programming c++ trying to multiply same indexes of matrices

[OverclockingNub]

why am i getting an error in my for loop?

[dcgreen2k]

The error is saying that you didn't declare the variable i before using it. In the code shown in the error message, you're assigning a value to i but the compiler doesn't know what to do with it because you didn't give i a type (short, int, long, etc.).

 

You can fix the error by replacing i = 0 with int i = 0.

[igormp]

You forgot to declare the type of "i".

 

Do "for (int i = 0..." instead

[AbydosOne]

17 minutes ago, OverclockingNub said:

why am i getting an error in my for loop?

Also, it should be:

value3[i] = value1[i] * value2[i];

Variable you're assigning to goes first, followed by the values/operations.

 

Second, you're working with 2D arrays, so you need to go one more loop level deep (essentially, you're asking C to do multiply an array by an array, which I hope is an error, but may just return a garbage pointer...).

 

Possibly what you're looking for?:

for (int i = 0; i < 3; i++) {
	for (int j = 0; j < 3; j++) {
		value3[i][j] = value1[i][j]*value2[i][j];
	}
}

// Forgive my for-loop syntax, I literally never use them as I find the notation ugly and hard to follow compared to (do-)while-loops.

 

[OverclockingNub]

2 hours ago, AbydosOne said:

Also, it should be:

value3[i] = value1[i] * value2[i];

Variable you're assigning to goes first, followed by the values/operations.

 

Second, you're working with 2D arrays, so you need to go one more loop level deep (essentially, you're asking C to do multiply an array by an array, which I hope is an error, but may just return a garbage pointer...).

 

Possibly what you're looking for?:

for (int i = 0; i < 3; i++) {
	for (int j = 0; j < 3; j++) {
		value3[i][j] = value1[i][j]*value2[i][j];
	}
}

// Forgive my for-loop syntax, I literally never use them as I find the notation ugly and hard to follow compared to (do-)while-loops.

 

thank you, how do i fix this error then?

[AbydosOne]

1 minute ago, OverclockingNub said:

how do i fix this error then?

You need to initialize the array the same way you did the others. Values don't matter, zero is a decent option, but setting it to something that indicates the value wasn't calculated properly (like -1) is also valid.

[OverclockingNub]

4 minutes ago, AbydosOne said:

You need to initialize the array the same way you did the others. Values don't matter, zero is a decent option, but setting it to something that indicates the value wasn't calculated properly (like -1) is also valid.

i did that and im stlll getting the error?

[Sauron]

2 hours ago, OverclockingNub said:

i did that and im stlll getting the error?

No you're not, that's a different error... you should read what the messages say before pasting them here. now, the program compiles, but when it runs it gives you a segmentation fault. Seg faults occur when you try to access a memory area that was not initialized by your program, commonly when you try to access an index beyond an array's limits.

 

Let's see if we can find where:

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

consider what happens to i in this for loop. Every time the program is at the start of the loop it checks if i is less than 3, then it increases the value of I by 1 and runs the code inside the loop. This means that when i is 2 the loop will continue and the code will run with i = 3. Because your arrays only have 3 elements and indexing starts from 0, this is out of bounds. The same of course applies to j.

 

-edit-

as @AbydosOne pointed out the third statement is applied after the loop execution, which means the variable would go out of bounds only if the comparison operator was <=. In this case the cause is a typo in the inner loop.

[AbydosOne]

45 minutes ago, Sauron said:

Every time the program is at the start of the loop it checks if i is less than 3, then it increases the value of I by 1 and runs the code inside the loop.

Is that how for-loops behave? Good lord, no wonder why I never used them... now enhanced for-loops, that's where the good stuff is. #Java

 

 

EDIT: So I had to fact check this: the third expression is incremented after the code block, not before it.

Quote

Statement 1 is executed (one time) before the execution of the code block.

Statement 2 defines the condition for executing the code block.

Statement 3 is executed (every time) after the code block has been executed.

https://www.w3schools.com/c/c_for_loop.php

[AbydosOne]

1 hour ago, OverclockingNub said:

i did that and im stlll getting the error?

 

i needs to be j; all variables should match in each for-loop.

 

Also, paste your code into a code-block instead of screenshotting it. It's easier for us to read and copy/edit.

[Sauron]

1 hour ago, AbydosOne said:

 

EDIT: So I had to fact check this: the third expression is incremented after the code block, not before it.

ah, sorry, you're right... I missed the typo

 

that's what I get for answering when I should have already been in bed