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C++ array help

Started by Frank Montanaro Oct 1, 2008 at 3:00 PM 3 replies 800+ views

Original Post

Oct 1, 2008 at 3:00 PM

Hey, I'm having a bit of array problems in this C++ thing, I was brought up on Java so this is really puzzling me as to why I am getting these results. Here is the code (just the basic functions used) followed by my output. My Methods: GLfloat* mulMatrix( GLfloat m1[], GLfloat m2[]){ GLfloat p[16]; int i; for(i = 0; i < 16; i++){ p = m1 * m2; printf("m1[%d] * m2[%d] = %f * %f = %f, ",i,i,m1,m2,p); } return p; } void print(GLfloat m[], int s){ int i; for(i = 0; i < s; i++){ printf("%f ,",m); if(i%4 == 0) printf("\n"); } } --------------- Calling the methods: GLfloat m1[16]= {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16}; GLfloat m2[16]= {2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2}; print(mulMatrix(m1,m2),16); ----------------- OUPUT: m1[0] * m2[0] = 1.000000 * 2.000000 = 2.000000, m1[1] * m2[1] = 2.000000 * 2.000000 = 4.000000, m1[2] * m2[2] = 3.000000 * 2.000000 = 6.000000, m1[3] * m2[3] = 4.000000 * 2.000000 = 8.000000, m1[4] * m2[4] = 5.000000 * 2.000000 = 10.000000, m1[5] * m2[5] = 6.000000 * 2.000000 = 12.000000, m1[6] * m2[6] = 7.000000 * 2.000000 = 14.000000, m1[7] * m2[7] = 8.000000 * 2.000000 = 16.000000, m1[8] * m2[8] = 9.000000 * 2.000000 = 18.000000, m1[9] * m2[9] = 10.000000 * 2.000000 = 20.000000, m1[10] * m2[10] = 11.000000 * 2.000000 = 22.000000, m1[11] * m2[11] = 12.000000 * 2.000000 = 24.000000, m1[12] * m2[12] = 13.000000 * 2.000000 = 26.000000, m1[13] * m2[13] = 14.000000 * 2.000000 = 28.000000, m1[14] * m2[14] = 15.000000 * 2.000000 = 30.000000, m1[15] * m2[15] = 16.000000 * 2.000000 = 32.000000, ----------seperated here for reading's sake 2.000000 , 0.000000 ,-1.939702 ,-1.939702 ,-1.939730 , -1.939706 ,0.000000 ,0.000000 ,0.000000 , 0.007812 ,22.000000 ,24.000000 ,26.000000 , freeglut (./plant): Unable to create direct context rendering for window 'plant' This may hurt performance. 28.000000 ,30.000000 ,32.000000 , ----------------------------- So, It seems to be doing all the stuff right inside of the method, but then when calling the print method, I get some funky answers except for the last 6 spaces in the array. I'm pretty confused as to why it isn't working. it works for another method I have that multiples a vector x matrix but not for this one which is matrix x matrix. Thanks, Frank

rufus74

Oct 1, 2008 at 3:12 PM

GLfloat* mulMatrix( GLfloat m1[], GLfloat m2[]){GLfloat p[16];int i;for(i = 0; i < 16; i++){p = m1 * m2;printf("m1[%d] * m2[%d] = %f * %f = %f, ",i,i,m1,m2,p);}return p;}

You are returning a variable that is temporary created inside your method. It goes out of scope.

I would do something like this:

void mulMatrix( GLfloat m1[], GLfloat m2[], GLfloat* p){int i;for(i = 0; i < 16; i++){p = m1 * m2;printf("m1[%d] * m2[%d] = %f * %f = %f, ",i,i,m1,m2,p);}} // .....GLfloat p[16];mulMatrix(m1,m2, p);print(p,16);

cheers.

Frank Montanaro

Author

Frank Montanaro

Oct 1, 2008 at 3:20 PM

Ahh so you pass the pointer instead of returning one! Sweet, thanks man!

-Frank

KulSeran

196

KulSeran

Oct 1, 2008 at 3:30 PM

You should make note that that isn't how matrices multiply together.
Wiki on Matrix

owiley

Oct 1, 2008 at 3:31 PM

you return one but you have to grab the whole memory else its just a value and not a reference. I think is how it works but i know its possible cause this is how how you return arrays

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