objective-c - Objective-C 中的动态二维 C 数组崩溃

Question

我正在使用二维数组形式的矩阵制作曲线拟合程序，但编译器随机抛出 BAD_ACCESS 错误，例如线程 1:EXC_BAD_ACCESS(代码= 1，地址= 0x13f800000)。该程序有时可以运行，有时会崩溃。任何想法，将不胜感激。谢谢。

- (void)main {
    NSLog(@"Enter size");

    scanf("%i", &matrixSize);

    float* temp;

    matrix = (float**)calloc(matrixSize, sizeof(float*));
    temp = (float*)calloc(matrixSize+1, sizeof(float));

    for (int i = 0; i < matrixSize+1; i++) {
        matrix[i] = temp + (i*(matrixSize+1));
    }

    [self enterPoints];
    [self elimination];
    [self setNeedsDisplay:YES];
    free(matrix);
    free(temp);
}

//points entered here

- (void)enterPoints {
    CGPoint *points = (CGPoint *)malloc(matrixSize * sizeof(CGPoint));

    for (int i = 0; i < matrixSize; i++) {
        scanf("%lf", &points[i].x);
        scanf("%lf", &points[i].y);
    }

    for (int j = 0; j < matrixSize; j++) {
        for (int i = 0; i < matrixSize+1; i++) {
            if (i == (matrixSize)) {
                matrix[i][j] = points[j].y;
            }
            else {
                matrix[i][j] = pow(points[j].x, (matrixSize-1)-i);
            }
        }
    }
    free(points);
}

//matrix reduction occurs here

- (void)elimination {

    for (int j = 0; j < matrixSize; j++) {
        double divideValue = matrix[j][j];
        for (int i = 0; i < matrixSize+1; i++) {
            matrix[i][j] /= divideValue;
        }
        for (int j1 = 0; j1 < matrixSize; j1++) {
            if (j1 == j) {
                if (j1 == matrixSize-1) {
                    break;
                }
                else {
                    j1++;
                }
            }
            double subValue = matrix[j][j1];
            for (int i = 0; i < matrixSize+1; i++) {
                matrix[i][j1] -= matrix[i][j]*subValue;
            }
        }
    }
}

//drawing the polynomial 

- (void)drawRect:(NSRect)dirtyRect {
    NSGraphicsContext * GraphicsContext = [NSGraphicsContext currentContext];
    CGContextRef context = (CGContextRef) [GraphicsContext graphicsPort];

    CGContextSetRGBStrokeColor(context, 0.0, 0.0, 0.0, 1.0);

    CGContextSetLineWidth(context, 3.0);

    CGContextMoveToPoint(context, 0, matrix[matrixSize][0]*100 + 100);

    [GraphicsContext saveGraphicsState];

    CGMutablePathRef path;

    path = CGPathCreateMutable();

    for (float i = -matrixSize; i < matrixSize; i+=.01) {
        float y = 0;

        for (int j = 0; j < matrixSize; j++) {
             y += matrix[matrixSize][j]*pow(i, j);
        }

        CGContextAddLineToPoint(context, i*100 + 100, y*100 + 100);   
    }

    CGContextStrokePath(context);

    [GraphicsContext restoreGraphicsState];
}

Answer 1

您没有为矩阵分配足够的内存。此行设置了整个数据区域，但您只分配了 matrixSize+1 元素，而不是 matrixSize*(matrixSize+1):

temp = (float*)calloc(matrixSize+1, sizeof(float));

因此，维护 matrixSize+1 列和 matrixSize 行:

matrix = (float**)calloc(matrixSize, sizeof(float*));
temp = (float*)calloc(matrixSize * (matrixSize+1), sizeof(float));

for (int i = 0; i < matrixSize; i++) {
    matrix[i] = temp + (i*(matrixSize+1));
}

以后使用这个的时候要小心。您的解决方式是错误的:

for (int j = 0; j < matrixSize; j++) {
    for (int i = 0; i < matrixSize+1; i++) {
        if (i == (matrixSize)) {
            matrix[i][j] = points[j].y;
        }
        else {
            matrix[i][j] = pow(points[j].x, (matrixSize-1)-i);
        }
    }
}

请注意，i 转到 matrixSize+1，但您将其用作行索引(只有 matrixSize 行)。我认为您打算使用matrix[j][i]而不是matrix[i][j]。您在构造初始矩阵时也可以这样做，但实际上我已将其更改为符合您的分配。

所以我发现你的程序中有两点缓冲区溢出。