# Givens Rotation trouble

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Hello, I did implement a QR decomposition using givens rotation, however the signs of the values seem to be wrong Maybe you can find some mistakes, I have check several documentations already
397         inline matrix4 givensrotation( const matrix4& m, uint i, uint j)
398         {
399                 const static float epsilon = 0.0001f;
400                 matrix4 g = m;
401
402                 float a = m[j][j];
403                 float b = m[j];
404
405                 float c;
406                 float s;
407                 float t;
408
409                 if(fabs(b) < epsilon)
410                 {
411                         c = copysign(1,a);
412                         s = 0.0f;
413                 }
414                 else if(fabs(a) < epsilon)
415                 {
416                         c = 0.0f;
417                         s = copysign(1,b);
418                 }
419                 else
420                 {
421                         t = copysign(sqrt(a*a+b*b),a);
422                         c = a/t;
423                         s = b/t;
424                 }
425
426                 for(uint k=0;k<4;++k)
427                 {
428                         float x = g[j][k];
429                         float y = g[k];
430
431                         g[j][k] = c*x+s*y;
432                         g[k] = -s*x+c*y;
433                 }
434
435                 return g;
436         };


As a result I get this Input: 9 2 3 3 5 6 7 8 9 6 1 2 7 4 5 6 Result: 15.362290 8.462279 6.900012 8.266995 0.000000 4.515510 3.457011 4.438562 0.000000 0.000000 4.943572 4.978882 0.000000 0.000000 0.000000 -0.408248 Result in matlab: -15.362290 -8.462279 -6.900012 -8.266995 0.000000 -4.515510 -3.457011 -4.438562 0.000000 0.000000 4.943572 4.978882 0.000000 0.000000 0.000000 0.408248

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I'm not sure why you do the copysigns; I don't do that in my QR code. Other than that, I would just make sure that the way you're indexing your matrix elements results in the Givens matrix being multiplied on the right rather than on the left (unless you derived your formulas for the opposite way).

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Rutin
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