Why is each element of the first row in my array off by 0.5? | Together C & C++ | Page 1

rapid herald Oct 30, 2022, 5:01 AM

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I am preforming matrix multiplication;

void matrix_multiply(int m, int n, int k, double A[m][n], double B[n][k], double C[m][k]){
    
    int i; // feeling like an i day
    int v;
    int q;
    
    for(i = 0; i < m; i++){
        for(v = 0; v < k; v++){
            C[i][v] = 0; // clear any garbage
            for(q = 0; q < n; q++){
                C[i][v] += (A[i][q] * B[q][v]);
            } 
            
        }
    }
    
    

}```

of the following two 2d-arrays:
```c
    double M1[3][3] = { {1, 2, 3},
                        {4, 5, 6},
                        {7, 8, 9} };

    double M2[3][3] = { {1, 1, 1},
                        {0, 2, 2},
                        {0, 0, 3}    };

And I get the following output: (see screenshot)

tired tundraBOT Oct 30, 2022, 5:01 AM

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When your question is answered use !solved to mark the question as resolved.

Remember to ask specific questions, provide necessary details, and reduce your question to its simplest form. For tips on how to ask a good question run !howto ask.

rapid herald Oct 30, 2022, 5:03 AM

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Like its only happens in that one case, every other time I try multiplying others it works

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and is not off by 0.5

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(this is what the output should be)

gaunt scroll Oct 30, 2022, 5:23 AM

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rapid herald (this is what the output should be)

@rapid herald I run the program and it gets this exact output

rapid herald Oct 30, 2022, 5:23 AM

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huh

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but how

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how would mine be broken

gaunt scroll Oct 30, 2022, 5:24 AM

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I have no idea, I plugged in your exact function. Idk if you do any weird subtraction when you print? Lol

rapid herald Oct 30, 2022, 5:25 AM

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gaunt scroll Oct 30, 2022, 5:27 AM

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Well, I would have to see print_matrix because otherwise I just see a black box (pun intended)

rapid herald Oct 30, 2022, 5:27 AM

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lol

gaunt scroll Oct 30, 2022, 5:27 AM

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What IDE or text editor are you using by the way, looks like it's in a terminal

rapid herald Oct 30, 2022, 5:27 AM

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its my schools custom made IDE

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i tried restarting it

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and it still had the same output

gaunt scroll Oct 30, 2022, 5:28 AM

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oh fancy. I like the contrast, wonder if I could do it with sublime

rapid herald Oct 30, 2022, 5:30 AM

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did it work for you?

gaunt scroll Oct 30, 2022, 5:30 AM

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I showed you the printout

rapid herald Oct 30, 2022, 5:31 AM

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sorry I forgot

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I did not send

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gaunt scroll Oct 30, 2022, 5:34 AM

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#

;compile

#include <stdio.h>

void matrix_multiply(int m, int n, int k, double A[m][n], double B[n][k], double C[m][k]){
    
    int i; // feeling like an i day
    int v;
    int q;
    
    for(i = 0; i < m; i++){
        for(v = 0; v < k; v++){
            C[i][v] = 0; // clear any garbage
            for(q = 0; q < n; q++){
                C[i][v] += (A[i][q] * B[q][v]);
            } 
            
        }
    }
    
    

}
void print_matrix(int rows, int columns, double M[rows][columns]){
    for(int i = 0; i < rows; i++){
        for(int j=0; j < columns; j++){
            printf("%7.2f ", M[i][j]);
        }
        printf("\n");
    }
}
int main(){
    double M1[3][3] = { {1, 2, 3},
                        {4, 5, 6},
                        {7, 8, 9} };

    double M2[3][3] = { {1, 1, 1},
                        {0, 2, 2},
                        {0, 0, 3}    };
    double output[3][3] = {{0}};
    
    matrix_multiply(3,3,3,M1,M2,output);
    
    int num_rows = sizeof(output) / sizeof(output[0]);
    int num_cols = sizeof(output[0]) / sizeof(output[0][0]);
    
    print_matrix(num_rows,num_cols,output);
    return 0;
}

tight valveBOT Oct 30, 2022, 5:35 AM

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Program Output

1.00    5.00   14.00 
   4.00   14.00   32.00 
   7.00   23.00   50.00

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Program Output

1.00    5.00   14.00 
   4.00   14.00   32.00 
   7.00   23.00   50.00

rapid herald Oct 30, 2022, 5:36 AM

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wtf

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what in the actuall hecc

gaunt scroll Oct 30, 2022, 5:36 AM

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What lol

rapid herald Oct 30, 2022, 5:36 AM

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why does mine not work then wtf

gaunt scroll Oct 30, 2022, 5:37 AM

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Ah, yeah that's problematic isn't it

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Have access to a debugger, or gdb?

rapid herald Oct 30, 2022, 5:38 AM

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no :9

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:(

gaunt scroll Oct 30, 2022, 5:38 AM

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Umm how do you compile your code

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Are you on windows or linux

rapid herald Oct 30, 2022, 5:38 AM

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im on windows but im compiling through a vm i think on linux

gaunt scroll Oct 30, 2022, 5:39 AM

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Using WSL or are you actually on a vm like VMware or something

rapid herald Oct 30, 2022, 5:39 AM

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I am not entirely sure

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probs not VMware

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idk

gaunt scroll Oct 30, 2022, 5:39 AM

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The entire application is proprietary university software?

rapid herald Oct 30, 2022, 5:39 AM

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yes

gaunt scroll Oct 30, 2022, 5:39 AM

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The virtual machine so to speak?

rapid herald Oct 30, 2022, 5:40 AM

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I guess so

gaunt scroll Oct 30, 2022, 5:40 AM

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Is there a shell or are you clicking on GUI shit

rapid herald Oct 30, 2022, 5:40 AM

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wait nvm sorry not allowed to leak that

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i forgot

gaunt scroll Oct 30, 2022, 5:40 AM

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leak?

rapid herald Oct 30, 2022, 5:41 AM

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welll

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idk if im alowed to share it idk

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im sure its ok

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maybe?

gaunt scroll Oct 30, 2022, 5:41 AM

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Well, sorry I already opened it, but it looks like you have a terminal (I call it a shell too)

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You're using gcc! You are compiling your code manually

rapid herald Oct 30, 2022, 5:41 AM

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oh idk what that is

gaunt scroll Oct 30, 2022, 5:42 AM

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It's a whole other world amigo. But the shell does look some form of linux or something

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Clean your program and compile again:
gcc -Wall -sdt=c18 -g3 -o vector_tester vector_tester.c matrix_and_vector.c -lm

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Then afterwards:
gdb ./vector_tester

rapid herald Oct 30, 2022, 5:43 AM

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clean my program?

gaunt scroll Oct 30, 2022, 5:43 AM

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By the way, your school should teach you how to use makefiles or Cmake

rapid herald Oct 30, 2022, 5:43 AM

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eventually

gaunt scroll Oct 30, 2022, 5:43 AM

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rm *.o vector_tester
This should clean up all your object files and your executable

rapid herald Oct 30, 2022, 5:44 AM

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rm: cannot remove '*.o': No such file or directory

gaunt scroll Oct 30, 2022, 5:44 AM

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what's ls show?

rapid herald Oct 30, 2022, 5:44 AM

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rm: cannot remove 'vector_tester': No such file or directory```

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oh wait

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this will be under matrix_tester

gaunt scroll Oct 30, 2022, 5:45 AM

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Go to the directory you build your program in

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Yeah glad I didn't make you remove random shit lmao

rapid herald Oct 30, 2022, 5:45 AM

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this?

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o wait thats my .h lol

gaunt scroll Oct 30, 2022, 5:46 AM

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rapid herald this?

mi amigo, where is the directory you ran the command:
gcc -Wall -sdt=c18 -o vector_tester vector_tester.c matrix_and_vector.c -lm

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You sent a screenshot earlier, I saw you have a terminal on the bottom of your screen. That had this command in it

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You need to type in your terminal for me

rapid herald Oct 30, 2022, 5:47 AM

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ok im there

gaunt scroll Oct 30, 2022, 5:47 AM

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Run the command ls and show me the printout

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Not the whole screen, just the terminal

rapid herald Oct 30, 2022, 5:48 AM

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there are like all my files ever

gaunt scroll Oct 30, 2022, 5:48 AM

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Okay

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find . -type f -name "vector_tester*"

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Run that command

rapid herald Oct 30, 2022, 5:48 AM

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it did something

gaunt scroll Oct 30, 2022, 5:48 AM

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what's the output amigo

rapid herald Oct 30, 2022, 5:48 AM

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./vector_tester.c```

gaunt scroll Oct 30, 2022, 5:49 AM

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Okay so you're in the directory. Looks like you don't have any object files and the executable vector_tester is not compiled

rapid herald Oct 30, 2022, 5:49 AM

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is that a W

gaunt scroll Oct 30, 2022, 5:49 AM

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Run the command in the same directory:
gcc -Wall -sdt=c18 -g3 -o vector_tester vector_tester.c matrix_and_vector.c -lm

rapid herald Oct 30, 2022, 5:49 AM

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gcc: error: unrecognized command-line option ‘-sdt=c18’; did you mean ‘-std=c18’?

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wait wut

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lol

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ah

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yes sorry

gaunt scroll Oct 30, 2022, 5:50 AM

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okay if you can't fix that, just delete -std=c18 idk what you need from that one

rapid herald Oct 30, 2022, 5:50 AM

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i see

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sorry im just really tired

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i saw it after i posted

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rn im just brain off dumping everything

gaunt scroll Oct 30, 2022, 5:50 AM

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It's okay. Did you run it and did it work

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find . -type f -name "vector_tester*" run this command again

rapid herald Oct 30, 2022, 5:51 AM

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it did not work

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'tis the same

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so there must be something randomly wrong with it

gaunt scroll Oct 30, 2022, 5:52 AM

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What is the gcc command saying?

rapid herald Oct 30, 2022, 5:52 AM

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nothing it just compiled normally

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oh i meant like

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the numbers

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are still 0.5 off

gaunt scroll Oct 30, 2022, 5:52 AM

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Oh okay

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We'll fix it. Run the find command again. I wanted to also have you run the program, but it's good you checked already

rapid herald Oct 30, 2022, 5:53 AM

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yay

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we will get this

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but like realisticly

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how on earth can I even get a decimal point when multiplying whole numbers

gaunt scroll Oct 30, 2022, 5:54 AM

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I don't know, that's why we have a debugger

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We can set things like watchpoints that should tell you when the value changes

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Or we can step through it line by line and examine values

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You need to find the path to the executable veector_tester which I believe is in your working directory.
Then run the command gdb ./vector_tester

rapid herald Oct 30, 2022, 5:55 AM

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btw

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its on

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matrix_tester

gaunt scroll Oct 30, 2022, 5:55 AM

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oh

rapid herald Oct 30, 2022, 5:55 AM

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vector testor is the first half of it

gaunt scroll Oct 30, 2022, 5:56 AM

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Okay well the important thing is, build everything with that -g3 flag I put in my initial command

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Seems like you have another program I got confused with because your -o vector_tester is saying to link and make an executable file named vector_tester

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So when you build your matrix program, make sure to also have the -g3. Otherwise debugger will have issues

rapid herald Oct 30, 2022, 5:57 AM

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okay

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ill do that now

gaunt scroll Oct 30, 2022, 5:58 AM

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You might have to clean the files again, removing the object files and the executable. Idk

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But just try what I said, and try running the gdb

rapid herald Oct 30, 2022, 5:58 AM

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whats gdb?

gaunt scroll Oct 30, 2022, 5:58 AM

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Another command, like gcc

rapid herald Oct 30, 2022, 5:59 AM

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it ran

gaunt scroll Oct 30, 2022, 5:59 AM

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gnu c compiler, gnu debugger

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start

rapid herald Oct 30, 2022, 5:59 AM

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bash: start: command not found

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D:

gaunt scroll Oct 30, 2022, 5:59 AM

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.. gdb should run, and then you should be in a new shell

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start isn't a bash command (your terminal)

rapid herald Oct 30, 2022, 6:00 AM

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oh weird

gaunt scroll Oct 30, 2022, 6:00 AM

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Your terminal runs the gdb command and it opens a new shell. In your current terminal, run which gdb

rapid herald Oct 30, 2022, 6:00 AM

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it just compiled like normally

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when I ran gdb

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it never made a new shell

gaunt scroll Oct 30, 2022, 6:00 AM

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gdb doesn't compile, gcc compiles

rapid herald Oct 30, 2022, 6:00 AM

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ohh

gaunt scroll Oct 30, 2022, 6:00 AM

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did you run gdb ./matrix_tester

rapid herald Oct 30, 2022, 6:01 AM

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gaunt scroll Oct 30, 2022, 6:01 AM

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YES

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THAT'S THE GDB SHELL

rapid herald Oct 30, 2022, 6:02 AM

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yaaay

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ok now what do I do

gaunt scroll Oct 30, 2022, 6:02 AM

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Find the file name and line number that corresponds to this:

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When you find that information, type this in the shell:
b FILE_NAME:LINE_NUMBER
Example: matrix_tester.c:42

rapid herald Oct 30, 2022, 6:03 AM

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matrix_and_vector.c

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line 409

gaunt scroll Oct 30, 2022, 6:03 AM

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b matrix_and_vector.c:409

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then run continue after

rapid herald Oct 30, 2022, 6:03 AM

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gaunt scroll Oct 30, 2022, 6:03 AM

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okay

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start

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then continue

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or maybe you have to say run then continue

rapid herald Oct 30, 2022, 6:04 AM

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gaunt scroll Oct 30, 2022, 6:04 AM

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Okay look, you've stopped code execution at the line that's printed

rapid herald Oct 30, 2022, 6:04 AM

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wut

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epic

gaunt scroll Oct 30, 2022, 6:05 AM

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you know what, let me show you some real magic

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Actually we'll hold off on that, might use that later

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Just run this:
p A[i][q]
p B[q][v]

rapid herald Oct 30, 2022, 6:06 AM

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two seperate lines or all in one?

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probs a dumb question sorry

gaunt scroll Oct 30, 2022, 6:06 AM

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Then
p A[i][q]*B[q][v]

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All separate lines

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p C[i][v]

rapid herald Oct 30, 2022, 6:07 AM

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gaunt scroll Oct 30, 2022, 6:07 AM

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p i
p q

rapid herald Oct 30, 2022, 6:08 AM

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gaunt scroll Oct 30, 2022, 6:09 AM

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p A[0][0] @ len ?

rapid herald Oct 30, 2022, 6:10 AM

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gaunt scroll Oct 30, 2022, 6:10 AM

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Oh woops

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Not len but 3

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OH WAIT
p (double *)A[i][q] maybe that works??

rapid herald Oct 30, 2022, 6:11 AM

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gaunt scroll Oct 30, 2022, 6:13 AM

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p ((double (*)[3]) A)[i][q]

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Okay I've never seen that syntax, and I'll admit that's a little bit cancerous

rapid herald Oct 30, 2022, 6:14 AM

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wait

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so there is a 0.5 in memory

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somewhere

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i know a little bit about pointers

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whats it doing in there

gaunt scroll Oct 30, 2022, 6:14 AM

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Yes, that's an element in your A array

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The first one, at [0][0]

rapid herald Oct 30, 2022, 6:15 AM

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ah i see i see

gaunt scroll Oct 30, 2022, 6:17 AM

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rapid herald I am preforming matrix multiplication; ```c void matrix_multiply(int m, int n,...

I mean, your first element here is 1, and the A matrix is passed in from M1 so that confirms it should be 1.0 ...

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Wait no, that screenshot.. where are you initializing these arrays??

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I guess i don't see M1 and M2

rapid herald Oct 30, 2022, 6:19 AM

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gaunt scroll Oct 30, 2022, 6:20 AM

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That's very weird.
d 1
finish
in your shell

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Then afterwords
p ((double (*)[3]) M1)[0][0]

rapid herald Oct 30, 2022, 6:21 AM

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this might be dumb but would running it on a different machine make a difference

gaunt scroll Oct 30, 2022, 6:22 AM

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So yeah just a sanity check, they are both saying the same thing. Same address, etcetera

rapid herald Oct 30, 2022, 6:22 AM

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no it wont

gaunt scroll Oct 30, 2022, 6:22 AM

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I mean, I ran it on two separate "machines" and got the "right" value, so idk probably

rapid herald Oct 30, 2022, 6:22 AM

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sorry yeah dumb question

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its on a vm tth

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tho

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it did not make a difference

gaunt scroll Oct 30, 2022, 6:25 AM

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🤷‍♂️ Yeah we checked it out, your matrix in the source code says 1, but at runtime it's saying 0.5. I'd bring it up to your teacher, TA, or professor

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Might need something more involved, I tried getting deep into it, weird bug. Maybe somebody else will read this at some point and give input

rapid herald Oct 30, 2022, 6:26 AM

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I can message my prof rn

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but I wanna sound like I know what Im doing

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so i need you to do my words for me

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hahahahhaha

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cus honestly idk what the hell we just did

gaunt scroll Oct 30, 2022, 6:26 AM

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Well figuring it out is my homework to you, good luck fam salute

rapid herald Oct 30, 2022, 6:27 AM

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shit

gaunt scroll Oct 30, 2022, 6:27 AM

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Everything I mentioned you can search online and it'll go in detail. you'll sound smarter because you'll be smarter, you got dis

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I only speak from experience 😜

rapid herald Oct 30, 2022, 6:27 AM

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its due tomorrow and I have so much other stuff I gotta do

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well

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the quick fix

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is to add 0.5 to everything

gaunt scroll Oct 30, 2022, 6:29 AM

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To everything? I can't think off the top of my head right now why the output would all be off by 0.5, but I think adjusting the inputs completely will be too much. Maybe one or two elements are off? Like the first one

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Idk, good luck, experiment with it. Or say fuck it and email the prof

rapid herald Oct 30, 2022, 6:31 AM

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rip

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well thanks for the help

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well, like time spend trying to

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!solved

tired tundraBOT Oct 30, 2022, 6:32 AM

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Thank you and let us know if you have any more questions!

rapid herald Oct 30, 2022, 6:32 AM

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but not really solved

gaunt scroll Oct 30, 2022, 6:32 AM

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I probably wouldn't have marked it solved lol. Sorry, weird bug

rapid herald Oct 30, 2022, 6:32 AM

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oh

gaunt scroll Oct 30, 2022, 6:32 AM

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idk who cares

rapid herald Oct 30, 2022, 6:32 AM

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me D;

gaunt scroll Oct 30, 2022, 6:32 AM

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message somebody else in class

#Why is each element of the first row in my array off by 0.5?