how to read files with different name using fid

I am trying to make averaging for data saved in files named:pfoil_1_var_1_562474.raw, pfoil_1_var_1_562476.raw .......pfoil_1_var_1_562494.raw.

I have written the following lines but an error was generated

clc
clear
N = 6710*6; % grid size (number of points in raw file)
fid = fopen('pfoil_1_var_1_562474.raw','r'); % rb = read binary
data = fread(fid,N,'single');
fclose(fid);
start=8;
data1=data(start:length(data));
p=data1;
step=2
a=562476;
b=562494;
for i=a:step:b
fid = fopen('pfoil_1_var_1_',sprintf('%d',i),'.raw','r'); % rb = read binary
data = fread(fid,N,'single');
fclose(fid);
start=8;
data1=data(start:length(data));
p2=data1;
p=p+p2;
end

the error generated is

Error using fopen
Invalid permission.

Error in readsubspace_p (line 14)
fid = fopen('pfoil_1_var_1_',sprintf('%d',i),'.raw','r'); % rb = read binary
 

How to write this line correclty so the name of the file to be read change within the for loop

fid = fopen('pfoil_1_var_1_',sprintf('%d',i),'.raw','r'); % rb = read binary

Answer 1

[Stephen23]

fnm = sprintf('pfoil_1_var_1_%d.raw',i);
fid = fopen(fnm,'r');

[zein]

I used the followning lines and it worked properly

    filename=['pfoil_1_var_1_',num2str(i),'.raw'];
    fid = fopen(filename,'rb'); % rb = read binary

[Stephen23]

I recommend using SPRINTF rather than NUM2STR and string concatenation: it is fewer commands, more efficient, and gives you more control.