使用atoi（）读取2个单独的整数值的行

Question

I have a header line in a file that represents a matrix I want to read, eg

R4 C4
1 0 0 0
0 1 0 0 
0 0 1 0
0 0 0 1

What I want to do is read the first line for the '4's in this case. But the numbers could be of arbitrary length (to some extent). After some searching I found that atoi() could do the trick (maybe):

int main ()
{
FILE * pFile;
FILE * pFile2;
pFile = fopen ("A.txt","r");
pFile2 = fopen ("B.txt","r");
char c;
int lincount = 0;
int rows;
int columns;
if (pFile == NULL) perror ("Error opening file");
else{
while ((c = fgetc(pFile)) != '\n')
{
    if(c == 'R'){
    c = fgetc(pFile);
    rows = atoi(c);
    }
    if(c == 'C'){
    c = fgetc(pFile);
    columns = atoi(c);
    break;
    }
}
lincount++;
printf("Rows is %d and Columns is %d\n", rows, columns);
}

The error I get at compiling is

warning: passing argument 1 of ‘atoi’ makes pointer from integer without a cast
[enabled by default]
/usr/include/stdlib.h:148:12: note: expected ‘const char *’ but argument is of type
‘char’

I don't understand how atoi() works or how to fix this, and the documentation doesn't help me because I don't understand from the examples that I have found how the input to atoi() could possibly be a pointer since they seem to just input characters in the examples.

Answer 1

First, atoi takes char * as argument. And you are providing char .

As you said that numbers can be of variable length. So it will be better if you make some change in the below part of your code.

Instead

if(c == 'R'){
    c = fgetc(pFile);
    rows = atoi(c);
    }
    if(c == 'C'){
    c = fgetc(pFile);
    columns = atoi(c);
    break;
    }

change it to

int row;
int column;

if(c == 'R'){
    fscanf(pFile, "%d", &row);
    //rows = atoi(c);   <----No need of atoi
    }
    if(c == 'C'){
    fscanf(pFile, "%d", &column);
    //columns = atoi(c);   <----No need of atoi
    break;
    }