Why does my program output random characters?

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C++

I'm working on an assignment where the program needs to print out a rectangular spiral matrix made out of characters with a given string.
string is :

   const char C[71] =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890 .,-!?()";

given 2 dimensional matrix:

char M[70][70];

When i compile the program on Xcode it outputs random characters, but when i compile it online on cpp shell, the matrix is printed out. Does anyone know the reason to this and how to solve it?

example of the random characters:

\340\215
\270\216
\354\217
\340\220
\370\221
\340\222
˓
\212\225
\250\226

֘
\245\232
\300\233
?\235
&\236
P\237
\270\240
\242
@\243

\256
\273\257
\243\260
\234\261
\247\262
\200\263
\344\264
X\266
Z\267

x\270
P\271
Z\272
\220\273
\375\274
\250\276
\300

here is my code:

 void showSetting() {
        cout<<"W="<<W<<", H="<<H<<endl;
        int i,startRow = 0, endRow, startCol = 0;
        int counter=0;
            
            endRow = H-1;
 
           while(startRow <= endRow && startCol <= W) {
        for (i = startCol; i < W; i++) {
            M[startCol][i] = C[counter];
            counter++;
        }
        startRow++;

        for (i = startRow; i < endRow; i++) {
            M[i][W] = C[counter];
            counter++;
        }
        W--;

        if (startRow <= endRow) {
            for (i = W; i >= startCol; i--) {
                M[endRow][i] =C[counter];
                counter++;
            }
            endRow--;
        }

        if (startCol <= W) {
            for (i = endRow; i >= startRow; i--) {
                M[i][startCol] = C[counter];
                counter++;
            }
            startCol++;
        }
             
    }
        
    
    for(i = 0; i <70; i++) {
        for (int j = 0; j <70; j++) {
            cout<<M[i][j];
        }
cout<<endl;
    }

What I have tried:

Since I'm new to programming I have don't what is the cause of this and how to solve it.