Código HTML:
var form=document.form;
for(var ctr = 0 ; ctr < current_form.length; ctr++){
if(form.cantidad[ctr].value==0 ){
alert("Ingrese la cantidad a solicitar");
form.cantidad[ctr].value="";
form.cantidad[ctr].focus();
return false;
}else{
current_form.submit()
}
}

.
. . . .

