Printf changes the outcome the program

Question

I'm doing logistic regression in C, but I have problems with calculating the gradient for the y-intercept. Here's the code: - sigmoid_j() and log_odds_pred() works perfectly. I'll explain the problem in more detail in the specific line - at the function definition of gradient_i()

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

char m = 2, n = 2;

double sigmoid_j(double); //Gives probability prediction for a training example
double log_odds_pred(double *theta, double *b, float x[2][2], int ex_number); //Gives log-odds prediction for a training example

void gradient_i(double *theta, double *b, float x[2][2], char y[], double *dj_db); //Calculates gradient of a parameter for feature n

int main(){

    //Please ignore why I reset m, n. I'll fix this later
    m = 2;
    n = 2;

    float x[2][2] = {{16, 23}, {0.3,1}};
    char y[] = {0,1};
    //Perfect parameters - these are just the ones that my
    double theta_init[] = {0.018,0.034};
    double b_init = -0.025;

    //TEST - GRADIENT
    double dj_db = 0;
    gradient_i(theta_init,&b_init,x,y,&dj_db);
    printf("Gradient of b: %lf\n",dj_db);

    return 0;
}

void gradient_i(double *theta, double *b, float x[2][2], char y[], double *dj_db){
    double grad = 0;
    double y_hat_j; //prediction/hypothesis
    for(int j=0;j<m;j++){
        y_hat_j = sigmoid_j(log_odds_pred(theta,b,x,j));
       //printf("h_%d, %lf\n", j, y_hat_j);
//and here's the problem. If I don't use printf, I'm getting 0.1176
//for the gradient of b (output of the function), which is wrong. If I do print this line,
//I'll get 0.86, which is the right result. I'm guessing it updates
//y_hat_j, but I don't understand why?
        grad += (y_hat_j - y[j]);
    }
    grad /= m;
    *dj_db = grad;
}

double sigmoid_j(double z_j){
    return 1/(1+exp(-z_j));
}

double log_odds_pred(double *theta, double *b, float x[2][2], int ex_number){
    double result;
    for(int i=0; i<n;i++){
        result += (*(theta+i)) * (x[i][ex_number]);
    }
    result += *b;
    return result;
}

Without a print statement in each loop, the prediction is not getting updated.