CODEWITHSUNDEEP
c++for-loop
M.Ionut
M.Ionut

Reputation: 187

How does for act when it receives no commands?

I wrote a for-loop without any instructions(except for the ones within the for syntax). Now, when I use a semi-colon immediately after the ending for bracket, the i variable stops at the value 4, as expected; whereas if i do not use a semi-colon, it stops at value 6. How come?

#include <iostream>

using namespace std;

int main()
{
    int x=0;
    int i;
    for(i=0;i<=3;i++) //if the semicolon is absent here, value of x is 6
                      //if present, value of x is 4, as expected
    x=x+i;
    cout<<"x="<<x;
    return 0;
}

Upvotes: 0

Views: 41

Answers (4)

Caleth
Caleth

Reputation: 62769

I wrote a for-loop without any instructions

No you didn't. It had one that you intended to be after the loop. A for loop always has a statement.

Now, when I use a semi-colon immediately after the ending for bracket, the i variable stops at the value 4, as expected;

;, on it's own, is a statement. So is {}

whereas if i do not use a semi-colon, it stops at value 6. How come?

Because it evaluates x = x + i as the loop body.

Unlike some other languages, whitespace does not delimit blocks. You can have misleading indentation. Applying an autoformatter would give you something like

int main()
{
    int x=0;
    int i;
    for(i=0;i<=3;i++)
        x=x+i;
    cout<<"x="<<x;
    return 0;
}

Upvotes: 1

Daniel Langr
Daniel Langr

Reputation: 23497

With a semicolon, your code is effectively the same as

int x = 0;
int i;
i = 4;
x = x + i; // x = 0 + 4;

since the for loop is equivalent to

for (i = 0; i <= 3; i++) { } // sets i to 4

Without semicolon:

int x = 0;
int i;
i = 0; x = x + i; // x = 0 + 0;
i = 1; x = x + i; // x = 0 + 1;
i = 2; x = x + i; // x = 1 + 2;
i = 3; x = x + i; // x = 3 + 3;

being for loop equivalent to:

for (i = 0; i <= 3; i++) { x = x + i; }

Read some good C++ book for beginners to learn more about the language syntax.

Upvotes: 2

IceFire
IceFire

Reputation: 4137

Without semicolon, your code is equivalent to

for(i = 0; i <= 3; i++)
{
    x = x + i;
}

With semicolon, it is a one-liner and x=x+i is never called.

This is why in some code style guides, you are asked to always use brackets with for.

Upvotes: 1

Blaze
Blaze

Reputation: 16876

This here

for(i=0;i<=3;i++)
x=x+i;

Is actually the same as this:

for(i=0;i<=3;i++){
    x=x+i;
}

The for always needs a statement after it, else it won't compile. To make it clear that the second line is the statement of the loop, it is often indented this way:

for(i=0;i<=3;i++)
    x=x+i;

With the semicolon the code is equivalent to this:

for(i=0;i<=3;i++){
    ; // does nothing, a so called "null statement"
}
x=x+i; // always happens once

Upvotes: 1

Related Questions