each time in your for loop.

Hope this helps!

answered Apr 22, 2017 at 16:19

haltode

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2 Comments

Joseph Over a year ago

So the your_condition is the same ending condition for both loops?

haltode Over a year ago

@Joseph this is not a "two for loop in one", this is a single for loop with multiple variables. Thus, the for loop only needs one condition. ;)

Dilip Kumar · Accepted Answer · 2017-04-22 16:22:33Z

0

for(j = i,k = 0; k < n2; j++,k++){
    initialisation  condition   increment or decrement

here they are considering two variables to keep track of.
Its not two for loops in a single for loop
the reason its done like that is to initialize and update variables each iteration.
Though you may have issues if you are using multiple type variables inside for loop for initialization

answered Apr 22, 2017 at 16:22

Dilip Kumar

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